15 Classes

15.1 General

A class is a data structure that may contain data members (constants and fields), function members (methods, properties, events, indexers, operators, instance constructors, finalizers, and static constructors), and nested types. Class types support inheritance, a mechanism whereby a derived class can extend and specialize a base class.

15.2 Class declarations

15.2.1 General

A class_declaration is a type_declaration (§14.7) that declares a new class.

class_declaration
    : attributes? class_modifier* 'partial'? 'class' identifier
        type_parameter_list? class_base? type_parameter_constraints_clause*
        class_body ';'?
    ;

A class_declaration consists of an optional set of attributes (§22), followed by an optional set of class_modifiers (§15.2.2), followed by an optional partial modifier (§15.2.7), followed by the keyword class and an identifier that names the class, followed by an optional type_parameter_list (§15.2.3), followed by an optional class_base specification (§15.2.4), followed by an optional set of type_parameter_constraints_clauses (§15.2.5), followed by a class_body (§15.2.6), optionally followed by a semicolon.

A class declaration shall not supply a type_parameter_constraints_clauses unless it also supplies a type_parameter_list.

A class declaration that supplies a type_parameter_list is a generic class declaration. Additionally, any class nested inside a generic class declaration or a generic struct declaration is itself a generic class declaration, since type arguments for the containing type shall be supplied to create a constructed type (§8.4).

15.2.2 Class modifiers

15.2.2.1 General

A class_declaration may optionally include a sequence of class modifiers:

class_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'abstract'
    | 'sealed'
    | 'static'
    | unsafe_modifier   // unsafe code support
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

It is a compile-time error for the same modifier to appear multiple times in a class declaration.

The new modifier is permitted on nested classes. It specifies that the class hides an inherited member by the same name, as described in §15.3.5. It is a compile-time error for the new modifier to appear on a class declaration that is not a nested class declaration.

The public, protected, internal, and private modifiers control the accessibility of the class. Depending on the context in which the class declaration occurs, some of these modifiers might not be permitted (§7.5.2).

When a partial type declaration (§15.2.7) includes an accessibility specification (via the public, protected, internal, and private modifiers), that specification shall agree with all other parts that include an accessibility specification. If no part of a partial type includes an accessibility specification, the type is given the appropriate default accessibility (§7.5.2).

The abstract, sealed, and static modifiers are discussed in the following subclauses.

15.2.2.2 Abstract classes

The abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class in the following ways:

• An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
• An abstract class is permitted (but not required) to contain abstract members.
• An abstract class cannot be sealed.

When a non-abstract class is derived from an abstract class, the non-abstract class shall include actual implementations of all inherited abstract members, thereby overriding those abstract members.

Example: In the following code

abstract class A
{
    public abstract void F();
}

abstract class B : A
{
    public void G() {}
}

class C : B
{
    public override void F()
    {
        // Actual implementation of F
    }
}

the abstract class A introduces an abstract method F. Class B introduces an additional method G, but since it doesn’t provide an implementation of F, B shall also be declared abstract. Class C overrides F and provides an actual implementation. Since there are no abstract members in C, C is permitted (but not required) to be non-abstract.

end example

If one or more parts of a partial type declaration (§15.2.7) of a class include the abstract modifier, the class is abstract. Otherwise, the class is non-abstract.

15.2.2.3 Sealed classes

The sealed modifier is used to prevent derivation from a class. A compile-time error occurs if a sealed class is specified as the base class of another class.

A sealed class cannot also be an abstract class.

Note: The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain run-time optimizations. In particular, because a sealed class is known to never have any derived classes, it is possible to transform virtual function member invocations on sealed class instances into non-virtual invocations. end note

If one or more parts of a partial type declaration (§15.2.7) of a class include the sealed modifier, the class is sealed. Otherwise, the class is unsealed.

15.2.2.4 Static classes

15.2.2.4.1 General

The static modifier is used to mark the class being declared as a static class. A static class shall not be instantiated, shall not be used as a type and shall contain only static members. Only a static class can contain declarations of extension methods (§15.6.10).

A static class declaration is subject to the following restrictions:

• A static class shall not include a sealed or abstract modifier. (However, since a static class cannot be instantiated or derived from, it behaves as if it was both sealed and abstract.)
• A static class shall not include a class_base specification (§15.2.4) and cannot explicitly specify a base class or a list of implemented interfaces. A static class implicitly inherits from type object.
• A static class shall only contain static members (§15.3.8).

  Note: All constants and nested types are classified as static members. end note

• A static class shall not have members with protected, private protected, or protected internal declared accessibility.

It is a compile-time error to violate any of these restrictions.

A static class has no instance constructors. It is not possible to declare an instance constructor in a static class, and no default instance constructor (§15.11.5) is provided for a static class.

The members of a static class are not automatically static, and the member declarations shall explicitly include a static modifier (except for constants and nested types). When a class is nested within a static outer class, the nested class is not a static class unless it explicitly includes a static modifier.

If one or more parts of a partial type declaration (§15.2.7) of a class include the static modifier, the class is static. Otherwise, the class is not static.

15.2.2.4.2 Referencing static class types

A namespace_or_type_name (§7.8) is permitted to reference a static class if

• The namespace_or_type_name is the T in a namespace_or_type_name of the form T.I, or
• The namespace_or_type-name is the T in a typeof_expression (§12.8.17) of the form typeof(T).

A primary_expression (§12.8) is permitted to reference a static class if

• The primary_expression is the E in a member_access (§12.8.7) of the form E.I.

In any other context, it is a compile-time error to reference a static class.

Note: For example, it is an error for a static class to be used as a base class, a constituent type (§15.3.7) of a member, a generic type argument, or a type parameter constraint. Likewise, a static class cannot be used in an array type, a new expression, a cast expression, an is expression, an as expression, a sizeof expression, or a default value expression. end note

15.2.3 Type parameters

A type parameter is a simple identifier that denotes a placeholder for a type argument supplied to create a constructed type. By constrast, a type argument (§8.4.2) is the type that is substituted for the type parameter when a constructed type is created.

type_parameter_list
    : '<' type_parameters '>'
  ;

type_parameters
    : attributes? type_parameter
    | type_parameters ',' attributes? type_parameter
    ;

type_parameter is defined in §8.5.

Each type parameter in a class declaration defines a name in the declaration space (§7.3) of that class. Thus, it cannot have the same name as another type parameter of that class or a member declared in that class. A type parameter cannot have the same name as the type itself.

Two partial generic type declarations (in the same program) contribute to the same unbound generic type if they have the same fully qualified name (which includes a generic_dimension_specifier (§12.8.17) for the number of type parameters) (§7.8.3). Two such partial type declarations shall specify the same name for each type parameter, in order.

15.2.4 Class base specification

15.2.4.1 General

A class declaration may include a class_base specification, which defines the direct base class of the class and the interfaces (§18) directly implemented by the class.

class_base
    : ':' class_type
    | ':' interface_type_list
    | ':' class_type ',' interface_type_list
    ;

interface_type_list
    : interface_type (',' interface_type)*
    ;

15.2.4.2 Base classes

When a class_type is included in the class_base, it specifies the direct base class of the class being declared. If a non-partial class declaration has no class_base, or if the class_base lists only interface types, the direct base class is assumed to be object. When a partial class declaration includes a base class specification, that base class specification shall reference the same type as all other parts of that partial type that include a base class specification. If no part of a partial class includes a base class specification, the base class is object. A class inherits members from its direct base class, as described in §15.3.4.

Example: In the following code

class A {}
class B : A {}

Class A is said to be the direct base class of B, and B is said to be derived from A. Since A does not explicitly specify a direct base class, its direct base class is implicitly object.

end example

For a constructed class type, including a nested type declared within a generic type declaration (§15.3.9.7), if a base class is specified in the generic class declaration, the base class of the constructed type is obtained by substituting, for each type_parameter in the base class declaration, the corresponding type_argument of the constructed type.

Example: Given the generic class declarations

class B<U,V> {...}
class G<T> : B<string,T[]> {...}

the base class of the constructed type G<int> would be B<string,int[]>.

end example

The base class specified in a class declaration can be a constructed class type (§8.4). A base class cannot be a type parameter on its own (§8.5), though it can involve the type parameters that are in scope.

Example:

class Base<T> {}

// Valid, non-constructed class with constructed base class
class Extend1 : Base<int> {}

// Error, type parameter used as base class
class Extend2<V> : V {}

// Valid, type parameter used as type argument for base class
class Extend3<V> : Base<V> {}

end example

The direct base class of a class type shall be at least as accessible as the class type itself (§7.5.5). For example, it is a compile-time error for a public class to derive from a private or internal class.

The direct base class of a class type shall not be any of the following types: System.Array, System.Delegate, System.Enum, System.ValueType or the dynamic type. Furthermore, a generic class declaration shall not use System.Attribute as a direct or indirect base class (§22.2.1).

In determining the meaning of the direct base class specification A of a class B, the direct base class of B is temporarily assumed to be object, which ensures that the meaning of a base class specification cannot recursively depend on itself.

Example: The following

class X<T>
{
    public class Y{}
}

class Z : X<Z.Y> {}

is in error since in the base class specification X<Z.Y> the direct base class of Z is considered to be object, and hence (by the rules of §7.8) Z is not considered to have a member Y.

end example

The base classes of a class are the direct base class and its base classes. In other words, the set of base classes is the transitive closure of the direct base class relationship.

Example: In the following:

class A {...}
class B<T> : A {...}
class C<T> : B<IComparable<T>> {...}
class D<T> : C<T[]> {...}

the base classes of D<int> are C<int[]>, B<IComparable<int[]>>, A, and object.

end example

Except for class object, every class has exactly one direct base class. The object class has no direct base class and is the ultimate base class of all other classes.

It is a compile-time error for a class to depend on itself. For the purpose of this rule, a class directly depends on its direct base class (if any) and directly depends on the nearest enclosing class within which it is nested (if any). Given this definition, the complete set of classes upon which a class depends is the transitive closure of the directly depends on relationship.

Example: The example

class A : A {}

is erroneous because the class depends on itself. Likewise, the example

class A : B {}
class B : C {}
class C : A {}

is in error because the classes circularly depend on themselves. Finally, the example

class A : B.C {}
class B : A
{
    public class C {}
}

results in a compile-time error because A depends on B.C (its direct base class), which depends on B (its immediately enclosing class), which circularly depends on A.

end example

A class does not depend on the classes that are nested within it.

Example: In the following code

class A
{
    class B : A {}
}

B depends on A (because A is both its direct base class and its immediately enclosing class), but A does not depend on B (since B is neither a base class nor an enclosing class of A). Thus, the example is valid.

end example

It is not possible to derive from a sealed class.

Example: In the following code

sealed class A {}
class B : A {} // Error, cannot derive from a sealed class

Class B is in error because it attempts to derive from the sealed class A.

end example

15.2.4.3 Interface implementations

A class_base specification may include a list of interface types, in which case the class is said to implement the given interface types. For a constructed class type, including a nested type declared within a generic type declaration (§15.3.9.7), each implemented interface type is obtained by substituting, for each type_parameter in the given interface, the corresponding type_argument of the constructed type.

The set of interfaces for a type declared in multiple parts (§15.2.7) is the union of the interfaces specified on each part. A particular interface can only be named once on each part, but multiple parts can name the same base interface(s). There shall only be one implementation of each member of any given interface.

Example: In the following:

partial class C : IA, IB {...}
partial class C : IC {...}
partial class C : IA, IB {...}

the set of base interfaces for class C is IA, IB, and IC.

end example

Typically, each part provides an implementation of the interface(s) declared on that part; however, this is not a requirement. A part can provide the implementation for an interface declared on a different part.

Example:

partial class X
{
    int IComparable.CompareTo(object o) {...}
}

partial class X : IComparable
{
    ...
}

end example

The base interfaces specified in a class declaration can be constructed interface types (§8.4, §18.2). A base interface cannot be a type parameter on its own, though it can involve the type parameters that are in scope.

Example: The following code illustrates how a class can implement and extend constructed types:

class C<U, V> {}
interface I1<V> {}
class D : C<string, int>, I1<string> {}
class E<T> : C<int, T>, I1<T> {}

end example

Interface implementations are discussed further in §18.6.

15.2.5 Type parameter constraints

Generic type and method declarations can optionally specify type parameter constraints by including type_parameter_constraints_clauses.

type_parameter_constraints_clauses
    : type_parameter_constraints_clause
    | type_parameter_constraints_clauses type_parameter_constraints_clause
    ;
    
type_parameter_constraints_clause
    : 'where' type_parameter ':' type_parameter_constraints
    ;

type_parameter_constraints
    : primary_constraint
    | secondary_constraints
    | constructor_constraint
    | primary_constraint ',' secondary_constraints
    | primary_constraint ',' constructor_constraint
    | secondary_constraints ',' constructor_constraint
    | primary_constraint ',' secondary_constraints ',' constructor_constraint
    ;

primary_constraint
    : class_type
    | 'class'
    | 'struct'
    | 'unmanaged'
    ;

secondary_constraints
    : interface_type
    | type_parameter
    | secondary_constraints ',' interface_type
    | secondary_constraints ',' type_parameter
    ;

constructor_constraint
    : 'new' '(' ')'
    ;

Each type_parameter_constraints_clause consists of the token where, followed by the name of a type parameter, followed by a colon and the list of constraints for that type parameter. There can be at most one where clause for each type parameter, and the where clauses can be listed in any order. Like the get and set tokens in a property accessor, the where token is not a keyword.

The list of constraints given in a where clause can include any of the following components, in this order: a single primary constraint, one or more secondary constraints, and the constructor constraint, new().

A primary constraint can be a class type, the reference type constraint class, the value type constraint struct, or the unmanaged type constraint unmanaged.

A secondary constraint can be a type_parameter or interface_type.

The reference type constraint specifies that a type argument used for the type parameter shall be a reference type. All class types, interface types, delegate types, array types, and type parameters known to be a reference type (as defined below) satisfy this constraint.

The value type constraint specifies that a type argument used for the type parameter shall be a non-nullable value type. All non-nullable struct types, enum types, and type parameters having the value type constraint satisfy this constraint. Note that although classified as a value type, a nullable value type (§8.3.12) does not satisfy the value type constraint. A type parameter having the value type constraint shall not also have the constructor_constraint, although it may be used as a type argument for another type parameter with a constructor_constraint.

Note: The System.Nullable<T> type specifies the non-nullable value type constraint for T. Thus, recursively constructed types of the forms T?? and Nullable<Nullable<T>> are prohibited. end note

Because unmanaged is not a keyword, in primary_constraint the unmanaged constraint is always syntactically ambiguous with class_type. For compatibility reasons, if a name lookup (§12.8.4) of the name unmanaged succeeds it is treated as a class_type. Otherwise it is treated as the unmanaged constraint.

The unmanaged type constraint specifies that a type argument used for the type parameter shall be a non-nullable unmanaged type (§8.8).

Pointer types are never allowed to be type arguments, and don’t satisfy any type constraints, even unmanaged, despite being unmanaged types.

If a constraint is a class type, an interface type, or a type parameter, that type specifies a minimal “base type” that every type argument used for that type parameter shall support. Whenever a constructed type or generic method is used, the type argument is checked against the constraints on the type parameter at compile-time. The type argument supplied shall satisfy the conditions described in §8.4.5.

A class_type constraint shall satisfy the following rules:

• The type shall be a class type.
• The type shall not be sealed.
• The type shall not be one of the following types: System.Array or System.ValueType.
• The type shall not be object.
• At most one constraint for a given type parameter may be a class type.

A type specified as an interface_type constraint shall satisfy the following rules:

• The type shall be an interface type.
• A type shall not be specified more than once in a given where clause.

In either case, the constraint may involve any of the type parameters of the associated type or method declaration as part of a constructed type, and may involve the type being declared.

Any class or interface type specified as a type parameter constraint shall be at least as accessible (§7.5.5) as the generic type or method being declared.

A type specified as a type_parameter constraint shall satisfy the following rules:

• The type shall be a type parameter.
• A type shall not be specified more than once in a given where clause.

In addition there shall be no cycles in the dependency graph of type parameters, where dependency is a transitive relation defined by:

• If a type parameter T is used as a constraint for type parameter S then S depends on T.
• If a type parameter S depends on a type parameter T and T depends on a type parameter U then S depends on U.

Given this relation, it is a compile-time error for a type parameter to depend on itself (directly or indirectly).

Any constraints shall be consistent among dependent type parameters. If type parameter S depends on type parameter T then:

• T shall not have the value type constraint. Otherwise, T is effectively sealed so S would be forced to be the same type as T, eliminating the need for two type parameters.
• If S has the value type constraint then T shall not have a class_type constraint.
• If S has a class_type constraint A and T has a class_type constraint B then there shall be an identity conversion or implicit reference conversion from A to B or an implicit reference conversion from B to A.
• If S also depends on type parameter U and U has a class_type constraint A and T has a class_type constraint B then there shall be an identity conversion or implicit reference conversion from A to B or an implicit reference conversion from B to A.

It is valid for S to have the value type constraint and T to have the reference type constraint. Effectively this limits T to the types System.Object, System.ValueType, System.Enum, and any interface type.

If the where clause for a type parameter includes a constructor constraint (which has the form new()), it is possible to use the new operator to create instances of the type (§12.8.16.2). Any type argument used for a type parameter with a constructor constraint shall be a value type, a non-abstract class having a public parameterless constructor, or a type parameter having the value type constraint or constructor constraint.

It is a compile-time error for type_parameter_constraints having a primary_constraint of struct or unmanaged to also have a constructor_constraint.

Example: The following are examples of constraints:

interface IPrintable
{
    void Print();
}

interface IComparable<T>
{
    int CompareTo(T value);
}

interface IKeyProvider<T>
{
    T GetKey();
}

class Printer<T> where T : IPrintable {...}
class SortedList<T> where T : IComparable<T> {...}

class Dictionary<K,V>
    where K : IComparable<K>
    where V : IPrintable, IKeyProvider<K>, new()
{
    ...
}

The following example is in error because it causes a circularity in the dependency graph of the type parameters:

class Circular<S,T>
    where S: T
    where T: S // Error, circularity in dependency graph
{
    ...
}

The following examples illustrate additional invalid situations:

class Sealed<S,T>
    where S : T
    where T : struct // Error, `T` is sealed
{
    ...
}

class A {...}
class B {...}

class Incompat<S,T>
    where S : A, T
    where T : B // Error, incompatible class-type constraints
{
    ...
}

class StructWithClass<S,T,U>
    where S : struct, T
    where T : U
    where U : A // Error, A incompatible with struct
{
    ...
}

end example

The dynamic erasure of a type C is type Cₓ constructed as follows:

• If C is a nested type Outer.Inner then Cₓ is a nested type Outerₓ.Innerₓ.
• If C Cₓis a constructed type G<A¹, ..., Aⁿ> with type arguments A¹, ..., Aⁿ then Cₓ is the constructed type G<A¹ₓ, ..., Aⁿₓ>.
• If C is an array type E[] then Cₓ is the array type Eₓ[].
• If C is dynamic then Cₓ is object.
• Otherwise, Cₓ is C.

The effective base class of a type parameter T is defined as follows:

Let R be a set of types such that:

• For each constraint of T that is a type parameter, R contains its effective base class.
• For each constraint of T that is a struct type, R contains System.ValueType.
• For each constraint of T that is an enumeration type, R contains System.Enum.
• For each constraint of T that is a delegate type, R contains its dynamic erasure.
• For each constraint of T that is an array type, R contains System.Array.
• For each constraint of T that is a class type, R contains its dynamic erasure.

Then

• If T has the value type constraint, its effective base class is System.ValueType.
• Otherwise, if R is empty then the effective base class is object.
• Otherwise, the effective base class of T is the most-encompassed type (§10.5.3) of set R. If the set has no encompassed type, the effective base class of T is object. The consistency rules ensure that the most-encompassed type exists.

If the type parameter is a method type parameter whose constraints are inherited from the base method the effective base class is calculated after type substitution.

These rules ensure that the effective base class is always a class_type.

The effective interface set of a type parameter T is defined as follows:

• If T has no secondary_constraints, its effective interface set is empty.
• If T has interface_type constraints but no type_parameter constraints, its effective interface set is the set of dynamic erasures of its interface_type constraints.
• If T has no interface_type constraints but has type_parameter constraints, its effective interface set is the union of the effective interface sets of its type_parameter constraints.
• If T has both interface_type constraints and type_parameter constraints, its effective interface set is the union of the set of dynamic erasures of its interface_type constraints and the effective interface sets of its type_parameter constraints.

A type parameter is known to be a reference type if it has the reference type constraint or its effective base class is not object or System.ValueType.

Values of a constrained type parameter type can be used to access the instance members implied by the constraints.

Example: In the following:

interface IPrintable
{
    void Print();
}

class Printer<T> where T : IPrintable
{
    void PrintOne(T x) => x.Print();
}

the methods of IPrintable can be invoked directly on x because T is constrained to always implement IPrintable.

end example

When a partial generic type declaration includes constraints, the constraints shall agree with all other parts that include constraints. Specifically, each part that includes constraints shall have constraints for the same set of type parameters, and for each type parameter, the sets of primary, secondary, and constructor constraints shall be equivalent. Two sets of constraints are equivalent if they contain the same members. If no part of a partial generic type specifies type parameter constraints, the type parameters are considered unconstrained.

Example:

partial class Map<K,V>
    where K : IComparable<K>
    where V : IKeyProvider<K>, new()
{
    ...
}

partial class Map<K,V>
    where V : IKeyProvider<K>, new()
    where K : IComparable<K>
{
    ...
}

partial class Map<K,V>
{
    ...
}

is correct because those parts that include constraints (the first two) effectively specify the same set of primary, secondary, and constructor constraints for the same set of type parameters, respectively.

end example

15.2.6 Class body

The class_body of a class defines the members of that class.

class_body
    : '{' class_member_declaration* '}'
    ;

15.2.7 Partial declarations

The modifier partial is used when defining a class, struct, or interface type in multiple parts. The partial modifier is a contextual keyword (§6.4.4) and only has special meaning immediately before one of the keywords class, struct, or interface.

Each part of a partial type declaration shall include a partial modifier and shall be declared in the same namespace or containing type as the other parts. The partial modifier indicates that additional parts of the type declaration might exist elsewhere, but the existence of such additional parts is not a requirement; it is valid for the only declaration of a type to include the partial modifier.

All parts of a partial type shall be compiled together such that the parts can be merged at compile-time. Partial types specifically do not allow already compiled types to be extended.

Nested types can be declared in multiple parts by using the partial modifier. Typically, the containing type is declared using partial as well, and each part of the nested type is declared in a different part of the containing type.

Example: The following partial class is implemented in two parts, which reside in different compilation units. The first part is machine generated by a database-mapping tool while the second part is manually authored:

public partial class Customer
{
    private int id;
    private string name;
    private string address;
    private List<Order> orders;

    public Customer()
    {
        ...
    }
}

// File: Customer2.cs
public partial class Customer
{
    public void SubmitOrder(Order orderSubmitted) => orders.Add(orderSubmitted);

    public bool HasOutstandingOrders() => orders.Count > 0;
}

When the two parts above are compiled together, the resulting code behaves as if the class had been written as a single unit, as follows:

public class Customer
{
    private int id;
    private string name;
    private string address;
    private List<Order> orders;

    public Customer()
    {
        ...
    }

    public void SubmitOrder(Order orderSubmitted) => orders.Add(orderSubmitted);

    public bool HasOutstandingOrders() => orders.Count > 0;
}

end example

The handling of attributes specified on the type or type parameters of different parts of a partial declaration is discussed in §22.3.

15.3 Class members

15.3.1 General

The members of a class consist of the members introduced by its class_member_declarations and the members inherited from the direct base class.

class_member_declaration
    : constant_declaration
    | field_declaration
    | method_declaration
    | property_declaration
    | event_declaration
    | indexer_declaration
    | operator_declaration
    | constructor_declaration
    | finalizer_declaration
    | static_constructor_declaration
    | type_declaration
    ;

The members of a class are divided into the following categories:

• Constants, which represent constant values associated with the class (§15.4).
• Fields, which are the variables of the class (§15.5).
• Methods, which implement the computations and actions that can be performed by the class (§15.6).
• Properties, which define named characteristics and the actions associated with reading and writing those characteristics (§15.7).
• Events, which define notifications that can be generated by the class (§15.8).
• Indexers, which permit instances of the class to be indexed in the same way (syntactically) as arrays (§15.9).
• Operators, which define the expression operators that can be applied to instances of the class (§15.10).
• Instance constructors, which implement the actions required to initialize instances of the class (§15.11)
• Finalizers, which implement the actions to be performed before instances of the class are permanently discarded (§15.13).
• Static constructors, which implement the actions required to initialize the class itself (§15.12).
• Types, which represent the types that are local to the class (§14.7).

A class_declaration creates a new declaration space (§7.3), and the type_parameters and the class_member_declarations immediately contained by the class_declaration introduce new members into this declaration space. The following rules apply to class_member_declarations:

• Instance constructors, finalizers, and static constructors shall have the same name as the immediately enclosing class. All other members shall have names that differ from the name of the immediately enclosing class.

• The name of a type parameter in the type_parameter_list of a class declaration shall differ from the names of all other type parameters in the same type_parameter_list and shall differ from the name of the class and the names of all members of the class.

• The name of a type shall differ from the names of all non-type members declared in the same class. If two or more type declarations share the same fully qualified name, the declarations shall have the partial modifier (§15.2.7) and these declarations combine to define a single type.

Note: Since the fully qualified name of a type declaration encodes the number of type parameters, two distinct types may share the same name as long as they have different number of type parameters. end note

• The name of a constant, field, property, or event shall differ from the names of all other members declared in the same class.

• The name of a method shall differ from the names of all other non-methods declared in the same class. In addition, the signature (§7.6) of a method shall differ from the signatures of all other methods declared in the same class, and two methods declared in the same class shall not have signatures that differ solely by in, out, and ref.

• The signature of an instance constructor shall differ from the signatures of all other instance constructors declared in the same class, and two constructors declared in the same class shall not have signatures that differ solely by ref and out.

• The signature of an indexer shall differ from the signatures of all other indexers declared in the same class.

• The signature of an operator shall differ from the signatures of all other operators declared in the same class.

The inherited members of a class (§15.3.4) are not part of the declaration space of a class.

Note: Thus, a derived class is allowed to declare a member with the same name or signature as an inherited member (which in effect hides the inherited member). end note

The set of members of a type declared in multiple parts (§15.2.7) is the union of the members declared in each part. The bodies of all parts of the type declaration share the same declaration space (§7.3), and the scope of each member (§7.7) extends to the bodies of all the parts. The accessibility domain of any member always includes all the parts of the enclosing type; a private member declared in one part is freely accessible from another part. It is a compile-time error to declare the same member in more than one part of the type, unless that member is a type having the partial modifier.

Example:

partial class A
{
    int x;                   // Error, cannot declare x more than once

    partial class Inner      // Ok, Inner is a partial type
    {
        int y;
    }
}

partial class A
{
    int x;                   // Error, cannot declare x more than once

    partial class Inner      // Ok, Inner is a partial type
    {
        int z;
    }
}

end example

Field initialization order can be significant within C# code, and some guarantees are provided, as defined in §15.5.6.1. Otherwise, the ordering of members within a type is rarely significant, but may be significant when interfacing with other languages and environments. In these cases, the ordering of members within a type declared in multiple parts is undefined.

15.3.2 The instance type

Each class declaration has an associated instance type. For a generic class declaration, the instance type is formed by creating a constructed type (§8.4) from the type declaration, with each of the supplied type arguments being the corresponding type parameter. Since the instance type uses the type parameters, it can only be used where the type parameters are in scope; that is, inside the class declaration. The instance type is the type of this for code written inside the class declaration. For non-generic classes, the instance type is simply the declared class.

Example: The following shows several class declarations along with their instance types:

class A<T>             // instance type: A<T>
{
    class B {}         // instance type: A<T>.B
    class C<U> {}      // instance type: A<T>.C<U>
}
class D {}             // instance type: D

end example

15.3.3 Members of constructed types

The non-inherited members of a constructed type are obtained by substituting, for each type_parameter in the member declaration, the corresponding type_argument of the constructed type. The substitution process is based on the semantic meaning of type declarations, and is not simply textual substitution.

Example: Given the generic class declaration

class Gen<T,U>
{
    public T[,] a;
    public void G(int i, T t, Gen<U,T> gt) {...}
    public U Prop { get {...} set {...} }
    public int H(double d) {...}
}

the constructed type Gen<int[],IComparable<string>> has the following members:

public int[,][] a;
public void G(int i, int[] t, Gen<IComparable<string>,int[]> gt) {...}
public IComparable<string> Prop { get {...} set {...} }
public int H(double d) {...}

The type of the member a in the generic class declaration Gen is “two-dimensional array of T”, so the type of the member a in the constructed type above is “two-dimensional array of single-dimensional array of int”, or int[,][].

end example

Within instance function members, the type of this is the instance type (§15.3.2) of the containing declaration.

All members of a generic class can use type parameters from any enclosing class, either directly or as part of a constructed type. When a particular closed constructed type (§8.4.3) is used at run-time, each use of a type parameter is replaced with the type argument supplied to the constructed type.

Example:

class C<V>
{
    public V f1;
    public C<V> f2 = null;

    public C(V x)
    {
        this.f1 = x;
        this.f2 = this;
    }
}

class Application
{
    static void Main()
    {
        C<int> x1 = new C<int>(1);
        Console.WriteLine(x1.f1);              // Prints 1

        C<double> x2 = new C<double>(3.1415);
        Console.WriteLine(x2.f1);              // Prints 3.1415
    }
}

end example

15.3.4 Inheritance

A class inherits the members of its direct base class. Inheritance means that a class implicitly contains all members of its direct base class, except for the instance constructors, finalizers, and static constructors of the base class. Some important aspects of inheritance are:

• Inheritance is transitive. If C is derived from B, and B is derived from A, then C inherits the members declared in B as well as the members declared in A.

• A derived class extends its direct base class. A derived class can add new members to those it inherits, but it cannot remove the definition of an inherited member.

• Instance constructors, finalizers, and static constructors are not inherited, but all other members are, regardless of their declared accessibility (§7.5). However, depending on their declared accessibility, inherited members might not be accessible in a derived class.

• A derived class can hide (§7.7.2.3) inherited members by declaring new members with the same name or signature. However, hiding an inherited member does not remove that member—it merely makes that member inaccessible directly through the derived class.

• An instance of a class contains a set of all instance fields declared in the class and its base classes, and an implicit conversion (§10.2.8) exists from a derived class type to any of its base class types. Thus, a reference to an instance of some derived class can be treated as a reference to an instance of any of its base classes.

• A class can declare virtual methods, properties, indexers, and events, and derived classes can override the implementation of these function members. This enables classes to exhibit polymorphic behavior wherein the actions performed by a function member invocation vary depending on the run-time type of the instance through which that function member is invoked.

The inherited members of a constructed class type are the members of the immediate base class type (§15.2.4.2), which is found by substituting the type arguments of the constructed type for each occurrence of the corresponding type parameters in the base_class_specification. These members, in turn, are transformed by substituting, for each type_parameter in the member declaration, the corresponding type_argument of the base_class_specification.

Example:

class B<U>
{
    public U F(long index) {...}
}

class D<T> : B<T[]>
{
    public T G(string s) {...}
}

In the code above, the constructed type D<int> has a non-inherited member public int G(string s) obtained by substituting the type argument int for the type parameter T. D<int> also has an inherited member from the class declaration B. This inherited member is determined by first determining the base class type B<int[]> of D<int> by substituting int for T in the base class specification B<T[]>. Then, as a type argument to B, int[] is substituted for U in public U F(long index), yielding the inherited member public int[] F(long index).

end example

15.3.5 The new modifier

A class_member_declaration is permitted to declare a member with the same name or signature as an inherited member. When this occurs, the derived class member is said to hide the base class member. See §7.7.2.3 for a precise specification of when a member hides an inherited member.

An inherited member M is considered to be available if M is accessible and there is no other inherited accessible member N that already hides M. Implicitly hiding an inherited member is not considered an error, but it does cause the compiler to issue a warning unless the declaration of the derived class member includes a new modifier to explicitly indicate that the derived member is intended to hide the base member. If one or more parts of a partial declaration (§15.2.7) of a nested type include the new modifier, no warning is issued if the nested type hides an available inherited member.

If a new modifier is included in a declaration that doesn’t hide an available inherited member, a warning to that effect is issued.

15.3.6 Access modifiers

A class_member_declaration can have any one of the permitted kinds of declared accessibility (§7.5.2): public, protected internal, protected, private protected, internal, or private. Except for the protected internal and private protected combinations, it is a compile-time error to specify more than one access modifier. When a class_member_declaration does not include any access modifiers, private is assumed.

15.3.7 Constituent types

Types that are used in the declaration of a member are called the constituent types of that member. Possible constituent types are the type of a constant, field, property, event, or indexer, the return type of a method or operator, and the parameter types of a method, indexer, operator, or instance constructor. The constituent types of a member shall be at least as accessible as that member itself (§7.5.5).

15.3.8 Static and instance members

Members of a class are either static members or instance members.

Note: Generally speaking, it is useful to think of static members as belonging to classes and instance members as belonging to objects (instances of classes). end note

When a field, method, property, event, operator, or constructor declaration includes a static modifier, it declares a static member. In addition, a constant or type declaration implicitly declares a static member. Static members have the following characteristics:

• When a static member M is referenced in a member_access (§12.8.7) of the form E.M, E shall denote a type that has a member M. It is a compile-time error for E to denote an instance.
• A static field in a non-generic class identifies exactly one storage location. No matter how many instances of a non-generic class are created, there is only ever one copy of a static field. Each distinct closed constructed type (§8.4.3) has its own set of static fields, regardless of the number of instances of the closed constructed type.
• A static function member (method, property, event, operator, or constructor) does not operate on a specific instance, and it is a compile-time error to refer to this in such a function member.

When a field, method, property, event, indexer, constructor, or finalizer declaration does not include a static modifier, it declares an instance member. (An instance member is sometimes called a non-static member.) Instance members have the following characteristics:

• When an instance member M is referenced in a member_access (§12.8.7) of the form E.M, E shall denote an instance of a type that has a member M. It is a binding-time error for E to denote a type.
• Every instance of a class contains a separate set of all instance fields of the class.
• An instance function member (method, property, indexer, instance constructor, or finalizer) operates on a given instance of the class, and this instance can be accessed as this (§12.8.13).

Example: The following example illustrates the rules for accessing static and instance members:

class Test
{
    int x;
    static int y;
    void F()
    {
        x = 1;               // Ok, same as this.x = 1
        y = 1;               // Ok, same as Test.y = 1
    }

    static void G()
    {
        x = 1;               // Error, cannot access this.x
        y = 1;               // Ok, same as Test.y = 1
    }

    static void Main()
    {
        Test t = new Test();
        t.x = 1;       // Ok
        t.y = 1;       // Error, cannot access static member through instance
        Test.x = 1;    // Error, cannot access instance member through type
        Test.y = 1;    // Ok
    }
}

The F method shows that in an instance function member, a simple_name (§12.8.4) can be used to access both instance members and static members. The G method shows that in a static function member, it is a compile-time error to access an instance member through a simple_name. The Main method shows that in a member_access (§12.8.7), instance members shall be accessed through instances, and static members shall be accessed through types.

end example

15.3.9 Nested types

15.3.9.1 General

A type declared within a class or struct is called a nested type. A type that is declared within a compilation unit or namespace is called a non-nested type.

Example: In the following example:

class A
{
    class B
    {
        static void F()
        {
            Console.WriteLine("A.B.F");
        }
    }
}

class B is a nested type because it is declared within class A, and class A is a non-nested type because it is declared within a compilation unit.

end example

15.3.9.2 Fully qualified name

The fully qualified name (§7.8.3) for a nested type declarationis S.N where S is the fully qualified name of the type declarationin which type N is declared and N is the unqualified name (§7.8.2) of the nested type declaration (including any generic_dimension_specifier (§12.8.17)).

15.3.9.3 Declared accessibility

Non-nested types can have public or internal declared accessibility and have internal declared accessibility by default. Nested types can have these forms of declared accessibility too, plus one or more additional forms of declared accessibility, depending on whether the containing type is a class or struct:

• A nested type that is declared in a class can have any of the permitted kinds of declared accessibility and, like other class members, defaults to private declared accessibility.
• A nested type that is declared in a struct can have any of three forms of declared accessibility (public, internal, or private) and, like other struct members, defaults to private declared accessibility.

Example: The example

public class List
{
    // Private data structure
    private class Node
    {
        public object Data;
        public Node Next;

        public Node(object data, Node next)
        {
            this.Data = data;
            this.Next = next;
        }
    }

    private Node first = null;
    private Node last = null;

    // Public interface
    public void AddToFront(object o) {...}
    public void AddToBack(object o) {...}
    public object RemoveFromFront() {...}
    public object RemoveFromBack() {...}
    public int Count { get {...} }
}

declares a private nested class Node.

end example

15.3.9.4 Hiding

A nested type may hide (§7.7.2.2) a base member. The new modifier (§15.3.5) is permitted on nested type declarations so that hiding can be expressed explicitly.

Example: The example

class Base
{
    public static void M()
    {
        Console.WriteLine("Base.M");
    }
}

class Derived: Base
{
    public new class M
    {
        public static void F()
        {
            Console.WriteLine("Derived.M.F");
        }
    }
}

class Test
{
    static void Main()
    {
        Derived.M.F();
    }
}

shows a nested class M that hides the method M defined in Base.

end example

15.3.9.5 this access

A nested type and its containing type do not have a special relationship with regard to this_access (§12.8.13). Specifically, this within a nested type cannot be used to refer to instance members of the containing type. In cases where a nested type needs access to the instance members of its containing type, access can be provided by providing the this for the instance of the containing type as a constructor argument for the nested type.

Example: The following example

class C
{
    int i = 123;
    public void F()
    {
        Nested n = new Nested(this);
        n.G();
    }

    public class Nested
    {
        C this_c;

        public Nested(C c)
        {
            this_c = c;
        }

        public void G()
        {
            Console.WriteLine(this_c.i);
        }
    }
}

class Test
{
    static void Main()
    {
        C c = new C();
        c.F();
    }
}

shows this technique. An instance of C creates an instance of Nested, and passes its own this to Nested’s constructor in order to provide subsequent access to C’s instance members.

end example

15.3.9.6 Access to private and protected members of the containing type

A nested type has access to all of the members that are accessible to its containing type, including members of the containing type that have private and protected declared accessibility.

Example: The example

class C
{
    private static void F() => Console.WriteLine("C.F");

    public class Nested
    {
        public static void G() => F();
    }
}

class Test
{
    static void Main() => C.Nested.G();
}

shows a class C that contains a nested class Nested. Within Nested, the method G calls the static method F defined in C, and F has private declared accessibility.

end example

A nested type also may access protected members defined in a base type of its containing type.

Example: In the following code

class Base
{
    protected void F() => Console.WriteLine("Base.F");
}

class Derived: Base
{
    public class Nested
    {
        public void G()
        {
            Derived d = new Derived();
            d.F(); // ok
        }
    }
}

class Test
{
    static void Main()
    {
        Derived.Nested n = new Derived.Nested();
        n.G();
    }
}

the nested class Derived.Nested accesses the protected method F defined in Derived’s base class, Base, by calling through an instance of Derived.

end example

15.3.9.7 Nested types in generic classes

A generic class declaration may contain nested type declarations. The type parameters of the enclosing class may be used within the nested types. A nested type declaration may contain additional type parameters that apply only to the nested type.

Every type declaration contained within a generic class declaration is implicitly a generic type declaration. When writing a reference to a type nested within a generic type, the containing constructed type, including its type arguments, shall be named. However, from within the outer class, the nested type may be used without qualification; the instance type of the outer class may be implicitly used when constructing the nested type.

Example: The following shows three different correct ways to refer to a constructed type created from Inner; the first two are equivalent:

class Outer<T>
{
    class Inner<U>
    {
        public static void F(T t, U u) {...}
    }

    static void F(T t)
    {
        Outer<T>.Inner<string>.F(t, "abc");    // These two statements have
        Inner<string>.F(t, "abc");             // the same effect
        Outer<int>.Inner<string>.F(3, "abc");  // This type is different
        Outer.Inner<string>.F(t, "abc");       // Error, Outer needs type arg
    }
}

end example

Although it is bad programming style, a type parameter in a nested type can hide a member or type parameter declared in the outer type.

Example:

class Outer<T>
{
    class Inner<T>                                  // Valid, hides Outer's T
    {
        public T t;                                 // Refers to Inner's T
    }
}

end example

15.3.10 Reserved member names

15.3.10.1 General

To facilitate the underlying C# run-time implementation, for each source member declaration that is a property, event, or indexer, the implementation shall reserve two method signatures based on the kind of the member declaration, its name, and its type (§15.3.10.2, §15.3.10.3, §15.3.10.4). It is a compile-time error for a program to declare a member whose signature matches a signature reserved by a member declared in the same scope, even if the underlying run-time implementation does not make use of these reservations.

The reserved names do not introduce declarations, thus they do not participate in member lookup. However, a declaration’s associated reserved method signatures do participate in inheritance (§15.3.4), and can be hidden with the new modifier (§15.3.5).

Note: The reservation of these names serves three purposes:

1. To allow the underlying implementation to use an ordinary identifier as a method name for get or set access to the C# language feature.
2. To allow other languages to interoperate using an ordinary identifier as a method name for get or set access to the C# language feature.
3. To help ensure that the source accepted by one conforming compiler is accepted by another, by making the specifics of reserved member names consistent across all C# implementations.

end note

The declaration of a finalizer (§15.13) also causes a signature to be reserved (§15.3.10.5).

15.3.10.2 Member names reserved for properties

For a property P (§15.7) of type T, the following signatures are reserved:

T get_P();
void set_P(T value);

Both signatures are reserved, even if the property is read-only or write-only.

Example: In the following code

class A
{
    public int P
    {
        get => 123;
    }
}

class B : A
{
    public new int get_P() => 456;

    public new void set_P(int value)
    {
    }
}

class Test
{
    static void Main()
    {
        B b = new B();
        A a = b;
        Console.WriteLine(a.P);
        Console.WriteLine(b.P);
        Console.WriteLine(b.get_P());
    }
}

A class A defines a read-only property P, thus reserving signatures for get_P and set_P methods. A class B derives from A and hides both of these reserved signatures. The example produces the output:

123
123
456

end example

15.3.10.3 Member names reserved for events

For an event E (§15.8) of delegate type T, the following signatures are reserved:

void add_E(T handler);
void remove_E(T handler);

15.3.10.4 Member names reserved for indexers

For an indexer (§15.9) of type T with parameter-list L, the following signatures are reserved:

T get_Item(L);
void set_Item(L, T value);

Both signatures are reserved, even if the indexer is read-only or write-only.

Furthermore the member name Item is reserved.

15.3.10.5 Member names reserved for finalizers

For a class containing a finalizer (§15.13), the following signature is reserved:

void Finalize();

15.4 Constants

A constant is a class member that represents a constant value: a value that can be computed at compile-time. A constant_declaration introduces one or more constants of a given type.

constant_declaration
    : attributes? constant_modifier* 'const' type constant_declarators ';'
    ;

constant_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    ;

A constant_declaration may include a set of attributes (§22), a new modifier (§15.3.5), and any one of the permitted kinds of declared accessibility (§15.3.6). The attributes and modifiers apply to all of the members declared by the constant_declaration. Even though constants are considered static members, a constant_declaration neither requires nor allows a static modifier. It is an error for the same modifier to appear multiple times in a constant declaration.

The type of a constant_declaration specifies the type of the members introduced by the declaration. The type is followed by a list of constant_declarators (§13.6.3), each of which introduces a new member. A constant_declarator consists of an identifier that names the member, followed by an “=” token, followed by a constant_expression (§12.23) that gives the value of the member.

The type specified in a constant declaration shall be sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, an enum_type, or a reference_type. Each constant_expression shall yield a value of the target type or of a type that can be converted to the target type by an implicit conversion (§10.2).

The type of a constant shall be at least as accessible as the constant itself (§7.5.5).

The value of a constant is obtained in an expression using a simple_name (§12.8.4) or a member_access (§12.8.7).

A constant can itself participate in a constant_expression. Thus, a constant may be used in any construct that requires a constant_expression.

Note: Examples of such constructs include case labels, goto case statements, enum member declarations, attributes, and other constant declarations. end note

Note: As described in §12.23, a constant_expression is an expression that can be fully evaluated at compile-time. Since the only way to create a non-null value of a reference_type other than string is to apply the new operator, and since the new operator is not permitted in a constant_expression, the only possible value for constants of reference_types other than string is null. end note

When a symbolic name for a constant value is desired, but when the type of that value is not permitted in a constant declaration, or when the value cannot be computed at compile-time by a constant_expression, a readonly field (§15.5.3) may be used instead.

Note: The versioning semantics of const and readonly differ (§15.5.3.3). end note

A constant declaration that declares multiple constants is equivalent to multiple declarations of single constants with the same attributes, modifiers, and type.

Example:

class A
{
    public const double X = 1.0, Y = 2.0, Z = 3.0;
}

is equivalent to

class A
{
    public const double X = 1.0;
    public const double Y = 2.0;
    public const double Z = 3.0;
}

end example

Constants are permitted to depend on other constants within the same program as long as the dependencies are not of a circular nature. The compiler automatically arranges to evaluate the constant declarations in the appropriate order.

Example: In the following code

class A
{
    public const int X = B.Z + 1;
    public const int Y = 10;
}

class B
{
    public const int Z = A.Y + 1;
}

the compiler first evaluates A.Y, then evaluates B.Z, and finally evaluates A.X, producing the values 10, 11, and 12.

end example

Constant declarations may depend on constants from other programs, but such dependencies are only possible in one direction.

Example: Referring to the example above, if A and B were declared in separate programs, it would be possible for A.X to depend on B.Z, but B.Z could then not simultaneously depend on A.Y. end example

15.5 Fields

15.5.1 General

A field is a member that represents a variable associated with an object or class. A field_declaration introduces one or more fields of a given type.

field_declaration
    : attributes? field_modifier* type variable_declarators ';'
    ;

field_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'static'
    | 'readonly'
    | 'volatile'
    | unsafe_modifier   // unsafe code support
    ;

variable_declarators
    : variable_declarator (',' variable_declarator)*
    ;

variable_declarator
    : identifier ('=' variable_initializer)?
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

A field_declaration may include a set of attributes (§22), a new modifier (§15.3.5), a valid combination of the four access modifiers (§15.3.6), and a static modifier (§15.5.2). In addition, a field_declaration may include a readonly modifier (§15.5.3) or a volatile modifier (§15.5.4), but not both. The attributes and modifiers apply to all of the members declared by the field_declaration. It is an error for the same modifier to appear multiple times in a field_declaration.

The type of a field_declaration specifies the type of the members introduced by the declaration. The type is followed by a list of variable_declarators, each of which introduces a new member. A variable_declarator consists of an identifier that names that member, optionally followed by an “=” token and a variable_initializer (§15.5.6) that gives the initial value of that member.

The type of a field shall be at least as accessible as the field itself (§7.5.5).

The value of a field is obtained in an expression using a simple_name (§12.8.4), a member_access (§12.8.7) or a base_access (§12.8.14). The value of a non-readonly field is modified using an assignment (§12.21). The value of a non-readonly field can be both obtained and modified using postfix increment and decrement operators (§12.8.15) and prefix increment and decrement operators (§12.9.6).

A field declaration that declares multiple fields is equivalent to multiple declarations of single fields with the same attributes, modifiers, and type.

Example:

class A
{
    public static int X = 1, Y, Z = 100;
}

is equivalent to

class A
{
    public static int X = 1;
    public static int Y;
    public static int Z = 100;
}

end example

15.5.2 Static and instance fields

When a field declaration includes a static modifier, the fields introduced by the declaration are static fields. When no static modifier is present, the fields introduced by the declaration are instance fields. Static fields and instance fields are two of the several kinds of variables (§9) supported by C#, and at times they are referred to as static variables and instance variables, respectively.

As explained in §15.3.8, each instance of a class contains a complete set of the instance fields of the class, while there is only one set of static fields for each non-generic class or closed constructed type, regardless of the number of instances of the class or closed constructed type.

15.5.3 Readonly fields

15.5.3.1 General

When a field_declaration includes a readonly modifier, the fields introduced by the declaration are readonly fields. Direct assignments to readonly fields can only occur as part of that declaration or in an instance constructor or static constructor in the same class. (A readonly field can be assigned to multiple times in these contexts.) Specifically, direct assignments to a readonly field are permitted only in the following contexts:

• In the variable_declarator that introduces the field (by including a variable_initializer in the declaration).
• For an instance field, in the instance constructors of the class that contains the field declaration; for a static field, in the static constructor of the class that contains the field declaration. These are also the only contexts in which it is valid to pass a readonly field as an out or ref parameter.

Attempting to assign to a readonly field or pass it as an out or ref parameter in any other context is a compile-time error.

15.5.3.2 Using static readonly fields for constants

A static readonly field is useful when a symbolic name for a constant value is desired, but when the type of the value is not permitted in a const declaration, or when the value cannot be computed at compile-time.

Example: In the following code

public class Color
{
    public static readonly Color Black = new Color(0, 0, 0);
    public static readonly Color White = new Color(255, 255, 255);
    public static readonly Color Red = new Color(255, 0, 0);
    public static readonly Color Green = new Color(0, 255, 0);
    public static readonly Color Blue = new Color(0, 0, 255);

    private byte red, green, blue;

    public Color(byte r, byte g, byte b)
    {
        red = r;
        green = g;
        blue = b;
    }
}

the Black, White, Red, Green, and Blue members cannot be declared as const members because their values cannot be computed at compile-time. However, declaring them static readonly instead has much the same effect.

end example

15.5.3.3 Versioning of constants and static readonly fields

Constants and readonly fields have different binary versioning semantics. When an expression references a constant, the value of the constant is obtained at compile-time, but when an expression references a readonly field, the value of the field is not obtained until run-time.

Example: Consider an application that consists of two separate programs:

namespace Program1
{
    public class Utils
    {
        public static readonly int x = 1;
    }
}

and

namespace Program2
{
    class Test
    {
        static void Main()
        {
            Console.WriteLine(Program1.Utils.X);
        }
    }
}

The Program1 and Program2 namespaces denote two programs that are compiled separately. Because Program1.Utils.X is declared as a static readonly field, the value output by the Console.WriteLine statement is not known at compile-time, but rather is obtained at run-time. Thus, if the value of X is changed and Program1 is recompiled, the Console.WriteLine statement will output the new value even if Program2 isn’t recompiled. However, had X been a constant, the value of X would have been obtained at the time Program2 was compiled, and would remain unaffected by changes in Program1 until Program2 is recompiled.

end example

15.5.4 Volatile fields

When a field_declaration includes a volatile modifier, the fields introduced by that declaration are volatile fields. For non-volatile fields, optimization techniques that reorder instructions can lead to unexpected and unpredictable results in multi-threaded programs that access fields without synchronization such as that provided by the lock_statement (§13.13). These optimizations can be performed by the compiler, by the run-time system, or by hardware. For volatile fields, such reordering optimizations are restricted:

• A read of a volatile field is called a volatile read. A volatile read has “acquire semantics”; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.
• A write of a volatile field is called a volatile write. A volatile write has “release semantics”; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence.

These restrictions ensure that all threads will observe volatile writes performed by any other thread in the order in which they were performed. A conforming implementation is not required to provide a single total ordering of volatile writes as seen from all threads of execution. The type of a volatile field shall be one of the following:

• A reference_type.
• A type_parameter that is known to be a reference type (§15.2.5).
• The type byte, sbyte, short, ushort, int, uint, char, float, bool, System.IntPtr, or System.UIntPtr.
• An enum_type having an enum_base type of byte, sbyte, short, ushort, int, or uint.

Example: The example

class Test
{
    public static int result;
    public static volatile bool finished;

    static void Thread2()
    {
        result = 143;
        finished = true;
    }

    static void Main()
    {
        finished = false;

        // Run Thread2() in a new thread
        new Thread(new ThreadStart(Thread2)).Start();    

        // Wait for Thread2() to signal that it has a result
        // by setting finished to true.
        for (;;)
        {
            if (finished)
            {
                Console.WriteLine($"result = {result}");
                return;
            }
        }
    }
}

produces the output:

result = 143

In this example, the method Main starts a new thread that runs the method Thread2. This method stores a value into a non-volatile field called result, then stores true in the volatile field finished. The main thread waits for the field finished to be set to true, then reads the field result. Since finished has been declared volatile, the main thread shall read the value 143 from the field result. If the field finished had not been declared volatile, then it would be permissible for the store to result to be visible to the main thread after the store to finished, and hence for the main thread to read the value 0 from the field result. Declaring finished as a volatile field prevents any such inconsistency.

end example

15.5.5 Field initialization

The initial value of a field, whether it be a static field or an instance field, is the default value (§9.3) of the field’s type. It is not possible to observe the value of a field before this default initialization has occurred, and a field is thus never “uninitialized”.

Example: The example

class Test
{
    static bool b;
    int i;

    static void Main()
    {
        Test t = new Test();
        Console.WriteLine($"b = {b}, i = {t.i}");
    }
}

produces the output

b = False, i = 0

because b and i are both automatically initialized to default values.

end example

15.5.6 Variable initializers

15.5.6.1 General

Field declarations may include variable_initializers. For static fields, variable initializers correspond to assignment statements that are executed during class initialization. For instance fields, variable initializers correspond to assignment statements that are executed when an instance of the class is created.

Example: The example

class Test
{
    static double x = Math.Sqrt(2.0);
    int i = 100;
    string s = "Hello";

    static void Main()
    {
        Test a = new Test();
        Console.WriteLine($"x = {x}, i = {a.i}, s = {a.s}");
    }
}

produces the output

x = 1.4142135623730951, i = 100, s = Hello

because an assignment to x occurs when static field initializers execute and assignments to i and s occur when the instance field initializers execute.

end example

The default value initialization described in §15.5.5 occurs for all fields, including fields that have variable initializers. Thus, when a class is initialized, all static fields in that class are first initialized to their default values, and then the static field initializers are executed in textual order. Likewise, when an instance of a class is created, all instance fields in that instance are first initialized to their default values, and then the instance field initializers are executed in textual order. When there are field declarations in multiple partial type declarations for the same type, the order of the parts is unspecified. However, within each part the field initializers are executed in order.

It is possible for static fields with variable initializers to be observed in their default value state.

Example: However, this is strongly discouraged as a matter of style. The example

class Test
{
    static int a = b + 1;
    static int b = a + 1;

    static void Main()
    {
        Console.WriteLine($"a = {a}, b = {b}");
    }
}

exhibits this behavior. Despite the circular definitions of a and b, the program is valid. It results in the output

a = 1, b = 2

because the static fields a and b are initialized to 0 (the default value for int) before their initializers are executed. When the initializer for a runs, the value of b is zero, and so a is initialized to 1. When the initializer for b runs, the value of a is already 1, and so b is initialized to 2.

end example

15.5.6.2 Static field initialization

The static field variable initializers of a class correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration (§15.5.6.1). Within a partial class, the meaning of “textual order” is specified by §15.5.6.1. If a static constructor (§15.12) exists in the class, execution of the static field initializers occurs immediately prior to executing that static constructor. Otherwise, the static field initializers are executed at an implementation-dependent time prior to the first use of a static field of that class.

Example: The example

class Test
{
    static void Main()
    {
        Console.WriteLine($"{B.Y} {A.X}");
    }

    public static int F(string s)
    {
        Console.WriteLine(s);
        return 1;
    }
}

class A
{
    public static int X = Test.F("Init A");
}

class B
{
    public static int Y = Test.F("Init B");
}

might produce either the output:

Init A
Init B
1 1

or the output:

Init B
Init A
1 1

because the execution of X’s initializer and Y’s initializer could occur in either order; they are only constrained to occur before the references to those fields. However, in the example:

class Test
{
    static void Main()
    {
        Console.WriteLine($"{B.Y} {A.X}");
    }

    public static int F(string s)
    {
        Console.WriteLine(s);
        return 1;
    }
}

class A
{
    static A() {}
    public static int X = Test.F("Init A");
}

class B
{
    static B() {}
    public static int Y = Test.F("Init B");
}

the output shall be:

Init B
Init A
1 1

because the rules for when static constructors execute (as defined in §15.12) provide that B’s static constructor (and hence B’s static field initializers) shall run before A’s static constructor and field initializers.

end example

15.5.6.3 Instance field initialization

The instance field variable initializers of a class correspond to a sequence of assignments that are executed immediately upon entry to any one of the instance constructors (§15.11.3) of that class. Within a partial class, the meaning of “textual order” is specified by §15.5.6.1. The variable initializers are executed in the textual order in which they appear in the class declaration (§15.5.6.1). The class instance creation and initialization process is described further in §15.11.

A variable initializer for an instance field cannot reference the instance being created. Thus, it is a compile-time error to reference this in a variable initializer, as it is a compile-time error for a variable initializer to reference any instance member through a simple_name.

Example: In the following code

class A
{
    int x = 1;
    int y = x + 1;     // Error, reference to instance member of this
}

the variable initializer for y results in a compile-time error because it references a member of the instance being created.

end example

15.6 Methods

15.6.1 General

A method is a member that implements a computation or action that can be performed by an object or class. Methods are declared using method_declarations:

method_declaration
    : attributes? method_modifiers return_type method_header method_body
    | attributes? ref_method_modifiers ref_kind ref_return_type method_header
      ref_method_body
    ;

method_modifiers
    : method_modifier* 'partial'?
    ;

ref_kind
    : 'ref'
    | 'ref' 'readonly'
    ;

ref_method_modifiers
    : ref_method_modifier*
    ;

method_header
    : member_name '(' formal_parameter_list? ')'
    | member_name type_parameter_list '(' formal_parameter_list? ')'
      type_parameter_constraints_clause*
    ;

method_modifier
    : ref_method_modifier
    | 'async'
    ;

ref_method_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'static'
    | 'virtual'
    | 'sealed'
    | 'override'
    | 'abstract'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;

return_type
    : ref_return_type
    | 'void'
    ;

ref_return_type
    : type
    ;

member_name
    : identifier
    | interface_type '.' identifier
    ;

method_body
    : block
    | '=>' null_conditional_invocation_expression ';'
    | '=>' expression ';'
    | ';'
    ;

ref_method_body
    : block
    | '=>' 'ref' variable_reference ';'
    | ';'
    ;

Grammar notes:

• unsafe_modifier (§23.2) is only available in unsafe code (§23).
• when recognising a method_body if both the null_conditional_invocation_expression and expression alternatives are applicable then the former shall be chosen.

Note: The overlapping of, and priority between, alternatives here is solely for descriptive convenience; the grammar rules could be elaborated to remove the overlap. ANTLR, and other grammar systems, adopt the same convenience and so method_body has the specified semantics automatically. end note

A method_declaration may include a set of attributes (§22) and one of the permitted kinds of declared accessibility (§15.3.6), the new (§15.3.5), static (§15.6.3), virtual (§15.6.4), override (§15.6.5), sealed (§15.6.6), abstract (§15.6.7), extern (§15.6.8) and async (§15.15) modifiers.

A declaration has a valid combination of modifiers if all of the following are true:

• The declaration includes a valid combination of access modifiers (§15.3.6).
• The declaration does not include the same modifier multiple times.
• The declaration includes at most one of the following modifiers: static, virtual, and override.
• The declaration includes at most one of the following modifiers: new and override.
• If the declaration includes the abstract modifier, then the declaration does not include any of the following modifiers: static, virtual, sealed, or extern.
• If the declaration includes the private modifier, then the declaration does not include any of the following modifiers: virtual, override, or abstract.
• If the declaration includes the sealed modifier, then the declaration also includes the override modifier.
• If the declaration includes the partial modifier, then it does not include any of the following modifiers: new, public, protected, internal, private, virtual, sealed, override, abstract, or extern.

Methods are classified according to what, if anything, they return:

• If ref is present, the method is returns-by-ref and returns a variable reference, that is optionally read-only;
• Otherwise, if return_type is void, the method is returns-no-value and does not return a value;
• Otherwise, the method is returns-by-value and returns a value.

The return_type of a returns-by-value or returns-no-value method declaration specifies the type of the result, if any, returned by the method. Only a returns-no-value method may include the partial modifier (§15.6.9). If the declaration includes the async modifier then return_type shall be void or the method returns-by-value and the return type is a task type (§15.15.1).

The ref_return_type of a returns-by-ref method declaration specifies the type of the variable referenced by the variable_reference returned by the method.

A generic method is a method whose declaration includes a type_parameter_list. This specifies the type parameters for the method. The optional type_parameter_constraints_clauses specify the constraints for the type parameters.

A generic method_declaration for an explicit interface member implementation shall not have any type_parameter_constraints_clauses; the declaration inherits any constraints from the constraints on the interface method.

Similarly, a method declaration with the override modifier shall not have any type_parameter_constraints_clauses and the constraints of the method’s type parameters are inherited from the virtual method being overridden.

The member_name specifies the name of the method. Unless the method is an explicit interface member implementation (§18.6.2), the member_name is simply an identifier.

For an explicit interface member implementation, the member_name consists of an interface_type followed by a “.” and an identifier. In this case, the declaration shall include no modifiers other than (possibly) extern or async.

The optional formal_parameter_list specifies the parameters of the method (§15.6.2).

The return_type or ref_return_type, and each of the types referenced in the formal_parameter_list of a method, shall be at least as accessible as the method itself (§7.5.5).

The method_body of a returns-by-value or returns-no-value method is either a semicolon, a block body or an expression body. A block body consists of a block, which specifies the statements to execute when the method is invoked. An expression body consists of =>, followed by a null_conditional_invocation_expression or expression, and a semicolon, and denotes a single expression to perform when the method is invoked.

For abstract and extern methods, the method_body consists simply of a semicolon. For partial methods the method_body may consist of either a semicolon, a block body or an expression body. For all other methods, the method_body is either a block body or an expression body.

If the method_body consists of a semicolon, the declaration shall not include the async modifier.

The ref_method_body of a returns-by-ref method is either a semicolon, a block body or an expression body. A block body consists of a block, which specifies the statements to execute when the method is invoked. An expression body consists of =>, followed by ref, a variable_reference, and a semicolon, and denotes a single variable_reference to evaluate when the method is invoked.

For abstract and extern methods, the ref_method_body consists simply of a semicolon; for all other methods, the ref_method_body is either a block body or an expression body.

The name, the number of type parameters, and the formal parameter list of a method define the signature (§7.6) of the method. Specifically, the signature of a method consists of its name, the number of its type parameters, and the number, parameter_mode_modifiers (§15.6.2.1), and types of its formal parameters. The return type is not part of a method’s signature, nor are the names of the formal parameters, the names of the type parameters, or the constraints. When a formal parameter type references a type parameter of the method, the ordinal position of the type parameter (not the name of the type parameter) is used for type equivalence.

The name of a method shall differ from the names of all other non-methods declared in the same class. In addition, the signature of a method shall differ from the signatures of all other methods declared in the same class, and two methods declared in the same class shall not have signatures that differ solely by in, out, and ref.

The method’s type_parameters are in scope throughout the method_declaration, and can be used to form types throughout that scope in return_type or ref_return_type, method_body or ref_method_body, and type_parameter_constraints_clauses but not in attributes.

All formal parameters and type parameters shall have different names.

15.6.2 Method parameters

15.6.2.1 General

The parameters of a method, if any, are declared by the method’s formal_parameter_list.

formal_parameter_list
    : fixed_parameters
    | fixed_parameters ',' parameter_array
    | parameter_array
    ;

fixed_parameters
    : fixed_parameter (',' fixed_parameter)*
    ;

fixed_parameter
    : attributes? parameter_modifier? type identifier default_argument?
    ;

default_argument
    : '=' expression
    ;

parameter_modifier
    : parameter_mode_modifier
    | 'this'
    ;

parameter_mode_modifier
    : 'ref'
    | 'out'
    | 'in'
    ;

parameter_array
    : attributes? 'params' array_type identifier
    ;

The formal parameter list consists of one or more comma-separated parameters of which only the last may be a parameter_array.

A fixed_parameter consists of an optional set of attributes (§22); an optional in, out, ref, or this modifier; a type; an identifier; and an optional default_argument. Each fixed_parameter declares a parameter of the given type with the given name. The this modifier designates the method as an extension method and is only allowed on the first parameter of a static method in a non-generic, non-nested static class. If the parameter is a struct type or a type parameter constrained to a struct, the this modifier may be combined with either the ref or in modifier, but not the out modifier. Extension methods are further described in §15.6.10. A fixed_parameter with a default_argument is known as an optional parameter, whereas a fixed_parameter without a default_argument is a required parameter. A required parameter shall not appear after an optional parameter in a formal_parameter_list.

A parameter with a ref, out or this modifier cannot have a default_argument. A parameter with an in modifier may have a default_argument. The expression in a default_argument shall be one of the following:

• a constant_expression
• an expression of the form new S() where S is a value type
• an expression of the form default(S) where S is a value type

The expression shall be implicitly convertible by an identity or nullable conversion to the type of the parameter.

If optional parameters occur in an implementing partial method declaration (§15.6.9), an explicit interface member implementation (§18.6.2), a single-parameter indexer declaration (§15.9), or in an operator declaration (§15.10.1) the compiler should give a warning, since these members can never be invoked in a way that permits arguments to be omitted.

A parameter_array consists of an optional set of attributes (§22), a params modifier, an array_type, and an identifier. A parameter array declares a single parameter of the given array type with the given name. The array_type of a parameter array shall be a single-dimensional array type (§17.2). In a method invocation, a parameter array permits either a single argument of the given array type to be specified, or it permits zero or more arguments of the array element type to be specified. Parameter arrays are described further in §15.6.2.4.

A parameter_array may occur after an optional parameter, but cannot have a default value – the omission of arguments for a parameter_array would instead result in the creation of an empty array.

Example: The following illustrates different kinds of parameters:

void M<T>(
    ref int i,
    decimal d,
    bool b = false,
    bool? n = false,
    string s = "Hello",
    object o = null,
    T t = default(T),
    params int[] a
) { }

In the formal_parameter_list for M, i is a required ref parameter, d is a required value parameter, b, s, o and t are optional value parameters and a is a parameter array.

end example

A method declaration creates a separate declaration space (§7.3) for parameters and type parameters. Names are introduced into this declaration space by the type parameter list and the formal parameter list of the method. The body of the method, if any, is considered to be nested within this declaration space. It is an error for two members of a method declaration space to have the same name. It is an error for the method declaration space and the local variable declaration space of a nested declaration space to contain elements with the same name.

A method invocation (§12.8.9.2) creates a copy, specific to that invocation, of the formal parameters and local variables of the method, and the argument list of the invocation assigns values or variable references to the newly created formal parameters. Within the block of a method, formal parameters can be referenced by their identifiers in simple_name expressions (§12.8.4).

The following kinds of formal parameters exist:

• Value parameters, which are declared without any modifiers.
• Input parameters, which are declared with the in modifier.
• Output parameters, which are declared with the out modifier.
• Reference parameters, which are declared with the ref modifier.
• Parameter arrays, which are declared with the params modifier.

Note: As described in §7.6, the in, out, and ref modifiers are part of a method’s signature, but the params modifier is not. end note

15.6.2.2 Value parameters

A parameter declared with no modifiers is a value parameter. A value parameter is a local variable that gets its initial value from the corresponding argument supplied in the method invocation.

When a formal parameter is a value parameter, the corresponding argument in a method invocation shall be an expression that is implicitly convertible (§10.2) to the formal parameter type.

A method is permitted to assign new values to a value parameter. Such assignments only affect the local storage location represented by the value parameter—they have no effect on the actual argument given in the method invocation.

15.6.2.3 By-reference parameters

15.6.2.3.1 General

A parameter declared with one of the in, ref or out modifiers is a by-reference parameter. A by-reference parameter is a local reference variable (§9.7); the initial referent is obtained from the corresponding argument supplied in the method invocation.

Note: The referent of a by-reference parameter can be changed using the ref assignment (= ref) operator.

When a formal parameter is a by-reference parameter, the corresponding argument in a method invocation shall consist of the corresponding keyword, in, ref, or out, followed by a variable_reference (§9.5) of the same type as the formal parameter. However, when the formal parameter is an in parameter, the argument may be an expression for which an implicit conversion (§10.2) exists from that argument expression to the type of the corresponding parameter.

By-reference parameters are not allowed on functions declared as an iterator (§15.14) or async function (§15.15).

In a method that takes multiple by-reference parameters, it is possible for multiple names to represent the same storage location.

15.6.2.3.2 Input parameters

A parameter declared with an in modifier is a input parameter. The argument corresponding to an in parameter is either a variable existing at the point of the method invocation, or one created by the implementation (§12.6.2.3) in the method invocation. A variable shall be definitely assigned before it can be passed as an argument for an input parameter. Within a method, an input parameter is always considered definitely assigned.

It is a compile-time error to modify the value of an input parameter.

Note: The primary purpose of input parameters is for efficiency. When the type of a method parameter is a large struct (in terms of memory requirements), it is useful to be able to avoid copying the whole value of the argument when calling the method. Input parameters allow methods to refer to existing values in memory, while providing protection against unwanted changes to those values. end note

15.6.2.3.3 Reference parameters

A parameter declared with a ref modifier is a reference parameter. A variable shall be definitely assigned before it can be passed as an argument for a reference parameter. Within a method, a reference parameter is always considered definitely assigned.

Example: The example

class Test
{
    static void Swap(ref int x, ref int y)
    {
        int temp = x;
        x = y;
        y = temp;
    }

    static void Main()
    {
        int i = 1, j = 2;
        Swap(ref i, ref j);
        Console.WriteLine($"i = {i}, j = {j}");
    }
}

produces the output

i = 2, j = 1

For the invocation of Swap in Main, x represents i and y represents j. Thus, the invocation has the effect of swapping the values of i and j.

end example

Example: In the following code

class A
{
    string s;
    void F(ref string a, ref string b)
    {
        s = "One";
        a = "Two";
        b = "Three";
    }

    void G()
    {
        F(ref s, ref s);
    }
}

the invocation of F in G passes a reference to s for both a and b. Thus, for that invocation, the names s, a, and b all refer to the same storage location, and the three assignments all modify the instance field s.

end example

15.6.2.3.4 Output parameters

A parameter declared with an out modifier is an output parameter. Within a method, just like a local variable, an output parameter is initially considered unassigned and shall be definitely assigned before its value is used. Every output parameter of a method shall be definitely assigned before the method returns.

A method declared as a partial method (§15.6.9) shall not have output parameters.

*Note: Output parameters are typically used in methods that produce multiple return values. end note

Example:

class Test
{
    static void SplitPath(string path, out string dir, out string name)
    {
        int i = path.Length;
        while (i > 0)
        {
            char ch = path[i - 1];
            if (ch == '\\' || ch == '/' || ch == ':')
            {
                break;
            }
            i--;
        }
        dir = path.Substring(0, i);
        name = path.Substring(i);
    }

    static void Main()
    {
        string dir, name;
        SplitPath(@"c:\Windows\System\hello.txt", out dir, out name);
        Console.WriteLine(dir);
        Console.WriteLine(name);
    }
}

The example produces the output:

c:\Windows\System\
hello.txt

Note that the dir and name variables can be unassigned before they are passed to SplitPath, and that they are considered definitely assigned following the call.

end example

15.6.2.4 Parameter arrays

A parameter declared with a params modifier is a parameter array. If a formal parameter list includes a parameter array, it shall be the last parameter in the list and it shall be of a single-dimensional array type.

Example: The types string[] and string[][] can be used as the type of a parameter array, but the type string[,] can not. end example

Note: It is not possible to combine the params modifier with the modifiers in, out, or ref. end note

A parameter array permits arguments to be specified in one of two ways in a method invocation:

• The argument given for a parameter array can be a single expression that is implicitly convertible (§10.2) to the parameter array type. In this case, the parameter array acts precisely like a value parameter.
• Alternatively, the invocation can specify zero or more arguments for the parameter array, where each argument is an expression that is implicitly convertible (§10.2) to the element type of the parameter array. In this case, the invocation creates an instance of the parameter array type with a length corresponding to the number of arguments, initializes the elements of the array instance with the given argument values, and uses the newly created array instance as the actual argument.

Except for allowing a variable number of arguments in an invocation, a parameter array is precisely equivalent to a value parameter (§15.6.2.2) of the same type.

Example: The example

class Test
{
    static void F(params int[] args)
    {
        Console.Write($"Array contains {args.Length} elements:");
        foreach (int i in args)
        {
            Console.Write($" {i}");
        }
        Console.WriteLine();
    }

    static void Main()
    {
        int[] arr = {1, 2, 3};
        F(arr);
        F(10, 20, 30, 40);
        F();
    }
}

produces the output

Array contains 3 elements: 1 2 3
Array contains 4 elements: 10 20 30 40
Array contains 0 elements:

The first invocation of F simply passes the array arr as a value parameter. The second invocation of F automatically creates a four-element int[] with the given element values and passes that array instance as a value parameter. Likewise, the third invocation of F creates a zero-element int[] and passes that instance as a value parameter. The second and third invocations are precisely equivalent to writing:

F(new int[] {10, 20, 30, 40});
F(new int[] {});

end example

When performing overload resolution, a method with a parameter array might be applicable, either in its normal form or in its expanded form (§12.6.4.2). The expanded form of a method is available only if the normal form of the method is not applicable and only if an applicable method with the same signature as the expanded form is not already declared in the same type.

Example: The example

class Test
{
    static void F(params object[] a) =>
        Console.WriteLine("F(object[])");

    static void F() =>
        Console.WriteLine("F()");

    static void F(object a0, object a1) =>
        Console.WriteLine("F(object,object)");

    static void Main()
    {
        F();
        F(1);
        F(1, 2);
        F(1, 2, 3);
        F(1, 2, 3, 4);
    }
}

produces the output

F()
F(object[])
F(object,object)
F(object[])
F(object[])

In the example, two of the possible expanded forms of the method with a parameter array are already included in the class as regular methods. These expanded forms are therefore not considered when performing overload resolution, and the first and third method invocations thus select the regular methods. When a class declares a method with a parameter array, it is not uncommon to also include some of the expanded forms as regular methods. By doing so, it is possible to avoid the allocation of an array instance that occurs when an expanded form of a method with a parameter array is invoked.

end example

An array is a reference type, so the value passed for a parameter array can be null.

Example: The example:

class Test
{
    static void F(params string[] array) =>
        Console.WriteLine(array == null);

    static void Main()
    {
        F(null);
        F((string) null);
    }
}

produces the output:

True
False

The second invocation produces False as it is equivalent to F(new string[] { null }) and passes an array containing a single null reference.

end example

When the type of a parameter array is object[], a potential ambiguity arises between the normal form of the method and the expanded form for a single object parameter. The reason for the ambiguity is that an object[] is itself implicitly convertible to type object. The ambiguity presents no problem, however, since it can be resolved by inserting a cast if needed.

Example: The example

class Test
{
    static void F(params object[] args)
    {
        foreach (object o in args)
        {
            Console.Write(o.GetType().FullName);
            Console.Write(" ");
        }
        Console.WriteLine();
    }

    static void Main()
    {
        object[] a = {1, "Hello", 123.456};
        object o = a;
        F(a);
        F((object)a);
        F(o);
        F((object[])o);
    }
}

produces the output

System.Int32 System.String System.Double
System.Object[]
System.Object[]
System.Int32 System.String System.Double

In the first and last invocations of F, the normal form of F is applicable because an implicit conversion exists from the argument type to the parameter type (both are of type object[]). Thus, overload resolution selects the normal form of F, and the argument is passed as a regular value parameter. In the second and third invocations, the normal form of F is not applicable because no implicit conversion exists from the argument type to the parameter type (type object cannot be implicitly converted to type object[]). However, the expanded form of F is applicable, so it is selected by overload resolution. As a result, a one-element object[] is created by the invocation, and the single element of the array is initialized with the given argument value (which itself is a reference to an object[]).

end example

15.6.3 Static and instance methods

When a method declaration includes a static modifier, that method is said to be a static method. When no static modifier is present, the method is said to be an instance method.

A static method does not operate on a specific instance, and it is a compile-time error to refer to this in a static method.

An instance method operates on a given instance of a class, and that instance can be accessed as this (§12.8.13).

The differences between static and instance members are discussed further in §15.3.8.

15.6.4 Virtual methods

When an instance method declaration includes a virtual modifier, that method is said to be a virtual method. When no virtual modifier is present, the method is said to be a non-virtual method.

The implementation of a non-virtual method is invariant: The implementation is the same whether the method is invoked on an instance of the class in which it is declared or an instance of a derived class. In contrast, the implementation of a virtual method can be superseded by derived classes. The process of superseding the implementation of an inherited virtual method is known as overriding that method (§15.6.5).

In a virtual method invocation, the run-time type of the instance for which that invocation takes place determines the actual method implementation to invoke. In a non-virtual method invocation, the compile-time type of the instance is the determining factor. In precise terms, when a method named N is invoked with an argument list A on an instance with a compile-time type C and a run-time type R (where R is either C or a class derived from C), the invocation is processed as follows:

• At binding-time, overload resolution is applied to C, N, and A, to select a specific method M from the set of methods declared in and inherited by C. This is described in §12.8.9.2.
• Then at run-time:
  • If M is a non-virtual method, M is invoked.
  • Otherwise, M is a virtual method, and the most derived implementation of M with respect to R is invoked.

For every virtual method declared in or inherited by a class, there exists a most derived implementation of the method with respect to that class. The most derived implementation of a virtual method M with respect to a class R is determined as follows:

• If R contains the introducing virtual declaration of M, then this is the most derived implementation of M with respect to R.
• Otherwise, if R contains an override of M, then this is the most derived implementation of M with respect to R.
• Otherwise, the most derived implementation of M with respect to R is the same as the most derived implementation of M with respect to the direct base class of R.

Example: The following example illustrates the differences between virtual and non-virtual methods:

class A
{
    public void F() => Console.WriteLine("A.F");
    public virtual void G() => Console.WriteLine("A.G");
}

class B : A
{
    public new void F() => Console.WriteLine("B.F");
    public override void G() => Console.WriteLine("B.G");
}

class Test
{
    static void Main()
    {
        B b = new B();
        A a = b;
        a.F();
        b.F();
        a.G();
        b.G();
    }
}

In the example, A introduces a non-virtual method F and a virtual method G. The class B introduces a new non-virtual method F, thus hiding the inherited F, and also overrides the inherited method G. The example produces the output:

A.F
B.F
B.G
B.G

Notice that the statement a.G() invokes B.G, not A.G. This is because the run-time type of the instance (which is B), not the compile-time type of the instance (which is A), determines the actual method implementation to invoke.

end example

Because methods are allowed to hide inherited methods, it is possible for a class to contain several virtual methods with the same signature. This does not present an ambiguity problem, since all but the most derived method are hidden.

Example: In the following code

class A
{
    public virtual void F() => Console.WriteLine("A.F");
}

class B : A
{
    public override void F() => Console.WriteLine("B.F");
}

class C : B
{
    public new virtual void F() => Console.WriteLine("C.F");
}

class D : C
{
    public override void F() => Console.WriteLine("D.F");
}

class Test
{
    static void Main()
    {
        D d = new D();
        A a = d;
        B b = d;
        C c = d;
        a.F();
        b.F();
        c.F();
        d.F();
    }
}

the C and D classes contain two virtual methods with the same signature: The one introduced by A and the one introduced by C. The method introduced by C hides the method inherited from A. Thus, the override declaration in D overrides the method introduced by C, and it is not possible for D to override the method introduced by A. The example produces the output:

B.F
B.F
D.F
D.F

Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less derived type in which the method is not hidden.

end example

15.6.5 Override methods

When an instance method declaration includes an override modifier, the method is said to be an override method. An override method overrides an inherited virtual method with the same signature. Whereas a virtual method declaration introduces a new method, an override method declaration specializes an existing inherited virtual method by providing a new implementation of that method.

The method overridden by an override declaration is known as the overridden base method For an override method M declared in a class C, the overridden base method is determined by examining each base class of C, starting with the direct base class of C and continuing with each successive direct base class, until in a given base class type at least one accessible method is located which has the same signature as M after substitution of type arguments. For the purposes of locating the overridden base method, a method is considered accessible if it is public, if it is protected, if it is protected internal, or if it is either internal or private protected and declared in the same program as C.

A compile-time error occurs unless all of the following are true for an override declaration:

• An overridden base method can be located as described above.
• There is exactly one such overridden base method. This restriction has effect only if the base class type is a constructed type where the substitution of type arguments makes the signature of two methods the same.
• The overridden base method is a virtual, abstract, or override method. In other words, the overridden base method cannot be static or non-virtual.
• The overridden base method is not a sealed method.
• There is an identity conversion between the return type of the overridden base method and the override method.
• The override declaration and the overridden base method have the same declared accessibility. In other words, an override declaration cannot change the accessibility of the virtual method. However, if the overridden base method is protected internal and it is declared in a different assembly than the assembly containing the override declaration then the override declaration’s declared accessibility shall be protected.
• The override declaration does not specify any type_parameter_constraints_clauses. Instead, the constraints are inherited from the overridden base method. Constraints that are type parameters in the overridden method may be replaced by type arguments in the inherited constraint. This can lead to constraints that are not valid when explicitly specified, such as value types or sealed types.

Example: The following demonstrates how the overriding rules work for generic classes:

abstract class C<T>
{
    public virtual T F() {...}
    public virtual C<T> G() {...}
    public virtual void H(C<T> x) {...}
}

class D : C<string>
{
    public override string F() {...}            // Ok
    public override C<string> G() {...}         // Ok
    public override void H(C<T> x) {...}        // Error, should be C<string>
}

class E<T,U> : C<U>
{
    public override U F() {...}                 // Ok
    public override C<U> G() {...}              // Ok
    public override void H(C<T> x) {...}        // Error, should be C<U>
}

end example

An override declaration can access the overridden base method using a base_access (§12.8.14).

Example: In the following code

class A
{
    int x;

    public virtual void PrintFields() => Console.WriteLine($"x = {x}");
}

class B : A
{
    int y;

    public override void PrintFields()
    {
        base.PrintFields();
        Console.WriteLine($"y = {y}");
    }
}

the base.PrintFields() invocation in B invokes the PrintFields method declared in A. A base_access disables the virtual invocation mechanism and simply treats the base method as a non-virtual method. Had the invocation in B been written ((A)this).PrintFields(), it would recursively invoke the PrintFields method declared in B, not the one declared in A, since PrintFields is virtual and the run-time type of ((A)this) is B.

end example

Only by including an override modifier can a method override another method. In all other cases, a method with the same signature as an inherited method simply hides the inherited method.

Example: In the following code

class A
{
    public virtual void F() {}
}

class B : A
{
    public virtual void F() {} // Warning, hiding inherited F()
}

the F method in B does not include an override modifier and therefore does not override the F method in A. Rather, the F method in B hides the method in A, and a warning is reported because the declaration does not include a new modifier.

end example

Example: In the following code

class A
{
    public virtual void F() {}
}

class B : A
{
    private new void F() {} // Hides A.F within body of B
}

class C : B
{
    public override void F() {} // Ok, overrides A.F
}

the F method in B hides the virtual F method inherited from A. Since the new F in B has private access, its scope only includes the class body of B and does not extend to C. Therefore, the declaration of F in C is permitted to override the F inherited from A.

end example

15.6.6 Sealed methods

When an instance method declaration includes a sealed modifier, that method is said to be a sealed method. A sealed method overrides an inherited virtual method with the same signature. A sealed method shall also be marked with the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method.

Example: The example

class A
{
    public virtual void F() => Console.WriteLine("A.F");
    public virtual void G() => Console.WriteLine("A.G");
}

class B : A
{
    public sealed override void F() => Console.WriteLine("B.F");
    public override void G()        => Console.WriteLine("B.G");
}

class C : B
{
    public override void G() => Console.WriteLine("C.G");
}

the class B provides two override methods: an F method that has the sealed modifier and a G method that does not. B’s use of the sealed modifier prevents C from further overriding F.

end example

15.6.7 Abstract methods

When an instance method declaration includes an abstract modifier, that method is said to be an abstract method. Although an abstract method is implicitly also a virtual method, it cannot have the modifier virtual.

An abstract method declaration introduces a new virtual method but does not provide an implementation of that method. Instead, non-abstract derived classes are required to provide their own implementation by overriding that method. Because an abstract method provides no actual implementation, the method body of an abstract method simply consists of a semicolon.

Abstract method declarations are only permitted in abstract classes (§15.2.2.2).

Example: In the following code

public abstract class Shape
{
    public abstract void Paint(Graphics g, Rectangle r);
}

public class Ellipse : Shape
{
    public override void Paint(Graphics g, Rectangle r) => g.DrawEllipse(r);
}

public class Box : Shape
{
    public override void Paint(Graphics g, Rectangle r) => g.DrawRect(r);
}

the Shape class defines the abstract notion of a geometrical shape object that can paint itself. The Paint method is abstract because there is no meaningful default implementation. The Ellipse and Box classes are concrete Shape implementations. Because these classes are non-abstract, they are required to override the Paint method and provide an actual implementation.

end example

It is a compile-time error for a base_access (§12.8.14) to reference an abstract method.

Example: In the following code

abstract class A
{
    public abstract void F();
}

class B : A
{
    // Error, base.F is abstract
    public override void F() => base.F();
}

a compile-time error is reported for the base.F() invocation because it references an abstract method.

end example

An abstract method declaration is permitted to override a virtual method. This allows an abstract class to force re-implementation of the method in derived classes, and makes the original implementation of the method unavailable.

Example: In the following code

class A
{
    public virtual void F() => Console.WriteLine("A.F");
}

abstract class B: A
{
    public abstract override void F();
}

class C : B
{
    public override void F() => Console.WriteLine("C.F");
}

class A declares a virtual method, class B overrides this method with an abstract method, and class C overrides the abstract method to provide its own implementation.

end example

15.6.8 External methods

When a method declaration includes an extern modifier, the method is said to be an external method. External methods are implemented externally, typically using a language other than C#. Because an external method declaration provides no actual implementation, the method body of an external method simply consists of a semicolon. An external method shall not be generic.

The mechanism by which linkage to an external method is achieved, is implementation-defined.

Example: The following example demonstrates the use of the extern modifier and the DllImport attribute:

class Path
{
    [DllImport("kernel32", SetLastError=true)]
    static extern bool CreateDirectory(string name, SecurityAttribute sa);

    [DllImport("kernel32", SetLastError=true)]
    static extern bool RemoveDirectory(string name);

    [DllImport("kernel32", SetLastError=true)]
    static extern int GetCurrentDirectory(int bufSize, StringBuilder buf);

    [DllImport("kernel32", SetLastError=true)]
    static extern bool SetCurrentDirectory(string name);
}

end example

15.6.9 Partial methods

When a method declaration includes a partial modifier, that method is said to be a partial method. Partial methods may only be declared as members of partial types (§15.2.7), and are subject to a number of restrictions.

Partial methods may be defined in one part of a type declaration and implemented in another. The implementation is optional; if no part implements the partial method, the partial method declaration and all calls to it are removed from the type declaration resulting from the combination of the parts.

Partial methods shall not define access modifiers; they are implicitly private. Their return type shall be void, and their parameters shall not have the out modifier. The identifier partial is recognized as a contextual keyword (§6.4.4) in a method declaration only if it appears immediately before the void keyword. A partial method cannot explicitly implement interface methods.

There are two kinds of partial method declarations: If the body of the method declaration is a semicolon, the declaration is said to be a defining partial method declaration. If the body is other than a semicolon, the declaration is said to be an implementing partial method declaration. Across the parts of a type declaration, there may be only one defining partial method declaration with a given signature, and there may be only one implementing partial method declaration with a given signature. If an implementing partial method declaration is given, a corresponding defining partial method declaration shall exist, and the declarations shall match as specified in the following:

• The declarations shall have the same modifiers (although not necessarily in the same order), method name, number of type parameters and number of parameters.
• Corresponding parameters in the declarations shall have the same modifiers (although not necessarily in the same order) and the same types, or identity convertible types (modulo differences in type parameter names).
• Corresponding type parameters in the declarations shall have the same constraints (modulo differences in type parameter names).

An implementing partial method declaration can appear in the same part as the corresponding defining partial method declaration.

Only a defining partial method participates in overload resolution. Thus, whether or not an implementing declaration is given, invocation expressions may resolve to invocations of the partial method. Because a partial method always returns void, such invocation expressions will always be expression statements. Furthermore, because a partial method is implicitly private, such statements will always occur within one of the parts of the type declaration within which the partial method is declared.

Note: The definition of matching defining and implementing partial method declarations does not require parameter names to match. This can produce surprising, albeit well defined, behaviour when named arguments (§12.6.2.1) are used. For example, given the defining partial method declaration for M in one file, and the implementing partial method declaration in another file:

// File P1.cs:
partial class P
{
    static partial void M(int x);
}

// File P2.cs:
partial class P
{
    static void Caller() => M(y: 0);
    static partial void M(int y) {}
}

is invalid as the invocation uses the argument name from the implementing and not the defining partial method declaration.

end note

If no part of a partial type declaration contains an implementing declaration for a given partial method, any expression statement invoking it is simply removed from the combined type declaration. Thus the invocation expression, including any subexpressions, has no effect at run-time. The partial method itself is also removed and will not be a member of the combined type declaration.

If an implementing declaration exists for a given partial method, the invocations of the partial methods are retained. The partial method gives rise to a method declaration similar to the implementing partial method declaration except for the following:

• The partial modifier is not included.

• The attributes in the resulting method declaration are the combined attributes of the defining and the implementing partial method declaration in unspecified order. Duplicates are not removed.

• The attributes on the parameters of the resulting method declaration are the combined attributes of the corresponding parameters of the defining and the implementing partial method declaration in unspecified order. Duplicates are not removed.

If a defining declaration but not an implementing declaration is given for a partial method M, the following restrictions apply:

• It is a compile-time error to create a delegate from M (§12.8.16.6).

• It is a compile-time error to refer to M inside an anonymous function that is converted to an expression tree type (§8.6).

• Expressions occurring as part of an invocation of M do not affect the definite assignment state (§9.4), which can potentially lead to compile-time errors.

• M cannot be the entry point for an application (§7.1).

Partial methods are useful for allowing one part of a type declaration to customize the behavior of another part, e.g., one that is generated by a tool. Consider the following partial class declaration:

partial class Customer
{
    string name;

    public string Name
    {
        get => name;
        set
        {
            OnNameChanging(value);
            name = value;
            OnNameChanged();
        }
    }

    partial void OnNameChanging(string newName);
    partial void OnNameChanged();
}

If this class is compiled without any other parts, the defining partial method declarations and their invocations will be removed, and the resulting combined class declaration will be equivalent to the following:

class Customer
{
    string name;

    public string Name
    {
        get => name;
        set => name = value;
    }
}

Assume that another part is given, however, which provides implementing declarations of the partial methods:

partial class Customer
{
    partial void OnNameChanging(string newName) =>
        Console.WriteLine($"Changing {name} to {newName}");

    partial void OnNameChanged() =>
        Console.WriteLine($"Changed to {name}");
}

Then the resulting combined class declaration will be equivalent to the following:

class Customer
{
    string name;

    public string Name
    {
        get => name;
        set
        {
            OnNameChanging(value);
            name = value;
            OnNameChanged();
        }
    }

    void OnNameChanging(string newName) =>
        Console.WriteLine($"Changing {name} to {newName}");

    void OnNameChanged() =>
        Console.WriteLine($"Changed to {name}");
}

15.6.10 Extension methods

When the first parameter of a method includes the this modifier, that method is said to be an extension method. Extension methods shall only be declared in non-generic, non-nested static classes. The first parameter of an extension method is restricted, as follows:

• It may have the parameter modifier in only if the parameter has a value type
• It may have the parameter modifier ref only if the parameter has a value type or is a generic type constrained to struct
• It shall not be a pointer type.

Example: The following is an example of a static class that declares two extension methods:

public static class Extensions
{
    public static int ToInt32(this string s) => Int32.Parse(s);

    public static T[] Slice<T>(this T[] source, int index, int count)
    {
        if (index < 0 || count < 0 || source.Length - index < count)
        {
            throw new ArgumentException();
        }
        T[] result = new T[count];
        Array.Copy(source, index, result, 0, count);
        return result;
    }
}

end example

An extension method is a regular static method. In addition, where its enclosing static class is in scope, an extension method may be invoked using instance method invocation syntax (§12.8.9.3), using the receiver expression as the first argument.

Example: The following program uses the extension methods declared above:

static class Program
{
    static void Main()
    {
        string[] strings = { "1", "22", "333", "4444" };
        foreach (string s in strings.Slice(1, 2))
        {
            Console.WriteLine(s.ToInt32());
        }
    }
}

The Slice method is available on the string[], and the ToInt32 method is available on string, because they have been declared as extension methods. The meaning of the program is the same as the following, using ordinary static method calls:

static class Program
{
    static void Main()
    {
        string[] strings = { "1", "22", "333", "4444" };
        foreach (string s in Extensions.Slice(strings, 1, 2))
        {
            Console.WriteLine(Extensions.ToInt32(s));
        }
    }
}

end example

15.6.11 Method body

The method body of a method declaration consists of either a block body, an expression body or a semicolon.

Abstract and external method declarations do not provide a method implementation, so their method bodies simply consist of a semicolon. For any other method, the method body is a block (§13.3) that contains the statements to execute when that method is invoked.

The effective return type of a method is void if the return type is void, or if the method is async and the return type is «TaskType» (§15.15.1). Otherwise, the effective return type of a non-async method is its return type, and the effective return type of an async method with return type «TaskType»<T>(§15.15.1) is T.

When the effective return type of a method is void and the method has a block body, return statements (§13.10.5) in the block shall not specify an expression. If execution of the block of a void method completes normally (that is, control flows off the end of the method body), that method simply returns to its caller.

When the effective return type of a method is void and the method has an expression body, the expression E shall be a statement_expression, and the body is exactly equivalent to a block body of the form { E; }.

For a returns-by-value method (§15.6.1), each return statement in that method’s body shall specify an expression that is implicitly convertible to the effective return type.

For a returns-by-ref method (§15.6.1), each return statement in that method’s body shall specify an expression whose type is that of the effective return type, and has a ref-safe-context of caller-context (§9.7.2).

For returns-by-value and returns-by-ref methods the endpoint of the method body shall not be reachable. In other words, control is not permitted to flow off the end of the method body.

Example: In the following code

class A
{
    public int F() {} // Error, return value required

    public int G()
    {
        return 1;
    }

    public int H(bool b)
    {
        if (b)
        {
            return 1;
        }
        else
        {
            return 0;
        }
    }

    public int I(bool b) => b ? 1 : 0;
}

the value-returning F method results in a compile-time error because control can flow off the end of the method body. The G and H methods are correct because all possible execution paths end in a return statement that specifies a return value. The I method is correct, because its body is equivalent to a block with just a single return statement in it.

end example

15.7 Properties

15.7.1 General

A property is a member that provides access to a characteristic of an object or a class. Examples of properties include the length of a string, the size of a font, the caption of a window, the name of a customer, and so on. Properties are a natural extension of fields—both are named members with associated types, and the syntax for accessing fields and properties is the same. However, unlike fields, properties do not denote storage locations. Instead, properties have accessors that specify the statements to be executed when their values are read or written. Properties thus provide a mechanism for associating actions with the reading and writing of an object’s characteristics; furthermore, they permit such characteristics to be computed.

Properties are declared using property_declarations:

property_declaration
    : attributes? property_modifier* type member_name property_body
    | attributes? property_modifier* ref_kind type member_name ref_property_body
    ;    

property_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'static'
    | 'virtual'
    | 'sealed'
    | 'override'
    | 'abstract'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;
    
property_body
    : '{' accessor_declarations '}' property_initializer?
    | '=>' expression ';'
    ;

property_initializer
    : '=' variable_initializer ';'
    ;

ref_property_body
    : '{' ref_get_accessor_declaration '}'
    | '=>' 'ref' variable_reference ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

There are two kinds of property_declaration:

• The first declares a non-ref-valued property. Its value has type type. This kind of property may be readable and/or writeable.
• The second declares a ref-valued property. Its value is a variable_reference (§9.5), that may be readonly, to a variable of type type. This kind of property is only readable.

A property_declaration may include a set of attributes (§22) and any one of the permitted kinds of declared accessibility (§15.3.6), the new (§15.3.5), static (§15.7.2), virtual (§15.6.4, §15.7.6), override (§15.6.5, §15.7.6), sealed (§15.6.6), abstract (§15.6.7, §15.7.6), and extern (§15.6.8) modifiers.

Property declarations are subject to the same rules as method declarations (§15.6) with regard to valid combinations of modifiers.

The member_name (§15.6.1) specifies the name of the property. Unless the property is an explicit interface member implementation, the member_name is simply an identifier. For an explicit interface member implementation (§18.6.2), the member_name consists of an interface_type followed by a “.” and an identifier.

The type of a property shall be at least as accessible as the property itself (§7.5.5).

A property_body may either consist of a statement body or an expression body. In a statement body, accessor_declarations, which shall be enclosed in “{” and “}” tokens, declare the accessors (§15.7.3) of the property. The accessors specify the executable statements associated with reading and writing the property.

In a property_body an expression body consisting of => followed by an expression E and a semicolon is exactly equivalent to the statement body { get { return E; } }, and can therefore only be used to specify read-only properties where the result of the get accessor is given by a single expression.

A property_initializer may only be given for an automatically implemented property (§15.7.4), and causes the initialization of the underlying field of such properties with the value given by the expression.

A ref_property_body may either consist of a statement body or an expression body. In a statement body a get_accessor_declaration declares the get accessor (§15.7.3) of the property. The accessor specifies the executable statements associated with reading the property.

In a ref_property_body an expression body consisting of => followed by ref, a variable_reference V and a semicolon is exactly equivalent to the statement body { get { return ref V; } }.

Note: Even though the syntax for accessing a property is the same as that for a field, a property is not classified as a variable. Thus, it is not possible to pass a property as an in, out, or ref argument unless the property is ref-valued and therefore returns a variable reference (§9.7). end note

When a property declaration includes an extern modifier, the property is said to be an external property. Because an external property declaration provides no actual implementation, each of its accessor_declarations consists of a semicolon.

15.7.2 Static and instance properties

When a property declaration includes a static modifier, the property is said to be a static property. When no static modifier is present, the property is said to be an instance property.

A static property is not associated with a specific instance, and it is a compile-time error to refer to this in the accessors of a static property.

An instance property is associated with a given instance of a class, and that instance can be accessed as this (§12.8.13) in the accessors of that property.

The differences between static and instance members are discussed further in §15.3.8.

15.7.3 Accessors

Note: This clause applies to both properties (§15.7) and indexers (§15.9). The clause is written in terms of properties, when reading for indexers substitute indexer/indexers for property/properties and consult the list of differences between properties and indexers given in §15.9.2. end note

The accessor_declarations of a property specify the executable statements associated with writing and/or reading that property.

accessor_declarations
    : get_accessor_declaration set_accessor_declaration?
    | set_accessor_declaration get_accessor_declaration?
    ;

get_accessor_declaration
    : attributes? accessor_modifier? 'get' accessor_body
    ;

set_accessor_declaration
    : attributes? accessor_modifier? 'set' accessor_body
    ;

accessor_modifier
    : 'protected'
    | 'internal'
    | 'private'
    | 'protected' 'internal'
    | 'internal' 'protected'
    | 'protected' 'private'
    | 'private' 'protected'
    ;

accessor_body
    : block
    | '=>' expression ';'
    | ';' 
    ;

ref_get_accessor_declaration
    : attributes? accessor_modifier? 'get' ref_accessor_body
    ;
    
ref_accessor_body
    : block
    | '=>' 'ref' variable_reference ';'
    | ';'
    ;

The accessor_declarations consist of a get_accessor_declaration, a set_accessor_declaration, or both. Each accessor declaration consists of optional attributes, an optional accessor_modifier, the token get or set, followed by an accessor_body.

For a ref-valued property the ref_get_accessor_declaration consists optional attributes, an optional accessor_modifier, the token get, followed by an ref_accessor_body.

The use of accessor_modifiers is governed by the following restrictions:

• An accessor_modifier shall not be used in an interface or in an explicit interface member implementation.
• For a property or indexer that has no override modifier, an accessor_modifier is permitted only if the property or indexer has both a get and set accessor, and then is permitted only on one of those accessors.
• For a property or indexer that includes an override modifier, an accessor shall match the accessor_modifier, if any, of the accessor being overridden.
• The accessor_modifier shall declare an accessibility that is strictly more restrictive than the declared accessibility of the property or indexer itself. To be precise:
  • If the property or indexer has a declared accessibility of public, the accessibility declared by accessor_modifier may be either private protected, protected internal, internal, protected, or private.
  • If the property or indexer has a declared accessibility of protected internal, the accessibility declared by accessor_modifier may be either private protected, protected private, internal, protected, or private.
  • If the property or indexer has a declared accessibility of internal or protected, the accessibility declared by accessor_modifier shall be either private protected or private.
  • If the property or indexer has a declared accessibility of private protected, the accessibility declared by accessor_modifier shall be private.
  • If the property or indexer has a declared accessibility of private, no accessor_modifier may be used.

For abstract and extern non-ref-valued properties, any accessor_body for each accessor specified is simply a semicolon. A non-abstract, non-extern property, but not an indexer, may also have the accessor_body for all accessors specified be a semicolon, in which case it is an automatically implemented property (§15.7.4). An automatically implemented property shall have at least a get accessor. For the accessors of any other non-abstract, non-extern property, the accessor_body is either:

• a block that specifies the statements to be executed when the corresponding accessor is invoked; or
• an expression body, which consists of => followed by an expression and a semicolon, and denotes a single expression to be executed when the corresponding accessor is invoked.

For abstract and extern ref-valued properties the ref_accessor_body is simply a semicolon. For the accessor of any other non-abstract, non-extern property, the ref_accessor_body is either:

• a block that specifies the statements to be executed when the get accessor is invoked; or
• an expression body, which consists of => followed by ref, a variable_reference and a semicolon. The variable reference is evaluated when the get accessor is invoked.

A get accessor for a non-ref-valued property corresponds to a parameterless method with a return value of the property type. Except as the target of an assignment, when such a property is referenced in an expression its get accessor is invoked to compute the value of the property (§12.2.2).

The body of a get accessor for a non-ref-valued property shall conform to the rules for value-returning methods described in §15.6.11. In particular, all return statements in the body of a get accessor shall specify an expression that is implicitly convertible to the property type. Furthermore, the endpoint of a get accessor shall not be reachable.

A get accessor for a ref-valued property corresponds to a parameterless method with a return value of a variable_reference to a variable of the property type. When such a property is referenced in an expression its get accessor is invoked to compute the variable_reference value of the property. That variable reference, like any other, is then used to read or, for non-readonly variable_references, write the referenced variable as required by the context.

Example: The following example illustrates a ref-valued property as the target of an assignment:

class Program
{
    static int field;
    static ref int Property => ref field;

    static void Main()
    {
        field = 10;
        Console.WriteLine(Property); // Prints 10
        Property = 20;               // This invokes the get accessor, then assigns
                                     // via the resulting variable reference
        Console.WriteLine(field);    // Prints 20
    }
}

end example

The body of a get accessor for a ref-valued property shall conform to the rules for ref-valued methods described in §15.6.11.

A set accessor corresponds to a method with a single value parameter of the property type and a void return type. The implicit parameter of a set accessor is always named value. When a property is referenced as the target of an assignment (§12.21), or as the operand of ++ or –- (§12.8.15, §12.9.6), the set accessor is invoked with an argument that provides the new value (§12.21.2). The body of a set accessor shall conform to the rules for void methods described in §15.6.11. In particular, return statements in the set accessor body are not permitted to specify an expression. Since a set accessor implicitly has a parameter named value, it is a compile-time error for a local variable or constant declaration in a set accessor to have that name.

Based on the presence or absence of the get and set accessors, a property is classified as follows:

• A property that includes both a get accessor and a set accessor is said to be a read-write property.
• A property that has only a get accessor is said to be a read-only property. It is a compile-time error for a read-only property to be the target of an assignment.
• A property that has only a set accessor is said to be a write-only property. Except as the target of an assignment, it is a compile-time error to reference a write-only property in an expression.

Note: The pre- and postfix ++ and -- operators and compound assignment operators cannot be applied to write-only properties, since these operators read the old value of their operand before they write the new one. end note

Example: In the following code

public class Button : Control
{
    private string caption;

    public string Caption
    {
        get => caption;
        set
        {
            if (caption != value)
            {
                caption = value;
                Repaint();
            }
        }
    }

    public override void Paint(Graphics g, Rectangle r)
    {
        // Painting code goes here
    }
}

the Button control declares a public Caption property. The get accessor of the Caption property returns the string stored in the private caption field. The set accessor checks if the new value is different from the current value, and if so, it stores the new value and repaints the control. Properties often follow the pattern shown above: The get accessor simply returns a value stored in a private field, and the set accessor modifies that private field and then performs any additional actions required to update fully the state of the object. Given the Button class above, the following is an example of use of the Caption property:

Button okButton = new Button();
okButton.Caption = "OK"; // Invokes set accessor
string s = okButton.Caption; // Invokes get accessor

Here, the set accessor is invoked by assigning a value to the property, and the get accessor is invoked by referencing the property in an expression.

end example

The get and set accessors of a property are not distinct members, and it is not possible to declare the accessors of a property separately.

Example: The example

class A
{
    private string name;

    // Error, duplicate member name
    public string Name
    { 
        get => name;
    }

    // Error, duplicate member name
    public string Name
    { 
        set => name = value;
    }
}

does not declare a single read-write property. Rather, it declares two properties with the same name, one read-only and one write-only. Since two members declared in the same class cannot have the same name, the example causes a compile-time error to occur.

end example

When a derived class declares a property by the same name as an inherited property, the derived property hides the inherited property with respect to both reading and writing.

Example: In the following code

class A
{
    public int P
    {
        set {...}
    }
}

class B : A
{
    public new int P
    {
        get {...}
    }
}

the P property in B hides the P property in A with respect to both reading and writing. Thus, in the statements

B b = new B();
b.P = 1;       // Error, B.P is read-only
((A)b).P = 1;  // Ok, reference to A.P

the assignment to b.P causes a compile-time error to be reported, since the read-only P property in B hides the write-only P property in A. Note, however, that a cast can be used to access the hidden P property.

end example

Unlike public fields, properties provide a separation between an object’s internal state and its public interface.

Example: Consider the following code, which uses a Point struct to represent a location:

class Label
{
    private int x, y;
    private string caption;

    public Label(int x, int y, string caption)
    {
        this.x = x;
        this.y = y;
        this.caption = caption;
    }

    public int X => x;
    public int Y => y;
    public Point Location => new Point(x, y);
    public string Caption => caption;
}

Here, the Label class uses two int fields, x and y, to store its location. The location is publicly exposed both as an X and a Y property and as a Location property of type Point. If, in a future version of Label, it becomes more convenient to store the location as a Point internally, the change can be made without affecting the public interface of the class:

class Label
{
    private Point location;
    private string caption;

    public Label(int x, int y, string caption)
    {
        this.location = new Point(x, y);
        this.caption = caption;
    }

    public int X => location.X;
    public int Y => location.Y;
    public Point Location => location;
    public string Caption => caption;
}

Had x and y instead been public readonly fields, it would have been impossible to make such a change to the Label class.

end example

Note: Exposing state through properties is not necessarily any less efficient than exposing fields directly. In particular, when a property is non-virtual and contains only a small amount of code, the execution environment might replace calls to accessors with the actual code of the accessors. This process is known as inlining, and it makes property access as efficient as field access, yet preserves the increased flexibility of properties. end note

Example: Since invoking a get accessor is conceptually equivalent to reading the value of a field, it is considered bad programming style for get accessors to have observable side-effects. In the example

class Counter
{
    private int next;

    public int Next => next++;
}

the value of the Next property depends on the number of times the property has previously been accessed. Thus, accessing the property produces an observable side effect, and the property should be implemented as a method instead.

The “no side-effects” convention for get accessors doesn’t mean that get accessors should always be written simply to return values stored in fields. Indeed, get accessors often compute the value of a property by accessing multiple fields or invoking methods. However, a properly designed get accessor performs no actions that cause observable changes in the state of the object.

end example

Properties can be used to delay initialization of a resource until the moment it is first referenced.

Example:

public class Console
{
    private static TextReader reader;
    private static TextWriter writer;
    private static TextWriter error;

    public static TextReader In
    {
        get
        {
            if (reader == null)
            {
                reader = new StreamReader(Console.OpenStandardInput());
            }
            return reader;
        }
    }

    public static TextWriter Out
    {
        get
        {
            if (writer == null)
            {
                writer = new StreamWriter(Console.OpenStandardOutput());
            }
            return writer;
        }
    }

    public static TextWriter Error
    {
        get
        {
            if (error == null)
            {
                error = new StreamWriter(Console.OpenStandardError());
            }
            return error;
        }
    }
...
}

The Console class contains three properties, In, Out, and Error, that represent the standard input, output, and error devices, respectively. By exposing these members as properties, the Console class can delay their initialization until they are actually used. For example, upon first referencing the Out property, as in

Console.Out.WriteLine("hello, world");

the underlying TextWriter for the output device is created. However, if the application makes no reference to the In and Error properties, then no objects are created for those devices.

end example

15.7.4 Automatically implemented properties

An automatically implemented property (or auto-property for short), is a non-abstract, non-extern, non-ref-valued property with semicolon-only accessor bodies. Auto-properties shall have a get accessor and may optionally have a set accessor.

When a property is specified as an automatically implemented property, a hidden backing field is automatically available for the property, and the accessors are implemented to read from and write to that backing field. The hidden backing field is inaccessible, it can be read and written only through the automatically implemented property accessors, even within the containing type. If the auto-property has no set accessor, the backing field is considered readonly (§15.5.3). Just like a readonly field, a read-only auto-property may also be assigned to in the body of a constructor of the enclosing class. Such an assignment assigns directly to the read-only backing field of the property.

An auto-property may optionally have a property_initializer, which is applied directly to the backing field as a variable_initializer (§17.7).

Example:

public class Point
{
    public int X { get; set; } // Automatically implemented
    public int Y { get; set; } // Automatically implemented
}

is equivalent to the following declaration:

public class Point
{
    private int x;
    private int y;

    public int X { get { return x; } set { x = value; } }
    public int Y { get { return y; } set { y = value; } }
}

end example

Example: In the following

public class ReadOnlyPoint
{
    public int X { get; }
    public int Y { get; }

    public ReadOnlyPoint(int x, int y)
    {
        X = x;
        Y = y;
    }
}

is equivalent to the following declaration:

public class ReadOnlyPoint
{
    private readonly int __x;
    private readonly int __y;
    public int X { get { return __x; } }
    public int Y { get { return __y; } }

    public ReadOnlyPoint(int x, int y)
    {
        __x = x;
        __y = y;
    }
}

The assignments to the read-only field are valid, because they occur within the constructor.

end example

Although the backing field is hidden, that field may have field-targeted attributes applied directly to it via the automatically implemented property’s property_declaration (§15.7.1).

Example: The following code

[Serializable]
public class Foo
{
    [field: NonSerialized]
    public string MySecret { get; set; }
}

results in the field-targeted attribute NonSerialized being applied to the compiler-generated backing field, as if the code had been written as follows:

[Serializable]
public class Foo
{
    [NonSerialized]
    private string _mySecretBackingField;
    public string MySecret
    {
        get { return _mySecretBackingField; }
        set { _mySecretBackingField = value; }
    }
}

end example

15.7.5 Accessibility

If an accessor has an accessor_modifier, the accessibility domain (§7.5.3) of the accessor is determined using the declared accessibility of the accessor_modifier. If an accessor does not have an accessor_modifier, the accessibility domain of the accessor is determined from the declared accessibility of the property or indexer.

The presence of an accessor_modifier never affects member lookup (§12.5) or overload resolution (§12.6.4). The modifiers on the property or indexer always determine which property or indexer is bound to, regardless of the context of the access.

Once a particular non-ref-valued property or non-ref-valued indexer has been selected, the accessibility domains of the specific accessors involved are used to determine if that usage is valid:

• If the usage is as a value (§12.2.2), the get accessor shall exist and be accessible.
• If the usage is as the target of a simple assignment (§12.21.2), the set accessor shall exist and be accessible.
• If the usage is as the target of compound assignment (§12.21.4), or as the target of the ++ or -- operators (§12.8.15, §12.9.6), both the get accessors and the set accessor shall exist and be accessible.

Example: In the following example, the property A.Text is hidden by the property B.Text, even in contexts where only the set accessor is called. In contrast, the property B.Count is not accessible to class M, so the accessible property A.Count is used instead.

class A
{
    public string Text
    {
        get => "hello";
        set { }
    }

    public int Count
    {
        get => 5;
        set { }
    }
}

class B : A
{
    private string text = "goodbye";
    private int count = 0;

    public new string Text
    {
        get => text;
        protected set => text = value;
    }

    protected new int Count
    {
        get => count;
        set => count = value;
    }
}

class M
{
    static void Main()
    {
        B b = new B();
        b.Count = 12;       // Calls A.Count set accessor
        int i = b.Count;    // Calls A.Count get accessor
        b.Text = "howdy";   // Error, B.Text set accessor not accessible
        string s = b.Text;  // Calls B.Text get accessor
    }
}

end example

Once a particular ref-valued property or ref-valued indexer has been selected; whether the usage is as a value, the target of a simple assignment, or the target of a compound assignment; the accessibility domain of the get accessor involved is used to determine if that usage is valid.

An accessor that is used to implement an interface shall not have an accessor_modifier. If only one accessor is used to implement an interface, the other accessor may be declared with an accessor_modifier:

Example:

public interface I
{
    string Prop { get; }
}

public class C : I
{
    public string Prop
    {
        get => "April";     // Must not have a modifier here
        internal set {...}  // Ok, because I.Prop has no set accessor
    }
}

end example

15.7.6 Virtual, sealed, override, and abstract accessors

Note: This clause applies to both properties (§15.7) and indexers (§15.9). The clause is written in terms of properties, when reading for indexers substitute indexer/indexers for property/properties and consult the list of differences between properties and indexers given in §15.9.2. end note

A virtual property declaration specifies that the accessors of the property are virtual. The virtual modifier applies to all non-private accessors of a property. When an accessor of a virtual property has the private accessor_modifier, the private accessor is implicitly not virtual.

An abstract property declaration specifies that the accessors of the property are virtual, but does not provide an actual implementation of the accessors. Instead, non-abstract derived classes are required to provide their own implementation for the accessors by overriding the property. Because an accessor for an abstract property declaration provides no actual implementation, its accessor_body simply consists of a semicolon. An abstract property shall not have a private accessor.

A property declaration that includes both the abstract and override modifiers specifies that the property is abstract and overrides a base property. The accessors of such a property are also abstract.

Abstract property declarations are only permitted in abstract classes (§15.2.2.2). The accessors of an inherited virtual property can be overridden in a derived class by including a property declaration that specifies an override directive. This is known as an overriding property declaration. An overriding property declaration does not declare a new property. Instead, it simply specializes the implementations of the accessors of an existing virtual property.

The override declaration and the overridden base property are required to have the same declared accessibility. In other words, an override declaration shall not change the accessibility of the base property. However, if the overridden base property is protected internal and it is declared in a different assembly than the assembly containing the override declaration then the override declaration’s declared accessibility shall be protected. If the inherited property has only a single accessor (i.e., if the inherited property is read-only or write-only), the overriding property shall include only that accessor. If the inherited property includes both accessors (i.e., if the inherited property is read-write), the overriding property can include either a single accessor or both accessors. There shall be an identity conversion between the type of the overriding and the inherited property.

An overriding property declaration may include the sealed modifier. Use of this modifier prevents a derived class from further overriding the property. The accessors of a sealed property are also sealed.

Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods. Specifically, the rules described in §15.6.4, §15.6.5, §15.6.6, and §15.6.7 apply as if accessors were methods of a corresponding form:

• A get accessor corresponds to a parameterless method with a return value of the property type and the same modifiers as the containing property.
• A set accessor corresponds to a method with a single value parameter of the property type, a void return type, and the same modifiers as the containing property.

Example: In the following code

abstract class A
{
    int y;

    public virtual int X
    {
        get => 0;
    }

    public virtual int Y
    {
        get => y;
        set => y = value;
    }

    public abstract int Z { get; set; }
}

X is a virtual read-only property, Y is a virtual read-write property, and Z is an abstract read-write property. Because Z is abstract, the containing class A shall also be declared abstract.

A class that derives from A is shown below:

class B : A
{
    int z;

    public override int X
    {
        get => base.X + 1;
    }

    public override int Y
    {
        set => base.Y = value < 0 ? 0: value;
    }

    public override int Z
    {
        get => z;
        set => z = value;
    }
}

Here, the declarations of X, Y, and Z are overriding property declarations. Each property declaration exactly matches the accessibility modifiers, type, and name of the corresponding inherited property. The get accessor of X and the set accessor of Y use the base keyword to access the inherited accessors. The declaration of Z overrides both abstract accessors—thus, there are no outstanding abstract function members in B, and B is permitted to be a non-abstract class.

end example

When a property is declared as an override, any overridden accessors shall be accessible to the overriding code. In addition, the declared accessibility of both the property or indexer itself, and of the accessors, shall match that of the overridden member and accessors.

Example:

public class B
{
    public virtual int P
    {
        get {...}
        protected set {...}
    }
}

public class D: B
{
    public override int P
    {
        get {...}            // Must not have a modifier here
        protected set {...}  // Must specify protected here
    }
}

end example

15.8 Events

15.8.1 General

An event is a member that enables an object or class to provide notifications. Clients can attach executable code for events by supplying event handlers.

Events are declared using event_declarations:

event_declaration
    : attributes? event_modifier* 'event' type variable_declarators ';'
    | attributes? event_modifier* 'event' type member_name
        '{' event_accessor_declarations '}'
    ;

event_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'static'
    | 'virtual'
    | 'sealed'
    | 'override'
    | 'abstract'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;

event_accessor_declarations
    : add_accessor_declaration remove_accessor_declaration
    | remove_accessor_declaration add_accessor_declaration
    ;

add_accessor_declaration
    : attributes? 'add' block
    ;

remove_accessor_declaration
    : attributes? 'remove' block
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

An event_declaration may include a set of attributes (§22) and any one of the permitted kinds of declared accessibility (§15.3.6), the new (§15.3.5), static (§15.6.3, §15.8.4), virtual (§15.6.4, §15.8.5), override (§15.6.5, §15.8.5), sealed (§15.6.6), abstract (§15.6.7, §15.8.5), and extern (§15.6.8) modifiers.

Event declarations are subject to the same rules as method declarations (§15.6) with regard to valid combinations of modifiers.

The type of an event declaration shall be a delegate_type (§8.2.8), and that delegate_type shall be at least as accessible as the event itself (§7.5.5).

An event declaration can include event_accessor_declarations. However, if it does not, for non-extern, non-abstract events, the compiler shall supply them automatically (§15.8.2); for extern events, the accessors are provided externally.

An event declaration that omits event_accessor_declarations defines one or more events—one for each of the variable_declarators. The attributes and modifiers apply to all of the members declared by such an event_declaration.

It is a compile-time error for an event_declaration to include both the abstract modifier and event_accessor_declarations.

When an event declaration includes an extern modifier, the event is said to be an external event. Because an external event declaration provides no actual implementation, it is an error for it to include both the extern modifier and event_accessor_declarations.

It is a compile-time error for a variable_declarator of an event declaration with an abstract or external modifier to include a variable_initializer.

An event can be used as the left operand of the += and -= operators. These operators are used, respectively, to attach event handlers to, or to remove event handlers from an event, and the access modifiers of the event control the contexts in which such operations are permitted.

The only operations that are permitted on an event by code that is outside the type in which that event is declared, are += and -=. Therefore, while such code can add and remove handlers for an event, it cannot directly obtain or modify the underlying list of event handlers.

In an operation of the form x += y or x –= y, when x is an event the result of the operation has type void (§12.21.5) (as opposed to having the type of x, with the value of x after the assignment, as for other the += and -= operators defined on non-event types). This prevents external code from indirectly examining the underlying delegate of an event.

Example: The following example shows how event handlers are attached to instances of the Button class:

public delegate void EventHandler(object sender, EventArgs e);

public class Button : Control
{
    public event EventHandler Click;
}

public class LoginDialog : Form
{
    Button okButton;
    Button cancelButton;

    public LoginDialog()
    {
        okButton = new Button(...);
        okButton.Click += new EventHandler(OkButtonClick);
        cancelButton = new Button(...);
        cancelButton.Click += new EventHandler(CancelButtonClick);
    }

    void OkButtonClick(object sender, EventArgs e)
    {
        // Handle okButton.Click event
    }

    void CancelButtonClick(object sender, EventArgs e)
    {
        // Handle cancelButton.Click event
    }
}

Here, the LoginDialog instance constructor creates two Button instances and attaches event handlers to the Click events.

end example

15.8.2 Field-like events

Within the program text of the class or struct that contains the declaration of an event, certain events can be used like fields. To be used in this way, an event shall not be abstract or extern, and shall not explicitly include event_accessor_declarations. Such an event can be used in any context that permits a field. The field contains a delegate (§20), which refers to the list of event handlers that have been added to the event. If no event handlers have been added, the field contains null.

Example: In the following code

public delegate void EventHandler(object sender, EventArgs e);

public class Button : Control
{
    public event EventHandler Click;

    protected void OnClick(EventArgs e)
    {
        EventHandler handler = Click;
        if (handler != null)
        {
            handler(this, e);
        }
    }

    public void Reset() => Click = null;
}

Click is used as a field within the Button class. As the example demonstrates, the field can be examined, modified, and used in delegate invocation expressions. The OnClick method in the Button class “raises” the Click event. The notion of raising an event is precisely equivalent to invoking the delegate represented by the event—thus, there are no special language constructs for raising events. Note that the delegate invocation is preceded by a check that ensures the delegate is non-null and that the check is made on a local copy to ensure thread safety.

Outside the declaration of the Button class, the Click member can only be used on the left-hand side of the += and –= operators, as in

b.Click += new EventHandler(...);

which appends a delegate to the invocation list of the Click event, and

Click –= new EventHandler(...);

which removes a delegate from the invocation list of the Click event.

end example

When compiling a field-like event, the compiler automatically creates storage to hold the delegate, and creates accessors for the event that add or remove event handlers to the delegate field. The addition and removal operations are thread safe, and may (but are not required to) be done while holding the lock (§13.13) on the containing object for an instance event, or the System.Type object (§12.8.17) for a static event.

Note: Thus, an instance event declaration of the form:

class X
{
    public event D Ev;
}

shall be compiled to something equivalent to:

class X
{
    private D __Ev; // field to hold the delegate

    public event D Ev
    {
        add
        {
            /* Add the delegate in a thread safe way */
        }
        remove
        {
            /* Remove the delegate in a thread safe way */
        }
    }
}

Within the class X, references to Ev on the left-hand side of the += and –= operators cause the add and remove accessors to be invoked. All other references to Ev are compiled to reference the hidden field __Ev instead (§12.8.7). The name “__Ev” is arbitrary; the hidden field could have any name or no name at all.

end note

15.8.3 Event accessors

Note: Event declarations typically omit event_accessor_declarations, as in the Button example above. For example, they might be included if the storage cost of one field per event is not acceptable. In such cases, a class can include event_accessor_declarations and use a private mechanism for storing the list of event handlers. end note

The event_accessor_declarations of an event specify the executable statements associated with adding and removing event handlers.

The accessor declarations consist of an add_accessor_declaration and a remove_accessor_declaration. Each accessor declaration consists of the token add or remove followed by a block. The block associated with an add_accessor_declaration specifies the statements to execute when an event handler is added, and the block associated with a remove_accessor_declaration specifies the statements to execute when an event handler is removed.

Each add_accessor_declaration and remove_accessor_declaration corresponds to a method with a single value parameter of the event type, and a void return type. The implicit parameter of an event accessor is named value. When an event is used in an event assignment, the appropriate event accessor is used. Specifically, if the assignment operator is += then the add accessor is used, and if the assignment operator is –= then the remove accessor is used. In either case, the right operand of the assignment operator is used as the argument to the event accessor. The block of an add_accessor_declaration or a remove_accessor_declaration shall conform to the rules for void methods described in §15.6.9. In particular, return statements in such a block are not permitted to specify an expression.

Since an event accessor implicitly has a parameter named value, it is a compile-time error for a local variable or constant declared in an event accessor to have that name.

Example: In the following code

class Control : Component
{
    // Unique keys for events
    static readonly object mouseDownEventKey = new object();
    static readonly object mouseUpEventKey = new object();

    // Return event handler associated with key
    protected Delegate GetEventHandler(object key) {...}

    // Add event handler associated with key
    protected void AddEventHandler(object key, Delegate handler) {...}

    // Remove event handler associated with key
    protected void RemoveEventHandler(object key, Delegate handler) {...}

    // MouseDown event
    public event MouseEventHandler MouseDown
    {
        add { AddEventHandler(mouseDownEventKey, value); }
        remove { RemoveEventHandler(mouseDownEventKey, value); }
    }

    // MouseUp event
    public event MouseEventHandler MouseUp
    {
        add { AddEventHandler(mouseUpEventKey, value); }
        remove { RemoveEventHandler(mouseUpEventKey, value); }
    }

    // Invoke the MouseUp event
    protected void OnMouseUp(MouseEventArgs args)
    {
        MouseEventHandler handler;
        handler = (MouseEventHandler)GetEventHandler(mouseUpEventKey);
        if (handler != null)
        {
            handler(this, args);
        }
    }
}

the Control class implements an internal storage mechanism for events. The AddEventHandler method associates a delegate value with a key, the GetEventHandler method returns the delegate currently associated with a key, and the RemoveEventHandler method removes a delegate as an event handler for the specified event. Presumably, the underlying storage mechanism is designed such that there is no cost for associating a null delegate value with a key, and thus unhandled events consume no storage.

end example

15.8.4 Static and instance events

When an event declaration includes a static modifier, the event is said to be a static event. When no static modifier is present, the event is said to be an instance event.

A static event is not associated with a specific instance, and it is a compile-time error to refer to this in the accessors of a static event.

An instance event is associated with a given instance of a class, and this instance can be accessed as this (§12.8.13) in the accessors of that event.

The differences between static and instance members are discussed further in §15.3.8.

15.8.5 Virtual, sealed, override, and abstract accessors

A virtual event declaration specifies that the accessors of that event are virtual. The virtual modifier applies to both accessors of an event.

An abstract event declaration specifies that the accessors of the event are virtual, but does not provide an actual implementation of the accessors. Instead, non-abstract derived classes are required to provide their own implementation for the accessors by overriding the event. Because an accessor for an abstract event declaration provides no actual implementation, it shall not provide event_accessor_declarations.

An event declaration that includes both the abstract and override modifiers specifies that the event is abstract and overrides a base event. The accessors of such an event are also abstract.

Abstract event declarations are only permitted in abstract classes (§15.2.2.2).

The accessors of an inherited virtual event can be overridden in a derived class by including an event declaration that specifies an override modifier. This is known as an overriding event declaration. An overriding event declaration does not declare a new event. Instead, it simply specializes the implementations of the accessors of an existing virtual event.

An overriding event declaration shall specify the exact same accessibility modifiers and name as the overridden event, there shall be an identity conversion between the type of the overriding and the overridden event, and both the add and remove accessors shall be specified within the declaration.

An overriding event declaration can include the sealed modifier. Use of this modifier prevents a derived class from further overriding the event. The accessors of a sealed event are also sealed.

It is a compile-time error for an overriding event declaration to include a new modifier.

Except for differences in declaration and invocation syntax, virtual, sealed, override, and abstract accessors behave exactly like virtual, sealed, override and abstract methods. Specifically, the rules described in §15.6.4, §15.6.5, §15.6.6, and §15.6.7 apply as if accessors were methods of a corresponding form. Each accessor corresponds to a method with a single value parameter of the event type, a void return type, and the same modifiers as the containing event.

15.9 Indexers

15.9.1 General

An indexer is a member that enables an object to be indexed in the same way as an array. Indexers are declared using indexer_declarations:

indexer_declaration
    : attributes? indexer_modifier* indexer_declarator indexer_body
    | attributes? indexer_modifier* ref_kind indexer_declarator ref_indexer_body
    ;

indexer_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'virtual'
    | 'sealed'
    | 'override'
    | 'abstract'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;

indexer_declarator
    : type 'this' '[' formal_parameter_list ']'
    | type interface_type '.' 'this' '[' formal_parameter_list ']'
    ;

indexer_body
    : '{' accessor_declarations '}' 
    | '=>' expression ';'
    ;  

ref_indexer_body
    : '{' ref_get_accessor_declaration '}'
    | '=>' 'ref' variable_reference ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

There are two kinds of indexer_declaration:

• The first declares a non-ref-valued indexer. Its value has type type. This kind of indexer may be readable and/or writeable.
• The second declares a ref-valued indexer. Its value is a variable_reference (§9.5), that may be readonly, to a variable of type type. This kind of indexer is only readable.

An indexer_declaration may include a set of attributes (§22) and any one of the permitted kinds of declared accessibility (§15.3.6), the new (§15.3.5), virtual (§15.6.4), override (§15.6.5), sealed (§15.6.6), abstract (§15.6.7), and extern (§15.6.8) modifiers.

Indexer declarations are subject to the same rules as method declarations (§15.6) with regard to valid combinations of modifiers, with the one exception being that the static modifier is not permitted on an indexer declaration.

The type of an indexer declaration specifies the element type of the indexer introduced by the declaration.

Note: As indexers are designed to be used in array element-like contexts, the term element type as defined for an array is also used with an indexer. end note

Unless the indexer is an explicit interface member implementation, the type is followed by the keyword this. For an explicit interface member implementation, the type is followed by an interface_type, a “.”, and the keyword this. Unlike other members, indexers do not have user-defined names.

The formal_parameter_list specifies the parameters of the indexer. The formal parameter list of an indexer corresponds to that of a method (§15.6.2), except that at least one parameter shall be specified, and that the this, ref, and out parameter modifiers are not permitted.

The type of an indexer and each of the types referenced in the formal_parameter_list shall be at least as accessible as the indexer itself (§7.5.5).

An indexer_body may either consist of a statement body (§15.7.1) or an expression body (§15.6.1). In a statement body, accessor_declarations, which shall be enclosed in “{” and “}” tokens, declare the accessors (§15.7.3) of the indexer. The accessors specify the executable statements associated with reading and writing indexer elements.

In a indexer_body an expression body consisting of “=>” followed by an expression E and a semicolon is exactly equivalent to the statement body { get { return E; } }, and can therefore only be used to specify read-only indexers where the result of the get accessor is given by a single expression.

A ref_indexer_body may either consist of a statement body or an expression body. In a statement body a get_accessor_declaration declares the get accessor (§15.7.3) of the property. The accessor specifies the executable statements associated with reading the property.

In a ref_indexer_body an expression body consisting of => followed by ref, a variable_reference V and a semicolon is exactly equivalent to the statement body { get { return ref V; } }.

Note: Even though the syntax for accessing an indexer element is the same as that for an array element, an indexer element is not classified as a variable. Thus, it is not possible to pass an indexer element as an in, out, or ref argument unless the indexer is ref-valued and therefore returns a reference (§9.7). end note

The formal_parameter_list of an indexer defines the signature (§7.6) of the indexer. Specifically, the signature of an indexer consists of the number and types of its formal parameters. The element type and names of the formal parameters are not part of an indexer’s signature.

The signature of an indexer shall differ from the signatures of all other indexers declared in the same class.

When an indexer declaration includes an extern modifier, the indexer is said to be an external indexer. Because an external indexer declaration provides no actual implementation, each of its accessor_declarations consists of a semicolon.

Example: The example below declares a BitArray class that implements an indexer for accessing the individual bits in the bit array.

class BitArray
{
    int[] bits;
    int length;

    public BitArray(int length)
    {
        if (length < 0)
        {
            throw new ArgumentException();
        }
        bits = new int[((length - 1) >> 5) + 1];
        this.length = length;
    }

    public int Length => length;

    public bool this[int index]
    {
        get
        {
            if (index < 0 || index >= length)
            {
                throw new IndexOutOfRangeException();
            }
            return (bits[index >> 5] & 1 << index) != 0;
        }
        set
        {
            if (index < 0 || index >= length)
            {
                throw new IndexOutOfRangeException();
            }
            if (value)
            {
                bits[index >> 5] |= 1 << index;
            }
            else
            {
                bits[index >> 5] &= ~(1 << index);
            }
        }
    }
}

An instance of the BitArray class consumes substantially less memory than a corresponding bool[] (since each value of the former occupies only one bit instead of the latter’s one byte), but it permits the same operations as a bool[].

The following CountPrimes class uses a BitArray and the classical “sieve” algorithm to compute the number of primes between 2 and a given maximum:

class CountPrimes
{
    static int Count(int max)
    {
        BitArray flags = new BitArray(max + 1);
        int count = 0;
        for (int i = 2; i <= max; i++)
        {
            if (!flags[i])
            {
                for (int j = i * 2; j <= max; j += i)
                {
                    flags[j] = true;
                }
                count++;
            }
        }
        return count;
    }

    static void Main(string[] args)
    {
        int max = int.Parse(args[0]);
        int count = Count(max);
        Console.WriteLine($"Found {count} primes between 2 and {max}");
    }
}

Note that the syntax for accessing elements of the BitArray is precisely the same as for a bool[].

The following example shows a 26×10 grid class that has an indexer with two parameters. The first parameter is required to be an upper- or lowercase letter in the range A–Z, and the second is required to be an integer in the range 0–9.

class Grid
{
    const int NumRows = 26;
    const int NumCols = 10;
    int[,] cells = new int[NumRows, NumCols];

    public int this[char row, int col]
    {
        get
        {
            row = Char.ToUpper(row);
            if (row < 'A' || row > 'Z')
            {
                throw new ArgumentOutOfRangeException("row");
            }
            if (col < 0 || col >= NumCols)
            {
                throw new ArgumentOutOfRangeException ("col");
            }
            return cells[row - 'A', col];
        }
        set
        {
            row = Char.ToUpper(row);
            if (row < 'A' || row > 'Z')
            {
                throw new ArgumentOutOfRangeException ("row");
            }
            if (col < 0 || col >= NumCols)
            {
                throw new ArgumentOutOfRangeException ("col");
            }
            cells[row - 'A', col] = value;
        }
    }
}

end example

15.9.2 Indexer and Property Differences

Indexers and properties are very similar in concept, but differ in the following ways:

• A property is identified by its name, whereas an indexer is identified by its signature.
• A property is accessed through a simple_name (§12.8.4) or a member_access (§12.8.7), whereas an indexer element is accessed through an element_access (§12.8.11.3).
• A property can be a static member, whereas an indexer is always an instance member.
• A get accessor of a property corresponds to a method with no parameters, whereas a get accessor of an indexer corresponds to a method with the same formal parameter list as the indexer.
• A set accessor of a property corresponds to a method with a single parameter named value, whereas a set accessor of an indexer corresponds to a method with the same formal parameter list as the indexer, plus an additional parameter named value.
• It is a compile-time error for an indexer accessor to declare a local variable or local constant with the same name as an indexer parameter.
• In an overriding property declaration, the inherited property is accessed using the syntax base.P, where P is the property name. In an overriding indexer declaration, the inherited indexer is accessed using the syntax base[E], where E is a comma-separated list of expressions.
• There is no concept of an “automatically implemented indexer”. It is an error to have a non-abstract, non-external indexer with semicolon accessors.

Aside from these differences, all rules defined in §15.7.3, §15.7.5 and §15.7.6 apply to indexer accessors as well as to property accessors.

This replacing of property/properties with indexer/indexers when reading §15.7.3, §15.7.5 and §15.7.6 applies to defined terms as well. Specifically, read-write property becomes read-write indexer, read-only property becomes read-only indexer, and write-only property becomes write-only indexer.

15.10 Operators

15.10.1 General

An operator is a member that defines the meaning of an expression operator that can be applied to instances of the class. Operators are declared using operator_declarations:

operator_declaration
    : attributes? operator_modifier+ operator_declarator operator_body
    ;

operator_modifier
    : 'public'
    | 'static'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;

operator_declarator
    : unary_operator_declarator
    | binary_operator_declarator
    | conversion_operator_declarator
    ;

unary_operator_declarator
    : type 'operator' overloadable_unary_operator '(' fixed_parameter ')'
    ;

overloadable_unary_operator
    : '+' | '-' | '!' | '~' | '++' | '--' | 'true' | 'false'
    ;

binary_operator_declarator
    : type 'operator' overloadable_binary_operator
        '(' fixed_parameter ',' fixed_parameter ')'
    ;

overloadable_binary_operator
    : '+'  | '-'  | '*'  | '/'  | '%'  | '&' | '|' | '^'  | '<<' 
    | right_shift | '==' | '!=' | '>' | '<' | '>=' | '<='
    ;

conversion_operator_declarator
    : 'implicit' 'operator' type '(' fixed_parameter ')'
    | 'explicit' 'operator' type '(' fixed_parameter ')'
    ;

operator_body
    : block
    | '=>' expression ';'
    | ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

There are three categories of overloadable operators: Unary operators (§15.10.2), binary operators (§15.10.3), and conversion operators (§15.10.4).

The operator_body is either a semicolon, a block body (§15.6.1) or an expression body (§15.6.1). A block body consists of a block, which specifies the statements to execute when the operator is invoked. The block shall conform to the rules for value-returning methods described in §15.6.11. An expression body consists of => followed by an expression and a semicolon, and denotes a single expression to perform when the operator is invoked.

For extern operators, the operator_body consists simply of a semicolon. For all other operators, the operator_body is either a block body or an expression body.

The following rules apply to all operator declarations:

• An operator declaration shall include both a public and a static modifier.
• The parameter(s) of an operator shall have no modifiers other than in.
• The signature of an operator (§15.10.2, §15.10.3, §15.10.4) shall differ from the signatures of all other operators declared in the same class.
• All types referenced in an operator declaration shall be at least as accessible as the operator itself (§7.5.5).
• It is an error for the same modifier to appear multiple times in an operator declaration.

Each operator category imposes additional restrictions, as described in the following subclauses.

Like other members, operators declared in a base class are inherited by derived classes. Because operator declarations always require the class or struct in which the operator is declared to participate in the signature of the operator, it is not possible for an operator declared in a derived class to hide an operator declared in a base class. Thus, the new modifier is never required, and therefore never permitted, in an operator declaration.

Additional information on unary and binary operators can be found in §12.4.

Additional information on conversion operators can be found in §10.5.

15.10.2 Unary operators

The following rules apply to unary operator declarations, where T denotes the instance type of the class or struct that contains the operator declaration:

• A unary +, -, !, or ~ operator shall take a single parameter of type T or T? and can return any type.
• A unary ++ or -- operator shall take a single parameter of type T or T? and shall return that same type or a type derived from it.
• A unary true or false operator shall take a single parameter of type T or T? and shall return type bool.

The signature of a unary operator consists of the operator token (+, -, !, ~, ++, --, true, or false) and the type of the single formal parameter. The return type is not part of a unary operator’s signature, nor is the name of the formal parameter.

The true and false unary operators require pair-wise declaration. A compile-time error occurs if a class declares one of these operators without also declaring the other. The true and false operators are described further in §12.24.

Example: The following example shows an implementation and subsequent usage of operator++ for an integer vector class:

public class IntVector
{
    public IntVector(int length) {...}
    public int Length { get { ... } }                      // Read-only property
    public int this[int index] { get { ... } set { ... } } // Read-write indexer

    public static IntVector operator++(IntVector iv)
    {
        IntVector temp = new IntVector(iv.Length);
        for (int i = 0; i < iv.Length; i++)
        {
            temp[i] = iv[i] + 1;
        }
        return temp;
    }
}

class Test
{
    static void Main()
    {
        IntVector iv1 = new IntVector(4); // Vector of 4 x 0
        IntVector iv2;
        iv2 = iv1++;              // iv2 contains 4 x 0, iv1 contains 4 x 1
        iv2 = ++iv1;              // iv2 contains 4 x 2, iv1 contains 4 x 2
    }
}

Note how the operator method returns the value produced by adding 1 to the operand, just like the postfix increment and decrement operators (§12.8.15), and the prefix increment and decrement operators (§12.9.6). Unlike in C++, this method should not modify the value of its operand directly as this would violate the standard semantics of the postfix increment operator (§12.8.15).

end example

15.10.3 Binary operators

The following rules apply to binary operator declarations, where T denotes the instance type of the class or struct that contains the operator declaration:

• A binary non-shift operator shall take two parameters, at least one of which shall have type T or T?, and can return any type.
• A binary << or >> operator (§12.11) shall take two parameters, the first of which shall have type T or T? and the second of which shall have type int or int?, and can return any type.

The signature of a binary operator consists of the operator token (+, -, *, /, %, &, |, ^, <<, >>, ==, !=, >, <, >=, or <=) and the types of the two formal parameters. The return type and the names of the formal parameters are not part of a binary operator’s signature.

Certain binary operators require pair-wise declaration. For every declaration of either operator of a pair, there shall be a matching declaration of the other operator of the pair. Two operator declarations match if identity conversions exist between their return types and their corresponding parameter types. The following operators require pair-wise declaration:

• operator == and operator !=
• operator > and operator <
• operator >= and operator <=

15.10.4 Conversion operators

A conversion operator declaration introduces a user-defined conversion (§10.5), which augments the pre-defined implicit and explicit conversions.

A conversion operator declaration that includes the implicit keyword introduces a user-defined implicit conversion. Implicit conversions can occur in a variety of situations, including function member invocations, cast expressions, and assignments. This is described further in §10.2.

A conversion operator declaration that includes the explicit keyword introduces a user-defined explicit conversion. Explicit conversions can occur in cast expressions, and are described further in §10.3.

A conversion operator converts from a source type, indicated by the parameter type of the conversion operator, to a target type, indicated by the return type of the conversion operator.

For a given source type S and target type T, if S or T are nullable value types, let S₀ and T₀ refer to their underlying types; otherwise, S₀ and T₀ are equal to S and T respectively. A class or struct is permitted to declare a conversion from a source type S to a target type T only if all of the following are true:

• S₀ and T₀ are different types.

• Either S₀ or T₀ is the instance type of the class or struct that contains the operator declaration.

• Neither S₀ nor T₀ is an interface_type.

• Excluding user-defined conversions, a conversion does not exist from S to T or from T to S.

For the purposes of these rules, any type parameters associated with S or T are considered to be unique types that have no inheritance relationship with other types, and any constraints on those type parameters are ignored.

Example: In the following:

class C<T> {...}

class D<T> : C<T>
{
    public static implicit operator C<int>(D<T> value) {...}     // Ok
    public static implicit operator C<string>(D<T> value) {...}  // Ok
    public static implicit operator C<T>(D<T> value) {...}       // Error
}

the first two operator declarations are permitted because T and int and string, respectively are considered unique types with no relationship. However, the third operator is an error because C<T> is the base class of D<T>.

end example

From the second rule, it follows that a conversion operator shall convert either to or from the class or struct type in which the operator is declared.

Example: It is possible for a class or struct type C to define a conversion from C to int and from int to C, but not from int to bool. end example

It is not possible to directly redefine a pre-defined conversion. Thus, conversion operators are not allowed to convert from or to object because implicit and explicit conversions already exist between object and all other types. Likewise, neither the source nor the target types of a conversion can be a base type of the other, since a conversion would then already exist. However, it is possible to declare operators on generic types that, for particular type arguments, specify conversions that already exist as pre-defined conversions.

Example:

struct Convertible<T>
{
    public static implicit operator Convertible<T>(T value) {...}
    public static explicit operator T(Convertible<T> value) {...}
}

when type object is specified as a type argument for T, the second operator declares a conversion that already exists (an implicit, and therefore also an explicit, conversion exists from any type to type object).

end example

In cases where a pre-defined conversion exists between two types, any user-defined conversions between those types are ignored. Specifically:

• If a pre-defined implicit conversion (§10.2) exists from type S to type T, all user-defined conversions (implicit or explicit) from S to T are ignored.
• If a pre-defined explicit conversion (§10.3) exists from type S to type T, any user-defined explicit conversions from S to T are ignored. Furthermore:
  • If either S or T is an interface type, user-defined implicit conversions from S to T are ignored.
  • Otherwise, user-defined implicit conversions from S to T are still considered.

For all types but object, the operators declared by the Convertible<T> type above do not conflict with pre-defined conversions.

Example:

void F(int i, Convertible<int> n)
{
    i = n;                    // Error
    i = (int)n;               // User-defined explicit conversion
    n = i;                    // User-defined implicit conversion
    n = (Convertible<int>)i;  // User-defined implicit conversion
}

However, for type object, pre-defined conversions hide the user-defined conversions in all cases but one:

void F(object o, Convertible<object> n)
{
    o = n;                       // Pre-defined boxing conversion
    o = (object)n;               // Pre-defined boxing conversion
    n = o;                       // User-defined implicit conversion
    n = (Convertible<object>)o;  // Pre-defined unboxing conversion
}

end example

User-defined conversions are not allowed to convert from or to interface_types. In particular, this restriction ensures that no user-defined transformations occur when converting to an interface_type, and that a conversion to an interface_type succeeds only if the object being converted actually implements the specified interface_type.

The signature of a conversion operator consists of the source type and the target type. (This is the only form of member for which the return type participates in the signature.) The implicit or explicit classification of a conversion operator is not part of the operator’s signature. Thus, a class or struct cannot declare both an implicit and an explicit conversion operator with the same source and target types.

Note: In general, user-defined implicit conversions should be designed to never throw exceptions and never lose information. If a user-defined conversion can give rise to exceptions (for example, because the source argument is out of range) or loss of information (such as discarding high-order bits), then that conversion should be defined as an explicit conversion. end note

Example: In the following code

public struct Digit
{
    byte value;

    public Digit(byte value)
    {
        if (value < 0 || value > 9)
        {
            throw new ArgumentException();
        }
        this.value = value;
    }

    public static implicit operator byte(Digit d) => d.value;
    public static explicit operator Digit(byte b) => new Digit(b);
}

the conversion from Digit to byte is implicit because it never throws exceptions or loses information, but the conversion from byte to Digit is explicit since Digit can only represent a subset of the possible values of a byte.

end example

15.11 Instance constructors

15.11.1 General

An instance constructor is a member that implements the actions required to initialize an instance of a class. Instance constructors are declared using constructor_declarations:

constructor_declaration
    : attributes? constructor_modifier* constructor_declarator constructor_body
    ;

constructor_modifier
    : 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | 'extern'
    | unsafe_modifier   // unsafe code support
    ;

constructor_declarator
    : identifier '(' formal_parameter_list? ')' constructor_initializer?
    ;

constructor_initializer
    : ':' 'base' '(' argument_list? ')'
    | ':' 'this' '(' argument_list? ')'
    ;

constructor_body
    : block
    | '=>' expression ';'
    | ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

A constructor_declaration may include a set of attributes (§22), any one of the permitted kinds of declared accessibility (§15.3.6), and an extern (§15.6.8) modifier. A constructor declaration is not permitted to include the same modifier multiple times.

The identifier of a constructor_declarator shall name the class in which the instance constructor is declared. If any other name is specified, a compile-time error occurs.

The optional formal_parameter_list of an instance constructor is subject to the same rules as the formal_parameter_list of a method (§15.6). As the this modifier for parameters only applies to extension methods (§15.6.10), no parameter in a constructor’s formal_parameter_list shall contain the this modifier. The formal parameter list defines the signature (§7.6) of an instance constructor and governs the process whereby overload resolution (§12.6.4) selects a particular instance constructor in an invocation.

Each of the types referenced in the formal_parameter_list of an instance constructor shall be at least as accessible as the constructor itself (§7.5.5).

The optional constructor_initializer specifies another instance constructor to invoke before executing the statements given in the constructor_body of this instance constructor. This is described further in §15.11.2.

When a constructor declaration includes an extern modifier, the constructor is said to be an external constructor. Because an external constructor declaration provides no actual implementation, its constructor_body consists of a semicolon. For all other constructors, the constructor_body consists of either

• a block, which specifies the statements to initialize a new instance of the class; or
• an expression body, which consists of => followed by an expression and a semicolon, and denotes a single expression to initialize a new instance of the class.

A constructor_body that is a block or expression body corresponds exactly to the block of an instance method with a void return type (§15.6.11).

Instance constructors are not inherited. Thus, a class has no instance constructors other than those actually declared in the class, with the exception that if a class contains no instance constructor declarations, a default instance constructor is automatically provided (§15.11.5).

Instance constructors are invoked by object_creation_expressions (§12.8.16.2) and through constructor_initializers.

15.11.2 Constructor initializers

All instance constructors (except those for class object) implicitly include an invocation of another instance constructor immediately before the constructor_body. The constructor to implicitly invoke is determined by the constructor_initializer:

• An instance constructor initializer of the form base(argument_list) (where argument_list is optional) causes an instance constructor from the direct base class to be invoked. That constructor is selected using argument_list and the overload resolution rules of §12.6.4. The set of candidate instance constructors consists of all the accessible instance constructors of the direct base class. If this set is empty, or if a single best instance constructor cannot be identified, a compile-time error occurs.
• An instance constructor initializer of the form this(argument_list) (where argument_list is optional) invokes another instance constructor from the same class. The constructor is selected using argument_list and the overload resolution rules of §12.6.4. The set of candidate instance constructors consists of all instance constructors declared in the class itself. If the resulting set of applicable instance constructors is empty, or if a single best instance constructor cannot be identified, a compile-time error occurs. If an instance constructor declaration invokes itself through a chain of one or more constructor initializers, a compile-time error occurs.

If an instance constructor has no constructor initializer, a constructor initializer of the form base() is implicitly provided.

Note: Thus, an instance constructor declaration of the form

C(...) {...}

is exactly equivalent to

C(...) : base() {...}

end note

The scope of the parameters given by the formal_parameter_list of an instance constructor declaration includes the constructor initializer of that declaration. Thus, a constructor initializer is permitted to access the parameters of the constructor.

Example:

class A
{
    public A(int x, int y) {}
}

class B: A
{
    public B(int x, int y) : base(x + y, x - y) {}
}

end example

An instance constructor initializer cannot access the instance being created. Therefore it is a compile-time error to reference this in an argument expression of the constructor initializer, as it is a compile-time error for an argument expression to reference any instance member through a simple_name.

15.11.3 Instance variable initializers

When an instance constructor has no constructor initializer, or it has a constructor initializer of the form base(...), that constructor implicitly performs the initializations specified by the variable_initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor. The variable initializers are executed in the textual order in which they appear in the class declaration (§15.5.6).

15.11.4 Constructor execution

Variable initializers are transformed into assignment statements, and these assignment statements are executed before the invocation of the base class instance constructor. This ordering ensures that all instance fields are initialized by their variable initializers before any statements that have access to that instance are executed.

Example: Given the following:

class A
{
    public A()
    {
        PrintFields();
    }

    public virtual void PrintFields() {}
}
class B: A
{
    int x = 1;
    int y;

    public B()
    {
        y = -1;
    }

    public override void PrintFields() =>
        Console.WriteLine($"x = {x}, y = {y}");
}

when new B() is used to create an instance of B, the following output is produced:

x = 1, y = 0

The value of x is 1 because the variable initializer is executed before the base class instance constructor is invoked. However, the value of y is 0 (the default value of an int) because the assignment to y is not executed until after the base class constructor returns. It is useful to think of instance variable initializers and constructor initializers as statements that are automatically inserted before the constructor_body. The example

class A
{
    int x = 1, y = -1, count;

    public A()
    {
        count = 0;
    }

    public A(int n)
    {
        count = n;
    }
}

class B : A
{
    double sqrt2 = Math.Sqrt(2.0);
    ArrayList items = new ArrayList(100);
    int max;

    public B(): this(100)
    {
        items.Add("default");
    }

    public B(int n) : base(n - 1)
    {
        max = n;
    }
}

contains several variable initializers; it also contains constructor initializers of both forms (base and this). The example corresponds to the code shown below, where each comment indicates an automatically inserted statement (the syntax used for the automatically inserted constructor invocations isn’t valid, but merely serves to illustrate the mechanism).

class A
{
    int x, y, count;
    public A()
    {
        x = 1;      // Variable initializer
        y = -1;     // Variable initializer
        object();   // Invoke object() constructor
        count = 0;
    }

    public A(int n)
    {
        x = 1;      // Variable initializer
        y = -1;     // Variable initializer
        object();   // Invoke object() constructor
        count = n;
    }
}

class B : A
{
    double sqrt2;
    ArrayList items;
    int max;
    public B() : this(100)
    {
        B(100);                      // Invoke B(int) constructor
        items.Add("default");
    }

    public B(int n) : base(n - 1)
    {
        sqrt2 = Math.Sqrt(2.0);      // Variable initializer
        items = new ArrayList(100);  // Variable initializer
        A(n - 1);                    // Invoke A(int) constructor
        max = n;
    }
}

end example

15.11.5 Default constructors

If a class contains no instance constructor declarations, a default instance constructor is automatically provided. That default constructor simply invokes a constructor of the direct base class, as if it had a constructor initializer of the form base(). If the class is abstract then the declared accessibility for the default constructor is protected. Otherwise, the declared accessibility for the default constructor is public.

Note: Thus, the default constructor is always of the form

protected C(): base() {}

or

public C(): base() {}

where C is the name of the class.

end note

If overload resolution is unable to determine a unique best candidate for the base-class constructor initializer then a compile-time error occurs.

Example: In the following code

class Message
{
    object sender;
    string text;
}

a default constructor is provided because the class contains no instance constructor declarations. Thus, the example is precisely equivalent to

class Message
{
    object sender;
    string text;

    public Message() : base() {}
}

end example

15.12 Static constructors

A static constructor is a member that implements the actions required to initialize a closed class. Static constructors are declared using static_constructor_declarations:

static_constructor_declaration
    : attributes? static_constructor_modifiers identifier '(' ')'
        static_constructor_body
    ;

static_constructor_modifiers
    : 'static'
    | 'static' 'extern' unsafe_modifier?
    | 'static' unsafe_modifier 'extern'?
    | 'extern' 'static' unsafe_modifier?
    | 'extern' unsafe_modifier 'static'
    | unsafe_modifier 'static' 'extern'?
    | unsafe_modifier 'extern' 'static'
    ;

static_constructor_body
    : block
    | '=>' expression ';'
    | ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

A static_constructor_declaration may include a set of attributes (§22) and an extern modifier (§15.6.8).

The identifier of a static_constructor_declaration shall name the class in which the static constructor is declared. If any other name is specified, a compile-time error occurs.

When a static constructor declaration includes an extern modifier, the static constructor is said to be an external static constructor. Because an external static constructor declaration provides no actual implementation, its static_constructor_body consists of a semicolon. For all other static constructor declarations, the static_constructor_body consists of either

• a block, which specifies the statements to execute in order to initialize the class; or
• an expression body, which consists of => followed by an expression and a semicolon, and denotes a single expression to execute in order to initialize the class.

A static_constructor_body that is a block or expression body corresponds exactly to the method_body of a static method with a void return type (§15.6.11).

Static constructors are not inherited, and cannot be called directly.

The static constructor for a closed class executes at most once in a given application domain. The execution of a static constructor is triggered by the first of the following events to occur within an application domain:

• An instance of the class is created.
• Any of the static members of the class are referenced.

If a class contains the Main method (§7.1) in which execution begins, the static constructor for that class executes before the Main method is called.

To initialize a new closed class type, first a new set of static fields (§15.5.2) for that particular closed type is created. Each of the static fields is initialized to its default value (§15.5.5). Next, the static field initializers (§15.5.6.2) are executed for those static fields. Finally, the static constructor is executed.

Example: The example

class Test
{
    static void Main()
    {
        A.F();
        B.F();
    }
}

class A
{
    static A()
    {
        Console.WriteLine("Init A");
    }

    public static void F()
    {
        Console.WriteLine("A.F");
    }
}

class B
{
    static B()
    {
        Console.WriteLine("Init B");
    }

    public static void F()
    {
        Console.WriteLine("B.F");
    }
}

must produce the output:

Init A
A.F
Init B
B.F

because the execution of A’s static constructor is triggered by the call to A.F, and the execution of B’s static constructor is triggered by the call to B.F.

end example

It is possible to construct circular dependencies that allow static fields with variable initializers to be observed in their default value state.

Example: The example

class A
{
    public static int X;

    static A()
    {
        X = B.Y + 1;
    }
}

class B
{
    public static int Y = A.X + 1;

    static B() {}

    static void Main()
    {
        Console.WriteLine($"X = {A.X}, Y = {B.Y}");
    }
}

produces the output

X = 1, Y = 2

To execute the Main method, the system first runs the initializer for B.Y, prior to class B’s static constructor. Y’s initializer causes A’s static constructor to be run because the value of A.X is referenced. The static constructor of A in turn proceeds to compute the value of X, and in doing so fetches the default value of Y, which is zero. A.X is thus initialized to 1. The process of running A’s static field initializers and static constructor then completes, returning to the calculation of the initial value of Y, the result of which becomes 2.

end example

Because the static constructor is executed exactly once for each closed constructed class type, it is a convenient place to enforce run-time checks on the type parameter that cannot be checked at compile-time via constraints (§15.2.5).

Example: The following type uses a static constructor to enforce that the type argument is an enum:

class Gen<T> where T : struct
{
    static Gen()
    {
        if (!typeof(T).IsEnum)
        {
            throw new ArgumentException("T must be an enum");
        }
    }
}

end example

15.13 Finalizers

Note: In an earlier version of this specification, what is now referred to as a “finalizer” was called a “destructor”. Experience has shown that the term “destructor” caused confusion and often resulted to incorrect expectations, especially to programmers knowing C++. In C++, a destructor is called in a determinate manner, whereas, in C#, a finalizer is not. To get determinate behavior from C#, one should use Dispose. end note

A finalizer is a member that implements the actions required to finalize an instance of a class. A finalizer is declared using a finalizer_declaration:

finalizer_declaration
    : attributes? '~' identifier '(' ')' finalizer_body
    | attributes? 'extern' unsafe_modifier? '~' identifier '(' ')'
      finalizer_body
    | attributes? unsafe_modifier 'extern'? '~' identifier '(' ')'
      finalizer_body
    ;

finalizer_body
    : block
    | '=>' expression ';'
    | ';'
    ;

unsafe_modifier (§23.2) is only available in unsafe code (§23).

A finalizer_declaration may include a set of attributes (§22).

The identifier of a finalizer_declarator shall name the class in which the finalizer is declared. If any other name is specified, a compile-time error occurs.

When a finalizer declaration includes an extern modifier, the finalizer is said to be an external finalizer. Because an external finalizer declaration provides no actual implementation, its finalizer_body consists of a semicolon. For all other finalizers, the finalizer_body consists of either

• a block, which specifies the statements to execute in order to finalize an instance of the class.
• or an expression body, which consists of => followed by an expression and a semicolon, and denotes a single expression to execute in order to finalize an instance of the class.

A finalizer_body that is a block or expression body corresponds exactly to the method_body of an instance method with a void return type (§15.6.11).

Finalizers are not inherited. Thus, a class has no finalizers other than the one that may be declared in that class.

Note: Since a finalizer is required to have no parameters, it cannot be overloaded, so a class can have, at most, one finalizer. end note

Finalizers are invoked automatically, and cannot be invoked explicitly. An instance becomes eligible for finalization when it is no longer possible for any code to use that instance. Execution of the finalizer for the instance may occur at any time after the instance becomes eligible for finalization (§7.9). When an instance is finalized, the finalizers in that instance’s inheritance chain are called, in order, from most derived to least derived. A finalizer may be executed on any thread. For further discussion of the rules that govern when and how a finalizer is executed, see §7.9.

Example: The output of the example

class A
{
    ~A()
    {
        Console.WriteLine("A's finalizer");
    }
}

class B : A
{
    ~B()
    {
        Console.WriteLine("B's finalizer");
    }
}

class Test
{
    static void Main()
    {
        B b = new B();
        b = null;
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }
}

is

B's finalizer
A's finalizer

since finalizers in an inheritance chain are called in order, from most derived to least derived.

end example

Finalizers are implemented by overriding the virtual method Finalize on System.Object. C# programs are not permitted to override this method or call it (or overrides of it) directly.

Example: For instance, the program

class A
{
    override protected void Finalize() {}  // Error
    public void F()
    {
        this.Finalize();                   // Error
    }
}

contains two errors.

end example

The compiler behaves as if this method, and overrides of it, do not exist at all.

Example: Thus, this program:

class A
{
    void Finalize() {}  // Permitted
}

is valid and the method shown hides System.Object’s Finalize method.

end example

For a discussion of the behavior when an exception is thrown from a finalizer, see §21.4.

15.14 Iterators

15.14.1 General

A function member (§12.6) implemented using an iterator block (§13.3) is called an iterator.

An iterator block may be used as the body of a function member as long as the return type of the corresponding function member is one of the enumerator interfaces (§15.14.2) or one of the enumerable interfaces (§15.14.3). It may occur as a method_body, operator_body or accessor_body, whereas events, instance constructors, static constructors and finalizer shall not be implemented as iterators.

When a function member is implemented using an iterator block, it is a compile-time error for the formal parameter list of the function member to specify any in, out, or ref parameters, or an parameter of a ref struct type.

15.14.2 Enumerator interfaces

The enumerator interfaces are the non-generic interface System.Collections.IEnumerator and all instantiations of the generic interface System.Collections.Generic.IEnumerator<T>. For the sake of brevity, in this subclause and its siblings these interfaces are referenced as IEnumerator and IEnumerator<T>, respectively.

15.14.3 Enumerable interfaces

The enumerable interfaces are the non-generic interface System.Collections.IEnumerable and all instantiations of the generic interface System.Collections.Generic.IEnumerable<T>. For the sake of brevity, in this subclause and its siblings these interfaces are referenced as IEnumerable and IEnumerable<T>, respectively.

15.14.4 Yield type

An iterator produces a sequence of values, all of the same type. This type is called the yield type of the iterator.

• The yield type of an iterator that returns IEnumerator or IEnumerable is object.
• The yield type of an iterator that returns IEnumerator<T> or IEnumerable<T> is T.

15.14.5 Enumerator objects

15.14.5.1 General

When a function member returning an enumerator interface type is implemented using an iterator block, invoking the function member does not immediately execute the code in the iterator block. Instead, an enumerator object is created and returned. This object encapsulates the code specified in the iterator block, and execution of the code in the iterator block occurs when the enumerator object’s MoveNext method is invoked. An enumerator object has the following characteristics:

• It implements IEnumerator and IEnumerator<T>, where T is the yield type of the iterator.
• It implements System.IDisposable.
• It is initialized with a copy of the argument values (if any) and instance value passed to the function member.
• It has four potential states, before, running, suspended, and after, and is initially in the before state.

An enumerator object is typically an instance of a compiler-generated enumerator class that encapsulates the code in the iterator block and implements the enumerator interfaces, but other methods of implementation are possible. If an enumerator class is generated by the compiler, that class will be nested, directly or indirectly, in the class containing the function member, it will have private accessibility, and it will have a name reserved for compiler use (§6.4.3).

An enumerator object may implement more interfaces than those specified above.

The following subclauses describe the required behavior of the MoveNext, Current, and Dispose members of the IEnumerator and IEnumerator<T> interface implementations provided by an enumerator object.

Enumerator objects do not support the IEnumerator.Reset method. Invoking this method causes a System.NotSupportedException to be thrown.

15.14.5.2 The MoveNext method

The MoveNext method of an enumerator object encapsulates the code of an iterator block. Invoking the MoveNext method executes code in the iterator block and sets the Current property of the enumerator object as appropriate. The precise action performed by MoveNext depends on the state of the enumerator object when MoveNext is invoked:

• If the state of the enumerator object is before, invoking MoveNext:
  • Changes the state to running.
  • Initializes the parameters (including this) of the iterator block to the argument values and instance value saved when the enumerator object was initialized.
  • Executes the iterator block from the beginning until execution is interrupted (as described below).
• If the state of the enumerator object is running, the result of invoking MoveNext is unspecified.
• If the state of the enumerator object is suspended, invoking MoveNext:
  • Changes the state to running.
  • Restores the values of all local variables and parameters (including this) to the values saved when execution of the iterator block was last suspended.

    Note: The contents of any objects referenced by these variables may have changed since the previous call to MoveNext. end note

  • Resumes execution of the iterator block immediately following the yield return statement that caused the suspension of execution and continues until execution is interrupted (as described below).
• If the state of the enumerator object is after, invoking MoveNext returns false.

When MoveNext executes the iterator block, execution can be interrupted in four ways: By a yield return statement, by a yield break statement, by encountering the end of the iterator block, and by an exception being thrown and propagated out of the iterator block.

• When a yield return statement is encountered (§9.4.4.20):
  • The expression given in the statement is evaluated, implicitly converted to the yield type, and assigned to the Current property of the enumerator object.
  • Execution of the iterator body is suspended. The values of all local variables and parameters (including this) are saved, as is the location of this yield return statement. If the yield return statement is within one or more try blocks, the associated finally blocks are not executed at this time.
  • The state of the enumerator object is changed to suspended.
  • The MoveNext method returns true to its caller, indicating that the iteration successfully advanced to the next value.
• When a yield break statement is encountered (§9.4.4.20):
  • If the yield break statement is within one or more try blocks, the associated finally blocks are executed.
  • The state of the enumerator object is changed to after.
  • The MoveNext method returns false to its caller, indicating that the iteration is complete.
• When the end of the iterator body is encountered:
  • The state of the enumerator object is changed to after.
  • The MoveNext method returns false to its caller, indicating that the iteration is complete.
• When an exception is thrown and propagated out of the iterator block:
  • Appropriate finally blocks in the iterator body will have been executed by the exception propagation.
  • The state of the enumerator object is changed to after.
  • The exception propagation continues to the caller of the MoveNext method.

15.14.5.3 The Current property

An enumerator object’s Current property is affected by yield return statements in the iterator block.

When an enumerator object is in the suspended state, the value of Current is the value set by the previous call to MoveNext. When an enumerator object is in the before, running, or after states, the result of accessing Current is unspecified.

For an iterator with a yield type other than object, the result of accessing Current through the enumerator object’s IEnumerable implementation corresponds to accessing Current through the enumerator object’s IEnumerator<T> implementation and casting the result to object.

15.14.5.4 The Dispose method

The Dispose method is used to clean up the iteration by bringing the enumerator object to the after state.

• If the state of the enumerator object is before, invoking Dispose changes the state to after.
• If the state of the enumerator object is running, the result of invoking Dispose is unspecified.
• If the state of the enumerator object is suspended, invoking Dispose:
  • Changes the state to running.
  • Executes any finally blocks as if the last executed yield return statement were a yield break statement. If this causes an exception to be thrown and propagated out of the iterator body, the state of the enumerator object is set to after and the exception is propagated to the caller of the Dispose method.
  • Changes the state to after.
• If the state of the enumerator object is after, invoking Dispose has no affect.

15.14.6 Enumerable objects

15.14.6.1 General

When a function member returning an enumerable interface type is implemented using an iterator block, invoking the function member does not immediately execute the code in the iterator block. Instead, an enumerable object is created and returned. The enumerable object’s GetEnumerator method returns an enumerator object that encapsulates the code specified in the iterator block, and execution of the code in the iterator block occurs when the enumerator object’s MoveNext method is invoked. An enumerable object has the following characteristics:

• It implements IEnumerable and IEnumerable<T>, where T is the yield type of the iterator.
• It is initialized with a copy of the argument values (if any) and instance value passed to the function member.

An enumerable object is typically an instance of a compiler-generated enumerable class that encapsulates the code in the iterator block and implements the enumerable interfaces, but other methods of implementation are possible. If an enumerable class is generated by the compiler, that class will be nested, directly or indirectly, in the class containing the function member, it will have private accessibility, and it will have a name reserved for compiler use (§6.4.3).

An enumerable object may implement more interfaces than those specified above.

Note: For example, an enumerable object may also implement IEnumerator and IEnumerator<T>, enabling it to serve as both an enumerable and an enumerator. Typically, such an implementation would return its own instance (to save allocations) from the first call to GetEnumerator. Subsequent invocations of GetEnumerator, if any, would return a new class instance, typically of the same class, so that calls to different enumerator instances will not affect each other. It cannot return the same instance even if the previous enumerator has already enumerated past the end of the sequence, since all future calls to an exhausted enumerator must throw exceptions. end note

15.14.6.2 The GetEnumerator method

An enumerable object provides an implementation of the GetEnumerator methods of the IEnumerable and IEnumerable<T> interfaces. The two GetEnumerator methods share a common implementation that acquires and returns an available enumerator object. The enumerator object is initialized with the argument values and instance value saved when the enumerable object was initialized, but otherwise the enumerator object functions as described in §15.14.5.

15.15 Async Functions

15.15.1 General

A method (§15.6) or anonymous function (§12.19) with the async modifier is called an async function. In general, the term async is used to describe any kind of function that has the async modifier.

It is a compile-time error for the formal parameter list of an async function to specify any in, out, or ref parameters, or any parameter of a ref struct type.

The return_type of an async method shall be either void or a task type. For an async method that produces a result value, a task type shall be generic. For an async method that does not produce a result value, a task type shall not be generic. Such types are referred to in this specification as «TaskType»<T> and «TaskType», respectively. The Standard library type System.Threading.Tasks.Task and types constructed from System.Threading.Tasks.Task<TResult> are task types, as well as a class, struct or interface type that is associated with a task builder type via the attribute System.Runtime.CompilerServices.AsyncMethodBuilderAttribute. Such types are referred to in this specification as «TaskBuilderType»<T> and «TaskBuilderType». A task type can have at most one type parameter and cannot be nested in a generic type.

An async method returning a task type is said to be task-returning.

Task types can vary in their exact definition, but from the language’s point of view, a task type is in one of the states incomplete, succeeded or faulted. A faulted task records a pertinent exception. A succeeded «TaskType»<T> records a result of type T. Task types are awaitable, and tasks can therefore be the operands of await expressions (§12.9.8).

Example: The task type MyTask<T> is associated with the task builder type MyTaskMethodBuilder<T> and the awaiter type Awaiter<T>:

using System.Runtime.CompilerServices; 
[AsyncMethodBuilder(typeof(MyTaskMethodBuilder<>))]
class MyTask<T>
{
    public Awaiter<T> GetAwaiter() { ... }
}

class Awaiter<T> : INotifyCompletion
{
    public void OnCompleted(Action completion) { ... }
    public bool IsCompleted { get; }
    public T GetResult() { ... }
}

end example

A task builder type is a class or struct type that corresponds to a specific task type (§15.15.2). The task builder type shall exactly match the declared accessibility of its corresponding task type.

Note: If the task type is declared internal, the the corresponding builder type must also be declared internal and be defined in the same assembly. If the task type is nested inside another type, the task buider type must also be nested in that same type. end note

An async function has the ability to suspend evaluation by means of await expressions (§12.9.8) in its body. Evaluation may later be resumed at the point of the suspending await expression by means of a resumption delegate. The resumption delegate is of type System.Action, and when it is invoked, evaluation of the async function invocation will resume from the await expression where it left off. The current caller of an async function invocation is the original caller if the function invocation has never been suspended or the most recent caller of the resumption delegate otherwise.

15.15.2 Task-type builder pattern

A task builder type can have at most one type parameter and cannot be nested in a generic type. A task builder type shall have the following members (for non-generic task builder types, SetResult has no parameters) with declared public accessibility:

class «TaskBuilderType»<T>
{
    public static «TaskBuilderType»<T> Create();
    public void Start<TStateMachine>(ref TStateMachine stateMachine)
                where TStateMachine : IAsyncStateMachine;
    public void SetStateMachine(IAsyncStateMachine stateMachine);
    public void SetException(Exception exception);
    public void SetResult(T result);
    public void AwaitOnCompleted<TAwaiter, TStateMachine>(
        ref TAwaiter awaiter, ref TStateMachine stateMachine)
        where TAwaiter : INotifyCompletion
        where TStateMachine : IAsyncStateMachine;
    public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
        ref TAwaiter awaiter, ref TStateMachine stateMachine)
        where TAwaiter : ICriticalNotifyCompletion
        where TStateMachine : IAsyncStateMachine;
    public «TaskType»<T> Task { get; }
}

The compiler generates code that uses the «TaskBuilderType» to implement the semantics of suspending and resuming the evaluation of the async function. The compiler uses the «TaskBuilderType» as follows:

• «TaskBuilderType».Create() is invoked to create an instance of the «TaskBuilderType», named builder in this list.
• builder.Start(ref stateMachine) is invoked to associate the builder with a compiler-generated state machine instance, stateMachine.
  • The builder shall call stateMachine.MoveNext() either in Start() or after Start() has returned to advance the state machine.
• After Start() returns, the async method invokes builder.Task for the task to return from the async method.
• Each call to stateMachine.MoveNext() will advance the state machine.
• If the state machine completes successfully, builder.SetResult() is called, with the method return value, if any.
• Otherwise, if an exception, e is thrown in the state machine, builder.SetException(e) is called.
• If the state machine reaches an await expr expression, expr.GetAwaiter() is invoked.
• If the awaiter implements ICriticalNotifyCompletion and IsCompleted is false, the state machine invokes builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine).
  • AwaitUnsafeOnCompleted() should call awaiter.UnsafeOnCompleted(action) with an Action that calls stateMachine.MoveNext() when the awaiter completes.
• Otherwise, the state machine invokes builder.AwaitOnCompleted(ref awaiter, ref stateMachine).
  • AwaitOnCompleted() should call awaiter.OnCompleted(action) with an Action that calls stateMachine.MoveNext() when the awaiter completes.
• SetStateMachine(IAsyncStateMachine) may be called by the compiler-generated IAsyncStateMachine implementation to identify the instance of the builder associated with a state machine instance, particularly for cases where the state machine is implemented as a value type.
  • If the builder calls stateMachine.SetStateMachine(stateMachine), the stateMachine will call builder.SetStateMachine(stateMachine) on the builder instance associated with stateMachine.

Note: For both SetResult(T result) and «TaskType»<T> Task { get; }, the parameter and argument respectively must be identity convertible to T. This allows a task-type builder to support types such as tuples, where two types that aren’t the same are identity convertible. end note

15.15.3 Evaluation of a task-returning async function

Invocation of a task-returning async function causes an instance of the returned task type to be generated. This is called the return task of the async function. The task is initially in an incomplete state.

The async function body is then evaluated until it is either suspended (by reaching an await expression) or terminates, at which point control is returned to the caller, along with the return task.

When the body of the async function terminates, the return task is moved out of the incomplete state:

• If the function body terminates as the result of reaching a return statement or the end of the body, any result value is recorded in the return task, which is put into a succeeded state.
• If the function body terminates because of an uncaught OperationCanceledException, the exception is recorded in the return task which is put into the canceled state.
• If the function body terminates as the result of any other uncaught exception (§13.10.6) the exception is recorded in the return task which is put into a faulted state.

15.15.4 Evaluation of a void-returning async function

If the return type of the async function is void, evaluation differs from the above in the following way: Because no task is returned, the function instead communicates completion and exceptions to the current thread’s synchronization context. The exact definition of synchronization context is implementation-dependent, but is a representation of “where” the current thread is running. The synchronization context is notified when evaluation of a void-returning async function commences, completes successfully, or causes an uncaught exception to be thrown.

This allows the context to keep track of how many void-returning async functions are running under it, and to decide how to propagate exceptions coming out of them.