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 benull
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 methodF
. ClassB
introduces an additional methodG
, but since it doesn’t provide an implementation ofF
,B
shall also be declared abstract. ClassC
overridesF
and provides an actual implementation. Since there are no abstract members inC
,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
orabstract
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
, orprotected 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 formT.I
, or - The namespace_or_type-name is the
T
in a typeof_expression (§12.8.18) of the formtypeof(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 formE.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 (§12.8.18) 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 ofB
, andB
is said to be derived fromA
. SinceA
does not explicitly specify a direct base class, its direct base class is implicitlyobject
.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 beB<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 ofZ
is considered to beobject
, and hence (by the rules of §7.8)Z
is not considered to have a memberY
.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>
areC<int[]>
,B<IComparable<int[]>>
,A
, andobject
.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 onB
(its immediately enclosing class), which circularly depends onA
.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 onA
(becauseA
is both its direct base class and its immediately enclosing class), butA
does not depend onB
(sinceB
is neither a base class nor an enclosing class ofA
). 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 classA
.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
isIA
,IB
, andIC
.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 forT
. Thus, recursively constructed types of the formsT??
andNullable<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
orSystem.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 parameterS
thenS
depends onT
. - If a type parameter
S
depends on a type parameterT
andT
depends on a type parameterU
thenS
depends onU
.
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 soS
would be forced to be the same type asT
, eliminating the need for two type parameters.- If
S
has the value type constraint thenT
shall not have a class_type constraint. - If
S
has a class_type constraintA
andT
has a class_type constraintB
then there shall be an identity conversion or implicit reference conversion fromA
toB
or an implicit reference conversion fromB
toA
. - If
S
also depends on type parameterU
andU
has a class_type constraintA
andT
has a class_type constraintB
then there shall be an identity conversion or implicit reference conversion fromA
toB
or an implicit reference conversion fromB
toA
.
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 (§12.8.17.2). 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 typeOuter.Inner
thenCₓ
is a nested typeOuterₓ.Innerₓ
. - If
C
Cₓ
is a constructed typeG<A¹, ..., Aⁿ>
with type argumentsA¹, ..., Aⁿ
thenCₓ
is the constructed typeG<A¹ₓ, ..., Aⁿₓ>
. - If
C
is an array typeE[]
thenCₓ
is the array typeEₓ[]
. - If
C
is dynamic thenCₓ
isobject
. - Otherwise,
Cₓ
isC
.
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
containsSystem.ValueType
. - For each constraint of
T
that is an enumeration type,R
containsSystem.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
containsSystem.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 isSystem.ValueType
. - Otherwise, if
R
is empty then the effective base class isobject
. - Otherwise, the effective base class of
T
is the most-encompassed type (§10.5.3) of setR
. If the set has no encompassed type, the effective base class ofT
isobject
. 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 onx
becauseT
is constrained to always implementIPrintable
.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
, andref
.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
andout
.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 declarationGen
is “two-dimensional array ofT
”, so the type of the membera
in the constructed type above is “two-dimensional array of single-dimensional array ofint
”, orint[,][]
.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 fromB
, andB
is derived fromA
, thenC
inherits the members declared inB
as well as the members declared inA
.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 publicint
G(string s)
obtained by substituting the type argumentint
for the type parameterT
.D<int>
also has an inherited member from the class declarationB
. This inherited member is determined by first determining the base class typeB<int[]>
ofD<int>
by substitutingint
forT
in the base class specificationB<T[]>
. Then, as a type argument toB
,int[]
is substituted forU
inpublic U F(long index)
, yielding the inherited memberpublic 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 formE.M
,E
shall denote a type that has a memberM
. It is a compile-time error forE
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 formE.M
,E
shall denote an instance of a type that has a memberM
. 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
(§12.8.14).
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. TheG
method shows that in a static function member, it is a compile-time error to access an instance member through a simple_name. TheMain
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 classA
, and classA
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 (§12.8.18)).
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
, orprivate
) and, like other struct members, defaults toprivate
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 methodM
defined inBase
.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 (§12.8.14). 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 ofNested
, and passes its own this toNested
’s constructor in order to provide subsequent access toC
’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 classNested
. WithinNested
, the methodG
calls the static methodF
defined inC
, andF
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 methodF
defined inDerived
’s base class,Base
, by calling through an instance ofDerived
.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:
- To allow the underlying implementation to use an ordinary identifier as a method name for get or set access to the C# language feature.
- To allow other languages to interoperate using an ordinary identifier as a method name for get or set access to the C# language feature.
- 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).
Certain names are reserved for use as operator method names (§15.3.10.6).
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 propertyP
, thus reserving signatures forget_P
andset_P
methods.A
classB
derives fromA
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();
15.3.10.6 Method names reserved for operators
The following method names are reserved. While many have corresponding operators in this specification, some are reserved for use by future versions, while some are reserved for interop with other languages.
Method Name | C# Operator |
---|---|
op_Addition |
+ (binary) |
op_AdditionAssignment |
(reserved) |
op_AddressOf |
(reserved) |
op_Assign |
(reserved) |
op_BitwiseAnd |
& (binary) |
op_BitwiseAndAssignment |
(reserved) |
op_BitwiseOr |
\| |
op_BitwiseOrAssignment |
(reserved) |
op_CheckedAddition |
(reserved for future use) |
op_CheckedDecrement |
(reserved for future use) |
op_CheckedDivision |
(reserved for future use) |
op_CheckedExplicit |
(reserved for future use) |
op_CheckedIncrement |
(reserved for future use) |
op_CheckedMultiply |
(reserved for future use) |
op_CheckedSubtraction |
(reserved for future use) |
op_CheckedUnaryNegation |
(reserved for future use) |
op_Comma |
(reserved) |
op_Decrement |
-- (prefix and postfix) |
op_Division |
/ |
op_DivisionAssignment |
(reserved) |
op_Equality |
== |
op_ExclusiveOr |
^ |
op_ExclusiveOrAssignment |
(reserved) |
op_Explicit |
explicit (narrowing) coercion |
op_False |
false |
op_GreaterThan |
> |
op_GreaterThanOrEqual |
>= |
op_Implicit |
implicit (widening) coercion |
op_Increment |
++ (prefix and postfix) |
op_Inequality |
!= |
op_LeftShift |
<< |
op_LeftShiftAssignment |
(reserved) |
op_LessThan |
< |
op_LessThanOrEqual |
<= |
op_LogicalAnd |
(reserved) |
op_LogicalNot |
! |
op_LogicalOr |
(reserved) |
op_MemberSelection |
(reserved) |
op_Modulus |
% |
op_ModulusAssignment |
(reserved) |
op_MultiplicationAssignment |
(reserved) |
op_Multiply |
* (binary) |
op_OnesComplement |
~ |
op_PointerDereference |
(reserved) |
op_PointerToMemberSelection |
(reserved) |
op_RightShift |
>> |
op_RightShiftAssignment |
(reserved) |
op_SignedRightShift |
(reserved) |
op_Subtraction |
- (binary) |
op_SubtractionAssignment |
(reserved) |
op_True |
true |
op_UnaryNegation |
- (unary) |
op_UnaryPlus |
+ (unary) |
op_UnsignedRightShift |
(reserved for future use) |
op_UnsignedRightShiftAssignment |
(reserved) |
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 thenew
operator, and since thenew
operator is not permitted in a constant_expression, the only possible value for constants of reference_types other thanstring
isnull
. 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
andreadonly
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 evaluatesB.Z
, and finally evaluatesA.X
, producing the values10
,11
, and12
.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
andB
were declared in separate programs, it would be possible forA.X
to depend onB.Z
, butB.Z
could then not simultaneously depend onA.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 (§12.8.15). 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 (§12.8.16) 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 output or reference parameter.
Attempting to assign to a readonly field or pass it as an output or reference 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
, andBlue
members cannot be declared as const members because their values cannot be computed at compile-time. However, declaring themstatic 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
andProgram2
namespaces denote two programs that are compiled separately. BecauseProgram1.Utils.X
is declared as astatic readonly
field, the value output by theConsole.WriteLine
statement is not known at compile-time, but rather is obtained at run-time. Thus, if the value ofX
is changed andProgram1
is recompiled, theConsole.WriteLine
statement will output the new value even ifProgram2
isn’t recompiled. However, hadX
been a constant, the value ofX
would have been obtained at the timeProgram2
was compiled, and would remain unaffected by changes inProgram1
untilProgram2
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
, orSystem.UIntPtr
. - An enum_type having an enum_base type of
byte
,sbyte
,short
,ushort
,int
, oruint
.
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($"result = {result}"); return; } } } }
produces the output:
result = 143
In this example, the method
Main
starts a new thread that runs the methodThread2
. This method stores a value into a non-volatile field calledresult
, then storestrue
in the volatile fieldfinished
. The main thread waits for the fieldfinished
to be set totrue
, then reads the fieldresult
. Sincefinished
has been declaredvolatile
, the main thread shall read the value143
from the fieldresult
. If the fieldfinished
had not been declaredvolatile
, then it would be permissible for the store toresult
to be visible to the main thread after the store tofinished
, and hence for the main thread to read the value 0 from the fieldresult
. Declaringfinished
as avolatile
field prevents any such inconsistency.end example
15.5.5 Field initialization
The initial value of a field, whether it be a static field or an instance field, is the default value (§9.3) of the field’s type. It is not possible to observe the value of a field before this default initialization has occurred, and a field is thus never “uninitialized”.
Example: The example
class Test { static bool b; int i; static void Main() { Test t = new Test(); Console.WriteLine($"b = {b}, i = {t.i}"); } }
produces the output
b = False, i = 0
because
b
andi
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($"x = {x}, i = {a.i}, s = {a.s}"); } }
produces the output
x = 1.4142135623730951, i = 100, s = Hello
because an assignment to
x
occurs when static field initializers execute and assignments toi
ands
occur when the instance field initializers execute.end example
The default value initialization described in §15.5.5 occurs for all fields, including fields that have variable initializers. Thus, when a class is initialized, all static fields in that class are first initialized to their default values, and then the static field initializers are executed in textual order. Likewise, when an instance of a class is created, all instance fields in that instance are first initialized to their default values, and then the instance field initializers are executed in textual order. When there are field declarations in multiple partial type declarations for the same type, the order of the parts is unspecified. However, within each part the field initializers are executed in order.
It is possible for static fields with variable initializers to be observed in their default value state.
Example: However, this is strongly discouraged as a matter of style. The example
class Test { static int a = b + 1; static int b = a + 1; static void Main() { Console.WriteLine($"a = {a}, b = {b}"); } }
exhibits this behavior. Despite the circular definitions of
a
andb
, the program is valid. It results in the outputa = 1, b = 2
because the static fields
a
andb
are initialized to0
(the default value forint
) before their initializers are executed. When the initializer fora
runs, the value ofb
is zero, and soa
is initialized to1
. When the initializer forb
runs, the value of a is already1
, and sob
is initialized to2
.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($"{B.Y} {A.X}"); } public static int F(string s) { Console.WriteLine(s); return 1; } } class A { public static int X = Test.F("Init A"); } class B { public static int Y = Test.F("Init B"); }
might produce either the output:
Init A Init B 1 1
or the output:
Init B Init A 1 1
because the execution of
X
’s initializer andY
’s initializer could occur in either order; they are only constrained to occur before the references to those fields. However, in the example:class Test { static void Main() { Console.WriteLine($"{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 henceB
’s static field initializers) shall run beforeA
’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 '(' parameter_list? ')'
| member_name type_parameter_list '(' 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
, andoverride
. - The declaration includes at most one of the following modifiers:
new
andoverride
. - If the declaration includes the
abstract
modifier, then the declaration does not include any of the following modifiers:static
,virtual
,sealed
, orextern
. - If the declaration includes the
private
modifier, then the declaration does not include any of the following modifiers:virtual
,override
, orabstract
. - If the declaration includes the
sealed
modifier, then the declaration also includes theoverride
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
, orextern
.
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 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 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 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 parameters. The return type is not part of a method’s signature, nor are the names of the parameters, the names of the type parameters, or the constraints. When a 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 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 parameter_list.
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 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 parameter_list.
A parameter with a ref
, out
or this
modifier cannot have a default_argument. An input parameter 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()
whereS
is a value type - an expression of the form
default(S)
whereS
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 parameter_list for
M
,i
is a requiredref
parameter,d
is a required value parameter,b
,s
,o
andt
are optional value parameters anda
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 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.
A method invocation (§12.8.10.2) creates a copy, specific to that invocation, of the parameters and local variables of the method, and the argument list of the invocation assigns values or variable references to the newly created parameters. Within the block of a method, parameters can be referenced by their identifiers in simple_name expressions (§12.8.4).
The following kinds of parameters exist:
- Value parameters (§15.6.2.2).
- Input parameters (§15.6.2.3.2).
- Output parameters (§15.6.2.3.4).
- Reference parameters (§15.6.2.3.3).
- Parameter arrays (§15.6.2.4).
Note: As described in §7.6, the
in
,out
, andref
modifiers are part of a method’s signature, but theparams
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.
For definite-assignment rules, see §9.2.5.
The corresponding argument in a method invocation shall be an expression that is implicitly convertible (§10.2) to the 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
Input, output, and reference parameters are by-reference parameters. 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 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 parameter. However, when the 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 an input parameter. The argument corresponding to an input 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. For definite-assignment rules, see §9.2.8.
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. For definite-assignment rules, see §9.2.6.
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
inMain
,x
representsi
andy
representsj
. Thus, the invocation has the effect of swapping the values ofi
andj
.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
inG
passes a reference tos
for botha
andb
. Thus, for that invocation, the namess
,a
, andb
all refer to the same storage location, and the three assignments all modify the instance fields
.end example
For a struct
type, within an instance method, instance accessor (§12.2.1), or instance constructor with a constructor initializer, the this
keyword behaves exactly as a reference parameter of the struct type (§12.8.14).
15.6.2.3.4 Output parameters
A parameter declared with an out
modifier is an output parameter. For definite-assignment rules, see §9.2.7.
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
andname
variables can be unassigned before they are passed toSplitPath
, 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 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[]
andstring[][]
can be used as the type of a parameter array, but the typestring[,]
can not. end example
Note: It is not possible to combine the
params
modifier with the modifiersin
,out
, orref
. 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 arrayarr
as a value parameter. The second invocation of F automatically creates a four-elementint[]
with the given element values and passes that array instance as a value parameter. Likewise, the third invocation ofF
creates a zero-elementint[]
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 toF(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 ofF
is applicable because an implicit conversion exists from the argument type to the parameter type (both are of typeobject[]
). Thus, overload resolution selects the normal form ofF
, and the argument is passed as a regular value parameter. In the second and third invocations, the normal form ofF
is not applicable because no implicit conversion exists from the argument type to the parameter type (typeobject
cannot be implicitly converted to typeobject[]
). However, the expanded form ofF
is applicable, so it is selected by overload resolution. As a result, a one-elementobject[]
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 anobject[]
).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
(§12.8.14).
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
, andA
, to select a specific methodM
from the set of methods declared in and inherited byC
. This is described in §12.8.10.2. - 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 ofM
with respect toR
is invoked.
- If
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 ofM
, then this is the most derived implementation ofM
with respect toR
. - Otherwise, if
R
contains an override ofM
, then this is the most derived implementation ofM
with respect toR
. - Otherwise, the most derived implementation of
M
with respect toR
is the same as the most derived implementation ofM
with respect to the direct base class ofR
.
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 methodF
and a virtual methodG
. The classB
introduces a new non-virtual methodF
, thus hiding the inheritedF
, and also overrides the inherited methodG
. The example produces the output:A.F B.F B.G B.G
Notice that the statement
a.G()
invokesB.G
, notA.G
. This is because the run-time type of the instance (which isB
), not the compile-time type of the instance (which isA
), 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
andD
classes contain two virtual methods with the same signature: The one introduced byA
and the one introduced byC
. The method introduced byC
hides the method inherited fromA
. Thus, the override declaration inD
overrides the method introduced byC
, and it is not possible forD
to override the method introduced byA
. 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 (§12.8.15).
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 inB
invokes the PrintFields method declared inA
. A base_access disables the virtual invocation mechanism and simply treats the base method as a non-virtual
method. Had the invocation inB
been written((A)this).PrintFields()
, it would recursively invoke thePrintFields
method declared inB
, not the one declared inA
, sincePrintFields
is virtual and the run-time type of((A)this)
isB
.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 inB
does not include anoverride
modifier and therefore does not override theF
method inA
. Rather, theF
method inB
hides the method inA
, 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 inB
hides the virtualF
method inherited fromA
. Since the newF
inB
has private access, its scope only includes the class body ofB
and does not extend toC
. Therefore, the declaration ofF
inC
is permitted to override theF
inherited fromA
.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: anF
method that has thesealed
modifier and aG
method that does not.B
’s use of thesealed
modifier preventsC
from further overridingF
.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. ThePaint
method is abstract because there is no meaningful default implementation. TheEllipse
andBox
classes are concreteShape
implementations. Because these classes are non-abstract, they are required to override thePaint
method and provide an actual implementation.end example
It is a compile-time error for a base_access (§12.8.15) 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, classB
overrides this method with an abstract method, and classC
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 theDllImport
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 be output parameters. 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
(§12.8.17.6).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 only be an input parameter if it has a value type
- It may only be a reference parameter if it has a value type or has 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 (§12.8.10.3), 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 thestring[]
, and theToInt32
method is available onstring
, 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. TheG
andH
methods are correct because all possible execution paths end in a return statement that specifies a return value. TheI
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, and the name of a customer. 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 or class’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
, orref
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 the accessor_bodys in its accessor_declarations shall be 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
(§12.8.14) 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 eitherprivate protected
,protected internal
,internal
,protected
, orprivate
. - If the property or indexer has a declared accessibility of
protected internal
, the accessibility declared by accessor_modifier may be eitherprivate protected
,protected private
,internal
,protected
, orprivate
. - If the property or indexer has a declared accessibility of
internal
orprotected
, the accessibility declared by accessor_modifier shall be eitherprivate protected
orprivate
. - If the property or indexer has a declared accessibility of
private protected
, the accessibility declared by accessor_modifier shall beprivate
. - If the property or indexer has a declared accessibility of
private
, no accessor_modifier may be used.
- If the property or indexer has a declared accessibility of
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 byref
, 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 –-
(§12.8.16, §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 publicCaption
property. The get accessor of the Caption property returns thestring
stored in the privatecaption
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 aprivate
field, and the set accessor modifies thatprivate
field and then performs any additional actions required to update fully the state of the object. Given theButton
class above, the following is an example of use of theCaption
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 inB
hides theP
property inA
with respect to both reading and writing. Thus, in the statementsB 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-onlyP
property inB
hides the write-onlyP
property inA
. Note, however, that a cast can be used to access the hiddenP
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 twoint
fields,x
andy
, to store its location. The location is publicly exposed both as anX
and aY
property and as aLocation
property of typePoint
. If, in a future version ofLabel
, it becomes more convenient to store the location as aPoint
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
andy
instead beenpublic readonly
fields, it would have been impossible to make such a change to theLabel
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
, andError
, that represent the standard input, output, and error devices, respectively. By exposing these members as properties, theConsole
class can delay their initialization until they are actually used. For example, upon first referencing theOut
property, as inConsole.Out.WriteLine("hello, world");
the underlying
TextWriter
for the output device is created. However, if the application makes no reference to theIn
andError
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_bodys. 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 (§12.8.16, §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 propertyB.Text
, even in contexts where only the set accessor is called. In contrast, the propertyB.Count
is not accessible to classM
, so the accessible propertyA.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, andZ
is an abstract read-write property. BecauseZ
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
, andZ
are overriding property declarations. Each property declaration exactly matches the accessibility modifiers, type, and name of the corresponding inherited property. The get accessor ofX
and the set accessor ofY
use the base keyword to access the inherited accessors. The declaration ofZ
overrides both abstract accessors—thus, there are no outstandingabstract
function members inB
, andB
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 twoButton
instances and attaches event handlers to theClick
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 theButton
class. As the example demonstrates, the field can be examined, modified, and used in delegate invocation expressions. TheOnClick
method in theButton
class “raises” theClick
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, theClick
member can only be used on the left-hand side of the+=
and–=
operators, as inb.Click += new EventHandler(...);
which appends a delegate to the invocation list of the
Click
event, andClick –= 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 (§12.8.18) 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 toEv
on the left-hand side of the+=
and–=
operators cause the add and remove accessors to be invoked. All other references toEv
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. TheAddEventHandler
method associates a delegate value with a key, theGetEventHandler
method returns the delegate currently associated with a key, and theRemoveEventHandler
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
(§12.8.14) 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' '[' parameter_list ']'
| type interface_type '.' 'this' '[' 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 parameter_list specifies the parameters of the indexer. The 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 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 an 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 indexer. The accessor specifies the executable statements associated with reading the indexer.
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
, orref
argument unless the indexer is ref-valued and therefore returns a reference (§9.7). end note
The 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 parameters. The element type and names of the 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 the accessor_bodys in its accessor_declarations shall be 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 correspondingbool[]
(since each value of the former occupies only one bit instead of the latter’s onebyte
), but it permits the same operations as abool[]
.The following
CountPrimes
class uses aBitArray
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 abool[]
.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 (§12.8.12.3).
- 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 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 parameter list as the indexer, plus an additional parameter namedvalue
. - 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
, whereP
is the property name. In an overriding indexer declaration, the inherited indexer is accessed using the syntaxbase[E]
, whereE
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 accessor_bodys.
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 astatic
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 typeT
orT?
and can return any type. - A unary
++
or--
operator shall take a single parameter of typeT
orT?
and shall return that same type or a type derived from it. - A unary
true
orfalse
operator shall take a single parameter of typeT
orT?
and shall return typebool
.
The signature of a unary operator consists of the operator token (+
, -
, !
, ~
, ++
, --
, true
, or false
) and the type of the single parameter. The return type is not part of a unary operator’s signature, nor is the name of the 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 (§12.8.16), 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 (§12.8.16).
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
orT?
, and can return any type. - A binary
<<
or>>
operator (§12.11) shall take two parameters, the first of which shall have typeT
or T? and the second of which shall have typeint
orint?
, and can return any type.
The signature of a binary operator consists of the operator token (+
, -
, *
, /
, %
, &
, |
, ^
, <<
, >>
, ==
, !=
, >
, <
, >=
, or <=
) and the types of the two parameters. The return type and the names of the 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
andT₀
are different types.Either
S₀
orT₀
is the instance type of the class or struct that contains the operator declaration.Neither
S₀
norT₀
is an interface_type.Excluding user-defined conversions, a conversion does not exist from
S
toT
or fromT
toS
.
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
andint
andstring
, respectively are considered unique types with no relationship. However, the third operator is an error becauseC<T>
is the base class ofD<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 fromC
toint
and fromint
toC
, but not fromint
tobool
. 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 forT
, 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 typeT
, all user-defined conversions (implicit or explicit) fromS
toT
are ignored. - If a pre-defined explicit conversion (§10.3) exists from type
S
to typeT
, any user-defined explicit conversions fromS
toT
are ignored. Furthermore:- If either
S
orT
is an interface type, user-defined implicit conversions fromS
toT
are ignored. - Otherwise, user-defined implicit conversions from
S
toT
are still considered.
- If either
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
tobyte
is implicit because it never throws exceptions or loses information, but the conversion frombyte
toDigit
is explicit sinceDigit
can only represent a subset of the possible values of abyte
.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 '(' 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 parameter_list of an instance constructor is subject to the same rules as the 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 parameter_list shall contain the this
modifier. The 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 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 (§12.8.17.2) 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 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 ofB
, 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 ofy
is 0 (the default value of anint
) because the assignment toy
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 exampleclass 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
andthis
). 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 toA.F
, and the execution ofB
’s static constructor is triggered by the call toB.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 forB.Y
, prior to classB
’s static constructor.Y
’s initializer causesA
’sstatic
constructor to be run because the value ofA.X
is referenced. The static constructor ofA
in turn proceeds to compute the value ofX
, and in doing so fetches the default value ofY
, which is zero.A.X
is thus initialized to 1. The process of runningA
’s static field initializers and static constructor then completes, returning to the calculation of the initial value ofY
, 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
’sFinalize
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 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
orIEnumerable
isobject
. - The yield type of an iterator that returns
IEnumerator<T>
orIEnumerable<T>
isT
.
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
andIEnumerator<T>
, whereT
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 thisyield return
statement. If theyield return
statement is within one or moretry
blocks, the associated finally blocks are not executed at this time. - The state of the enumerator object is changed to suspended.
- The
MoveNext
method returnstrue
to its caller, indicating that the iteration successfully advanced to the next value.
- The expression given in the statement is evaluated, implicitly converted to the yield type, and assigned to the
- When a
yield break
statement is encountered (§9.4.4.20):- If the
yield break
statement is within one or moretry
blocks, the associatedfinally
blocks are executed. - The state of the enumerator object is changed to after.
- The
MoveNext
method returnsfalse
to its caller, indicating that the iteration is complete.
- If the
- When the end of the iterator body is encountered:
- The state of the enumerator object is changed to after.
- The
MoveNext
method returnsfalse
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.
- Appropriate
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 ayield 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 theDispose
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
andIEnumerable<T>
, whereT
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
andIEnumerator<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 toGetEnumerator
. Subsequent invocations ofGetEnumerator
, 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 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 typeMyTaskMethodBuilder<T>
and the awaiter typeAwaiter<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 declaredinternal
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», namedbuilder
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 inStart()
or afterStart()
has returned to advance the state machine.
- The builder shall call
- After
Start()
returns, theasync
method invokesbuilder.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
andIsCompleted
is false, the state machine invokesbuilder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine)
.AwaitUnsafeOnCompleted()
should callawaiter.UnsafeOnCompleted(action)
with anAction
that callsstateMachine.MoveNext()
when the awaiter completes.
- Otherwise, the state machine invokes
builder.AwaitOnCompleted(ref awaiter, ref stateMachine)
.AwaitOnCompleted()
should callawaiter.OnCompleted(action)
with anAction
that callsstateMachine.MoveNext()
when the awaiter completes.
SetStateMachine(IAsyncStateMachine)
may be called by the compiler-generatedIAsyncStateMachine
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)
, thestateMachine
will callbuilder.SetStateMachine(stateMachine)
on the builder instance associated withstateMachine
.
- If the builder calls
Note: For both
SetResult(T result)
and«TaskType»<T> Task { get; }
, the parameter and argument respectively must be identity convertible toT
. 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.
ECMA C# draft specification