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Nullable Reference Types Specification

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Note

This article is a feature specification. The specification serves as the design document for the feature. It includes proposed specification changes, along with information needed during the design and development of the feature. These articles are published until the proposed spec changes are finalized and incorporated in the current ECMA specification.

There may be some discrepancies between the feature specification and the completed implementation. Those differences are captured in the pertinent language design meeting (LDM) notes.

You can learn more about the process for adopting feature speclets into the C# language standard in the article on the specifications.

This is a work in progress - several parts are missing or incomplete.

This feature adds two new kinds of nullable types (nullable reference types and nullable generic types) to the existing nullable value types, and introduces a static flow analysis for purpose of null-safety.

Syntax

Nullable reference types and nullable type parameters

Nullable reference types and nullable type parameters have the same syntax T? as the short form of nullable value types, but do not have a corresponding long form.

For the purposes of the specification, the current nullable_type production is renamed to nullable_value_type, and nullable_reference_type and nullable_type_parameter productions are added:

type
    : value_type
    | reference_type
    | nullable_type_parameter
    | type_parameter
    | type_unsafe
    ;

reference_type
    : ...
    | nullable_reference_type
    ;

nullable_reference_type
    : non_nullable_reference_type '?'
    ;

non_nullable_reference_type
    : reference_type
    ;

nullable_type_parameter
    : non_nullable_non_value_type_parameter '?'
    ;

non_nullable_non_value_type_parameter
    : type_parameter
    ;

The non_nullable_reference_type in a nullable_reference_type must be a nonnullable reference type (class, interface, delegate or array).

The non_nullable_non_value_type_parameter in nullable_type_parameter must be a type parameter that isn't constrained to be a value type.

Nullable reference types and nullable type parameters cannot occur in the following positions:

  • as a base class or interface
  • as the receiver of a member_access
  • as the type in an object_creation_expression
  • as the delegate_type in a delegate_creation_expression
  • as the type in an is_expression, a catch_clause or a type_pattern
  • as the interface in a fully qualified interface member name

A warning is given on a nullable_reference_type and nullable_type_parameter in a disabled nullable annotation context.

class and class? constraint

The class constraint has a nullable counterpart class?:

primary_constraint
    : ...
    | 'class' '?'
    ;

A type parameter constrained with class (in an enabled annotation context) must be instantiated with a nonnullable reference type.

A type parameter constrained with class? (or class in a disabled annotation context) may either be instantiated with a nullable or nonnullable reference type.

A warning is given on a class? constraint in a disabled annotation context.

The behavior of ? annotations on type parameters that aren't constrained to be either a struct or class is covered in Unconstrained type parameter annotations.

notnull constraint

A type parameter constrained with notnull may not be a nullable type (nullable value type, nullable reference type or nullable type parameter).

primary_constraint
    : ...
    | 'notnull'
    ;

default constraint

The default constraint can be used on a method override or explicit implementation to disambiguate T? meaning "nullable type parameter" from "nullable value type" (Nullable<T>). Lacking the default constraint a T? syntax in an override or explicit implementation will be interpreted as Nullable<T>

See Unconstrained type parameter annotations.

The null-forgiving operator

The post-fix ! operator is called the null-forgiving operator. It can be applied on a primary_expression or within a null_conditional_expression:

primary_expression
    : ...
    | null_forgiving_expression
    ;

null_forgiving_expression
    : primary_expression '!'
    ;

null_conditional_expression
    : primary_expression null_conditional_operations_no_suppression suppression?
    ;

null_conditional_operations_no_suppression
    : null_conditional_operations? '?' '.' identifier type_argument_list?
    | null_conditional_operations? '?' '[' argument_list ']'
    | null_conditional_operations '.' identifier type_argument_list?
    | null_conditional_operations '[' argument_list ']'
    | null_conditional_operations '(' argument_list? ')'
    ;

null_conditional_operations
    : null_conditional_operations_no_suppression suppression?
    ;

suppression
    : '!'
    ;

For example:

var v = expr!;
expr!.M();
_ = a?.b!.c;

The postfix ! operator has no runtime effect - it evaluates to the result of the underlying expression. Its only role is to change the null state of the expression to "not null", and to limit warnings given on its use.

Nullable compiler directives

#nullable directives control the nullable annotation and warning contexts.

pp_directive
    : ...
    | pp_nullable
    ;

pp_nullable
    : whitespace? '#' whitespace? 'nullable' whitespace nullable_action (whitespace nullable_target)? pp_new_line
    ;

nullable_action
    : 'disable'
    | 'enable'
    | 'restore'
    ;

nullable_target
    : 'warnings'
    | 'annotations'
    ;

#pragma warning directives are expanded to allow changing the nullable warning context:

pragma_warning_body
    : ...
    | 'warning' whitespace warning_action whitespace 'nullable'
    ;

For example:

#pragma warning disable nullable

Nullable contexts

Every line of source code has a nullable annotation context and a nullable warning context. These control whether nullable annotations have effect, and whether nullability warnings are given. The annotation context of a given line is either disabled or enabled. The warning context of a given line is either disabled or enabled.

Both contexts can be specified at the project level (outside of C# source code), or anywhere within a source file via #nullable pre-processor directives. If no project level settings are provided the default is for both contexts to be disabled.

The #nullable directive controls the annotation and warning contexts within the source text, and take precedence over the project-level settings.

A directive sets the context(s) it controls for subsequent lines of code, until another directive overrides it, or until the end of the source file.

The effect of the directives is as follows:

  • #nullable disable: Sets the nullable annotation and warning contexts to disabled
  • #nullable enable: Sets the nullable annotation and warning contexts to enabled
  • #nullable restore: Restores the nullable annotation and warning contexts to project settings
  • #nullable disable annotations: Sets the nullable annotation context to disabled
  • #nullable enable annotations: Sets the nullable annotation context to enabled
  • #nullable restore annotations: Restores the nullable annotation context to project settings
  • #nullable disable warnings: Sets the nullable warning context to disabled
  • #nullable enable warnings: Sets the nullable warning context to enabled
  • #nullable restore warnings: Restores the nullable warning context to project settings

Nullability of types

A given type can have one of three nullabilities: oblivious, nonnullable, and nullable.

Nonnullable types may cause warnings if a potential null value is assigned to them. Oblivious and nullable types, however, are "null-assignable" and can have null values assigned to them without warnings.

Values of oblivious and nonnullable types can be dereferenced or assigned without warnings. Values of nullable types, however, are "null-yielding" and may cause warnings when dereferenced or assigned without proper null checking.

The default null state of a null-yielding type is "maybe null" or "maybe default". The default null state of a non-null-yielding type is "not null".

The kind of type and the nullable annotation context it occurs in determine its nullability:

  • A nonnullable value type S is always nonnullable
  • A nullable value type S? is always nullable
  • An unannotated reference type C in a disabled annotation context is oblivious
  • An unannotated reference type C in an enabled annotation context is nonnullable
  • A nullable reference type C? is nullable (but a warning may be yielded in a disabled annotation context)

Type parameters additionally take their constraints into account:

  • A type parameter T where all constraints (if any) are either nullable types or the class? constraint is nullable
  • A type parameter T where at least one constraint is either oblivious or nonnullable or one of the struct or class or notnull constraints is
    • oblivious in a disabled annotation context
    • nonnullable in an enabled annotation context
  • A nullable type parameter T? is nullable, but a warning is yielded in a disabled annotation context if T isn't a value type

Oblivious vs nonnullable

A type is deemed to occur in a given annotation context when the last token of the type is within that context.

Whether a given reference type C in source code is interpreted as oblivious or nonnullable depends on the annotation context of that source code. But once established, it is considered part of that type, and "travels with it" e.g. during substitution of generic type arguments. It is as if there is an annotation like ? on the type, but invisible.

Constraints

Nullable reference types can be used as generic constraints.

class? is a new constraint denoting "possibly nullable reference type", whereas class in an enabled annotation context denotes "nonnullable reference type".

default is a new constraint denoting a type parameter that isn't known to be a reference or value type. It can only be used on overridden and explicitly implemented methods. With this constraint, T? means a nullable type parameter, as opposed to being a shorthand for Nullable<T>.

notnull is a new constraint denoting a type parameter that is nonnullable.

The nullability of a type argument or of a constraint does not impact whether the type satisfies the constraint, except where that is already the case today (nullable value types do not satisfy the struct constraint). However, if the type argument does not satisfy the nullability requirements of the constraint, a warning may be given.

Null state and null tracking

Every expression in a given source location has a null state, which indicated whether it is believed to potentially evaluate to null. The null state is either "not null", "maybe null", or "maybe default". The null state is used to determine whether a warning should be given about null-unsafe conversions and dereferences.

The distinction between "maybe null" and "maybe default" is subtle and applies to type parameters. The distinction is that a type parameter T which has the state "maybe null" means the value is in the domain of legal values for T however that legal value may include null. Where as a "maybe default" means that the value may be outside the legal domain of values for T.

Example:

// The value `t` here has the state "maybe null". It's possible for `T` to be instantiated
// with `string?` in which case `null` would be within the domain of legal values here. The 
// assumption though is the value provided here is within the legal values of `T`. Hence 
// if `T` is `string` then `null` will not be a value, just as we assume that `null` is not
// provided for a normal `string` parameter
void M<T>(T t)
{
    // There is no guarantee that default(T) is within the legal values for T hence the 
    // state *must* be "maybe-default" and hence `local` must be `T?`
    T? local = default(T);
}

Null tracking for variables

For certain expressions denoting variables, fields or properties, the null state is tracked between occurrences, based on assignments to them, tests performed on them and the control flow between them. This is similar to how definite assignment is tracked for variables. The tracked expressions are the ones of the following form:

tracked_expression
    : simple_name
    | this
    | base
    | tracked_expression '.' identifier
    ;

Where the identifiers denote fields or properties.

The null state for tracked variables is "not null" in unreachable code. This follows other decisions around unreachable code like considering all locals to be definitely assigned.

Describe null state transitions similar to definite assignment

Null state for expressions

The null state of an expression is derived from its form and type, and from the null state of variables involved in it.

Literals

The null state of a null literal depends on the target type of the expression. If the target type is a type parameter constrained to a reference type then it's "maybe default". Otherwise it is "maybe null".

The null state of a default literal depends on the target type of the default literal. A default literal with target type T has the same null state as the default(T) expression.

The null state of any other literal is "not null".

Simple names

If a simple_name is not classified as a value, its null state is "not null". Otherwise it is a tracked expression, and its null state is its tracked null state at this source location.

Member access

If a member_access is not classified as a value, its null state is "not null". Otherwise, if it is a tracked expression, its null state is its tracked null state at this source location. Otherwise its null state is the default null state of its type.

var person = new Person();

// The receiver is a tracked expression hence the member_access of the property 
// is tracked as well 
if (person.FirstName is not null)
{
    Use(person.FirstName);
}

// The return of an invocation is not a tracked expression hence the member_access
// of the return is also not tracked
if (GetAnonymous().FirstName is not null)
{
    // Warning: Cannot convert null literal to non-nullable reference type.
    Use(GetAnonymous().FirstName);
}

void Use(string s) 
{ 
    // ...
}

public class Person
{
    public string? FirstName { get; set; }
    public string? LastName { get; set; }

    private static Person s_anonymous = new Person();
    public static Person GetAnonymous() => s_anonymous;
}

Invocation expressions

If an invocation_expression invokes a member that is declared with one or more attributes for special null behavior, the null state is determined by those attributes. Otherwise the null state of the expression is the default null state of its type.

The null state of an invocation_expression is not tracked by the compiler.


// The result of an invocation_expression is not tracked
if (GetText() is not null)
{
    // Warning: Converting null literal or possible null value to non-nullable type.
    string s = GetText();
    // Warning: Dereference of a possibly null reference.
    Use(s);
}

// Nullable friendly pattern
if (GetText() is string s)
{
    Use(s);
}

string? GetText() => ... 
Use(string s) {  }

Element access

If an element_access invokes an indexer that is declared with one or more attributes for special null behavior, the null state is determined by those attributes. Otherwise the null state of the expression is the default null state of its type.

object?[] array = ...;
if (array[0] != null)
{
    // Warning: Converting null literal or possible null value to non-nullable type.
    object o = array[0];
    // Warning: Dereference of a possibly null reference.
    Console.WriteLine(o.ToString());
}

// Nullable friendly pattern
if (array[0] is {} o)
{
    Console.WriteLine(o.ToString());
}

Base access

If B denotes the base type of the enclosing type, base.I has the same null state as ((B)this).I and base[E] has the same null state as ((B)this)[E].

Default expressions

default(T) has the null state based on the properties of the type T:

  • If the type is a nonnullable type then it has the null state "not null"
  • Else if the type is a type parameter then it has the null state "maybe default"
  • Else it has the null state "maybe null"

Null-conditional expressions ?.

A null_conditional_expression has the null state based on the expression type. Note that this refers to the type of the null_conditional_expression, not the original type of the member being invoked:

  • If the type is a nullable value type then it has the null state "maybe null"
  • Else if the type is a nullable type parameter then it has the null state "maybe default"
  • Else it has the null state "maybe null"

Cast expressions

If a cast expression (T)E invokes a user-defined conversion, then the null state of the expression is the default null state for the type of the user-defined conversion. Otherwise:

  • If T is a nonnullable value type then T has the null state "not null"
  • Else if T is a nullable value type then T has the null state "maybe null"
  • Else if T is a nullable type in the form U? where U is a type parameter then T has the null state "maybe default"
  • Else if T is a nullable type, and E has null state "maybe null" or "maybe default", then T has the null state "maybe null"
  • Else if T is a type parameter, and E has null state "maybe null" or "maybe default", then T has the null state "maybe default"
  • Else T has the same null state as E

Unary and binary operators

If a unary or binary operator invokes an user-defined operator then the null state of the expression is the default null state for the type of the user-defined operator. Otherwise it is the null state of the expression.

Something special to do for binary + over strings and delegates?

Await expressions

The null state of await E is the default null state of its type.

The as operator

The null state of an E as T expression depends first on properties of the type T. If the type of T is nonnullable then the null state is "not null". Otherwise the null state depends on the conversion from the type of E to type T:

  • If the conversion is an identity, boxing, implicit reference, or implicit nullable conversion, then the null state is the null state of E
  • Else if T is a type parameter then it has the null state "maybe default"
  • Else it has the null state "maybe null"

The null-coalescing operator

The null state of E1 ?? E2 is the null state of E2

The conditional operator

The null state of E1 ? E2 : E3 is based on the null state of E2 and E3:

  • If both are "not null" then the null state is "not null"
  • Else if either is "maybe default" then the null state is "maybe default"
  • Else the null state is "not null"

Query expressions

The null state of a query expression is the default null state of its type.

Additional work needed here

Assignment operators

E1 = E2 and E1 op= E2 have the same null state as E2 after any implicit conversions have been applied.

Expressions that propagate null state

(E), checked(E) and unchecked(E) all have the same null state as E.

Expressions that are never null

The null state of the following expression forms is always "not null":

  • this access
  • interpolated strings
  • new expressions (object, delegate, anonymous object and array creation expressions)
  • typeof expressions
  • nameof expressions
  • anonymous functions (anonymous methods and lambda expressions)
  • null-forgiving expressions
  • is expressions

Nested functions

Nested functions (lambdas and local functions) are treated like methods, except in regards to their captured variables. The initial state of a captured variable inside a lambda or local function is the intersection of the nullable state of the variable at all the "uses" of that nested function or lambda. A use of a local function is either a call to that function, or where it is converted to a delegate. A use of a lambda is the point at which it is defined in source.

Type inference

nullable implicitly typed local variables

var infers an annotated type for reference types, and type parameters that aren't constrained to be a value type. For instance:

  • in var s = ""; the var is inferred as string?.
  • in var t = new T(); with an unconstrained T the var is inferred as T?.

Generic type inference

Generic type inference is enhanced to help decide whether inferred reference types should be nullable or not. This is a best effort. It may yield warnings regarding nullability constraints, and may lead to nullable warnings when the inferred types of the selected overload are applied to the arguments.

The first phase

Nullable reference types flow into the bounds from the initial expressions, as described below. In addition, two new kinds of bounds, namely null and default are introduced. Their purpose is to carry through occurrences of null or default in the input expressions, which may cause an inferred type to be nullable, even when it otherwise wouldn't.

The determination of what bounds to add in the first phase are enhanced as follows:

If an argument Ei has a reference type, the type U used for inference depends on the null state of Ei as well as its declared type:

  • If the declared type is a nonnullable reference type U0 or a nullable reference type U0? then
    • if the null state of Ei is "not null" then U is U0
    • if the null state of Ei is "maybe null" then U is U0?
  • Otherwise if Ei has a declared type, U is that type
  • Otherwise if Ei is null then U is the special bound null
  • Otherwise if Ei is default then U is the special bound default
  • Otherwise no inference is made.

Exact, upper-bound and lower-bound inferences

In inferences from the type U to the type V, if V is a nullable reference type V0?, then V0 is used instead of V in the following clauses.

  • If V is one of the unfixed type variables, U is added as an exact, upper or lower bound as before
  • Otherwise, if U is null or default, no inference is made
  • Otherwise, if U is a nullable reference type U0?, then U0 is used instead of U in the subsequent clauses.

The essence is that nullability that pertains directly to one of the unfixed type variables is preserved into its bounds. For the inferences that recurse further into the source and target types, on the other hand, nullability is ignored. It may or may not match, but if it doesn't, a warning will be issued later if the overload is chosen and applied.

Fixing

The spec currently does not do a good job of describing what happens when multiple bounds are identity convertible to each other, but are different. This may happen between object and dynamic, between tuple types that differ only in element names, between types constructed thereof and now also between C and C? for reference types.

In addition we need to propagate "nullness" from the input expressions to the result type.

To handle these we add more phases to fixing, which is now:

  1. Gather all the types in all the bounds as candidates, removing ? from all that are nullable reference types
  2. Eliminate candidates based on requirements of exact, lower and upper bounds (keeping null and default bounds)
  3. Eliminate candidates that do not have an implicit conversion to all the other candidates
  4. If the remaining candidates do not all have identity conversions to one another, then type inference fails
  5. Merge the remaining candidates as described below
  6. If the resulting candidate is a reference type and all of the exact bounds or any of the lower bounds are nullable reference types, null or default, then ? is added to the resulting candidate, making it a nullable reference type.

Merging is described between two candidate types. It is transitive and commutative, so the candidates can be merged in any order with the same ultimate result. It is undefined if the two candidate types are not identity convertible to each other.

The Merge function takes two candidate types and a direction (+ or -):

  • Merge(T, T, d) = T
  • Merge(S, T?, +) = Merge(S?, T, +) = Merge(S, T, +)?
  • Merge(S, T?, -) = Merge(S?, T, -) = Merge(S, T, -)
  • Merge(C<S1,...,Sn>, C<T1,...,Tn>, +) = C<Merge(S1, T1, d1),...,Merge(Sn, Tn, dn)>, where
    • di = + if the i'th type parameter of C<...> is covariant
    • di = - if the i'th type parameter of C<...> is contra- or invariant
  • Merge(C<S1,...,Sn>, C<T1,...,Tn>, -) = C<Merge(S1, T1, d1),...,Merge(Sn, Tn, dn)>, where
    • di = - if the i'th type parameter of C<...> is covariant
    • di = + if the i'th type parameter of C<...> is contra- or invariant
  • Merge((S1 s1,..., Sn sn), (T1 t1,..., Tn tn), d) = (Merge(S1, T1, d)n1,...,Merge(Sn, Tn, d) nn), where
    • ni is absent if si and ti differ, or if both are absent
    • ni is si if si and ti are the same
  • Merge(object, dynamic) = Merge(dynamic, object) = dynamic

Warnings

Potential null assignment

Potential null dereference

Constraint nullability mismatch

Nullable types in disabled annotation context

Override and implementation nullability mismatch

Attributes for special null behavior