csharplang/proposals/nullable-reference-types-specification.md

Nullable Reference Types Specification

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

Syntax

Nullable reference types

Nullable reference types 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 a nullable_reference_type production is added:

reference_type
    : ...
    | nullable_reference_type
    ;
    
nullable_reference_type
    : non_nullable_reference_type '?'
    ;
    
non_nullable_reference_type
    : type
    ;

The non_nullable_reference_type in a nullable_reference_type must be a non-nullable reference type (class, interface, delegate or array), or a type parameter that is constrained to be a non-nullable reference type (through the class constraint, or a class other than object).

Nullable reference types 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 where the nullable annotation context is disabled.

Nullable class constraint

The class constraint has a nullable counterpart class?:

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

Null-forgiving expressions

The post-fix ! operator is called the null-forgiving operator.

primary_expression
    : ...
    | null_forgiving_expression
    ;
    
null_forgiving_expression
    : primary_expression '!'
    ;

The primary_expression must be of a reference type.

Nullable compiler directives

#nullable directives control the nullable annotation and warning contexts.

pp_directive
    : ...
    | pp_nullable
    ;
    
pp_nullable
    : whitespace? '#' whitespace? 'nullable' whitespace nullable_action pp_new_line
    ;
    
nullable_action
    : 'disable'
    | 'enable'
    | 'restore'
    | 'safeonly'
    ;

#pragma warning directives are expanded to allow changing the nullable warning context, and to allow individual warnings to be enabled on even when they're disabled by default:

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

warning_action
    : ...
    | 'enable'
    ;

Note that the new form of pragma_warning_body uses nullable_action, not warning_action.

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, safeonly or enabled.

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

The #nullable directive controls both the annotation and warning contexts within the source text, and take precedence over the project-level settings. The #pragma warning ... nullable directives control only the warning context, leaving the annotation context unchanged.

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 safeonly: Sets the nullable annotation context to enabled and the warning context to safeonly
• #pragma warning disable nullable: Sets the nullable warning context to disabled
• #pragma warning enable nullable: Sets the nullable warning context to enabled
• #pragma warning restore nullable: Restores the nullable warning context to project settings
• #pragma warning safeonly nullable: Sets the nullable warning context to safeonly

Nullability of types

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

A given type can have one of four nullabilities: Oblivious, nonnullable, nullable and unknown. Nonnullable and unknown types may yield warnings if a potential null value is assigned to them. Values of nullable and unknown types may yield warnings when dereferenced without proper null checking.

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? in a disabled annotation context is nullable (but a warning is yielded)
• A nullable reference type C? in an enabled annotation context is nullable

Type parameters additionally take their constraints into account:

• An unconstrained type parameter T is unknown
• A type parameter T where all constraints are nullable, unknown or the class? constraint is unknown
• A type parameter T where at least one constraint is oblivious or nonnullable or one of the struct or class constraints is
  • oblivious in a disabled annotation context
  • nonnullable in an enabled annotation context
• A nullable type parameter T? where at least one of T's constraints is oblivious or nonnullable or one of the struct or class constraints, is
  • nullable in a disabled annotation context (but a warning is yielded)
  • nullable in an enabled annotation context

For a type parameter T, T? is only allowed if T is known to be a value type or known to be a reference type.

Constraints

Nullable reference types can be used as generic constraints. Furthermore object is now valid as an explicit constraint. Absence of a constraint is now equivalent to an object? constraint (instead of object), but (unlike object before) object? is not prohibited as an explicit constraint.

class? is a new constraint denoting "possibly nullable reference type", whereas class denotes "nonnullable reference type".

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" or "maybe null". The null state is used to determine whether a warning should be given about null-unsafe conversions and dereferences.

Null tracking for variables

For certain expressions denoting variables 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.

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 is "maybe null". The null state of a default literal that is being converted to a type that is known not to be a nonnullable value type is "maybe null". 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, if the nullability of its type is nullable or unknown, the null state is "maybe null". Otherwise the null state is "not null".

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, if the nullability of the type of the expression is nullable or unknown, the null state is "maybe null". Otherwise the null state is "not null".

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, if the nullability of the type of the expression is nullable or unknown, the null state is "maybe null". Otherwise the null state is "not null".

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 "non-null" if T is known to be a nonnullable value type. Otherwise it has the null state "maybe null".

Null-conditional expressions

A null_conditional_expression has the null state "maybe null".

Cast expressions

Await expressions

More expressions

Expressions that propagate null state

(E), E++, E--, checked(E), unchecked(E), +E, -E, !E, ~E, ++E, --E, all have the same null state as E.

Other expressions

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

• this_access
• interpolated_string_expression
• object_creation_expression
• delegate_creation_expression
• anonymous_object_creation_expression
• array_creation_expression
• typeof_expression
• nameof_expression
• anonymous_method_expression
• null_forgiving_expression

The null-forgiving operator

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, and to limit warnings given on its use.

Warnings

Type inference

Attributes for special null behavior