Nullable Reference Types Specification

***This is a work in progress - several parts are missing or incomplete. An updated version of this document can be found in the C# 9 folder. ***

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' '?'
    ;

The null-forgiving operator

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.

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.

nullable implicitly typed local variables

var infers an annotated type for reference types. For instance, in var s = ""; the var is inferred as string?.

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'
    ;

#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 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 four nullabilities: Oblivious, nonnullable, nullable and unknown.

Nonnullable and unknown 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.

Oblivious and nonnullable types can be dereferenced or assigned without warnings. Values of nullable and unknown 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". 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 null-yielding types (nullable and unknown) or the class? constraint is unknown
A type parameter T where at least one constraint is either 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.

Nested functions

Nested functions (lambdas and local functions) are treated like methods, except in regards to their captured variables. The default 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. A use of a function is either a call to that function, or where it is converted to a delegate.

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. 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 its null state is the default null state of its type.

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.

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.

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

If a cast expression (T)E invokes a user-defined conversion, then the null state of the expression is the default null state for its type. Otherwise, if T is null-yielding (nullable or unknown) then the null state is "maybe null". Otherwise the null state is the same as the null state of E.

Await expressions

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

The `as` operator

An as expression has the null state "maybe null".

The null-coalescing operator

E1 ?? E2 has the same null state as E2

The conditional operator

The null state of E1 ? E2 : E3 is "not null" if the null state of both E2 and E3 are "not null". Otherwise it is "maybe null".

Query expressions

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

Assignment operators

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

Unary and binary operators

If a unary or binary operator invokes an user-defined operator 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.

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

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

Type inference

Type inference for `var`

The type inferred for local variables declared with var is informed by the null state of the initializing expression.

var x = E;

If the type of E is a nullable reference type C? and the null state of E is "not null" then the type inferred for x is C. Otherwise, the inferred type is the type of E.

The nullability of the type inferred for x is determined as described above, based on the annotation context of the var, just as if the type had been given explicitly in that position.

Type inference for `var?`

The type inferred for local variables declared with var? is independent of the null state of the initializing expression.

var? x = E;

If the type T of E is a nullable value type or a nullable reference type then the type inferred for x is T. Otherwise, if T is a nonnullable value type S the type inferred is S?. Otherwise, if T is a nonnullable reference type C the type inferred is C?. Otherwise, the declaration is illegal.

The nullability of the type inferred for x is always nullable.

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, and does not in and of itself yield warnings, but may lead to nullable warnings when the inferred types of the selected overload are applied to the arguments.

The type inference does not rely on the annotation context of incoming types. Instead a type is inferred which acquires its own annotation context from where it "would have been" if it had been expressed explicitly. This underscores the role of type inference as a convenience for what you could have written yourself.

More precisely, the annotation context for an inferred type argument is the context of the token that would have been followed by the <...> type parameter list, had there been one; i.e. the name of the generic method being called. For query expressions that translate to such calls, the context is taken from the initial contextual keyword of the query clause from which the call is generated.

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. This works even for nullable value types, which are enhanced to pick up "nullness" in the inference process.

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:

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

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Nullable Reference Types Specification

Syntax