Out Variable Declarations

The out variable declaration feature enables a variable to be declared at the location that it is being passed as an out argument.

argument_value
    : 'out' type identifier
    | ...
    ;

A variable declared this way is called an out variable. You may use the contextual keyword var for the variable's type. The scope will be the same as for a pattern-variable introduced via pattern-matching.

According to Language Specification (section 7.6.7 Element access) the argument-list of an element-access (indexing expression) does not contain ref or out arguments. However, they are permitted by the compiler for various scenarios, for example indexers declared in metadata that accept out.

Within the scope of a local variable introduced by a local-variable-declaration, it is a compile-time error to refer to that local variable in a textual position that precedes its declaration.

It is also an error to reference implicitly-typed (§8.5.1) out variable in the same argument list that immediately contains its declaration.

Overload resolution is modified as follows:

We add a new conversion:

There is a conversion from expression from an implicitly-typed out variable declaration to every type.

Also

The type of an explicitly-typed out variable argument is the declared type.

and

An implicitly-typed out variable argument has no type.

Neither conversion from expression is better when the argument is an implicitly-typed out variable declaration. (this needs to be woven into the form of the specification)

The type of an implicitly-typed out variable is the type of the corresponding parameter in the signature of the method.

The new syntax node DeclarationExpressionSyntax is added to represent the declaration in an out var argument.

Discussion

There is a discussion thread for this feature at https://github.com/dotnet/roslyn/issues/6183.

Open issues and TODOs:

Tracked at https://github.com/dotnet/roslyn/issues/11566.