declaration-expressions.md

Declaration expressions

Support declaration assignments as expressions.

Motivation

Allow initialization at the point of declaration in more cases, simplifying code, and allowing var to be used.

SpecialType ReferenceType =>
    (var st = _type.SpecialType).IsValueType() ? SpecialType.None : st;

Allow declarations for ref arguments, similar to out var.

Convert(source, destination, ref List<Diagnostic> diagnostics = null);

Detailed design

Expressions are extended to include declaration assignment. Precedence is the same as assignment.

expression
    : non_assignment_expression
    | assignment
    | declaration_assignment_expression // new
    ;
declaration_assignment_expression // new
    : declaration_expression '=' local_variable_initializer
    ;
declaration_expression // C# 7.0
    | type variable_designation
    ;

The declaration assignment is of a single local.

The type of a declaration assignment expression is the type of the declaration. If the type is var, the inferred type is the type of the initializing expression.

The declaration assignment expression may be an l-value, for ref argument values in particular.

If the declaration assignment expression declares a value type, and the expression is an r-value, the value of the expression is a copy.

The declaration assignment expression may declare a ref local. There is an ambiguity when ref is used for a declaration expression in a ref argument. The local variable initializer determines whether the declaration is a ref local.

F(ref int x = IntFunc());    // int x;
F(ref int y = RefIntFunc()); // ref int y;

The scope of locals declared in declaration assignment expressions is the same the scope of corresponding declaration expressions from C#7.0.

It is a compile time error to refer to a local in text preceding the declaration expression.

Alternatives

No change. This feature is just syntactic shorthand after all.

More general sequence expressions: see #377.

To allow use of var in more cases, allow separate declaration and assignment of var locals, and infer the type from assignments from all code paths.

See also

See Basic Declaration Expression in #595.

See Deconstruction Declaration in the deconstruction feature.