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Discards

Discards are variables which you can assign to, but cannot read from. They don't have names. Instead, they are represented by an '_' (underscore). In C#7.0, they can appear in the following contexts:

  • out variable declarations, such as bool found = TryGetValue(out var _) or bool found = TryGetValue(out _)
  • deconstruction assignments, such as (x, _) = deconstructable;
  • deconstruction declarations, such as (var x, var _) = deconstructable;
  • is patterns, such as x is int _
  • switch/case patterns, such as case int _:

The principal representation of discards is an _ (underscore) designation in a declaration expression. For example, int _ in an out variable declaration or var (_, _, x) in a deconstruction declaration.

The second representation of discards is using the expression _ as a short-hand for var _, when no variable named _ is in scope. It is allowed in out vars, deconstruction assignments and declarations, and plain assignments (_ = IgnoredReturn();). It is, however, not allowed in C#7.0 patterns. When a variable named _ does exist in scope, then the expression _ is simply a reference to that variable, as it did in earlier versions of C#.

###Grammar changes

declaration_expression
	: type variable_designation
	;

variable_designation
	: single_variable_designation
	| parenthesized_variable_designation
	| discard_designation // new
	;

discard_designation // new
	: '_'
	;

References C# Language Design Notes for Oct 25 and 26, 2016 C# Design Notes for Oct 18, 2016 Design proposal for discards Note that those notes used to refer to discards as "wildcards".