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Lambda discard parameters

proposals/csharp-9.0/lambda-discard-parameters.md

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Lambda discard parameters

[!INCLUDESpecletdisclaimer]

Champion issue: https://github.com/dotnet/csharplang/issues/111

Summary

Allow discards (_) to be used as parameters of lambdas and anonymous methods. For example:

  • lambdas: (_, _) => 0, (int _, int _) => 0
  • anonymous methods: delegate(int _, int _) { return 0; }

Motivation

Unused parameters do not need to be named. The intent of discards is clear, i.e. they are unused/discarded.

Detailed design

Method parameters - §15.6.2 In the parameter list of a lambda or anonymous method with more than one parameter named _, such parameters are discard parameters. Note: if a single parameter is named _ then it is a regular parameter for backwards compatibility reasons.

Discard parameters do not introduce any names to any scopes. Note this implies they do not cause any _ (underscore) names to be hidden.

Simple names (§12.8.4) If K is zero and the simple_name appears within a block and if the block's (or an enclosing block's) local variable declaration space (Declarations - §7.3) contains a local variable, parameter (with the exception of discard parameters) or constant with name I, then the simple_name refers to that local variable, parameter or constant and is classified as a variable or value.

Scopes - §7.7 With the exception of discard parameters, the scope of a parameter declared in a lambda_expression (§12.19) is the anonymous_function_body of that lambda_expression With the exception of discard parameters, the scope of a parameter declared in an anonymous_method_expression (§12.19) is the block of that anonymous_method_expression.