C# Language Specification

Anonymous function signatures

The optional anonymous-function-signature of an anonymous function defines the names and optionally the types of the formal parameters for the anonymous function. The scope of the parameters of the anonymous function is the anonymous-function-body. (§3.7) Together with the parameter list (if given) the anonymous-method-body constitutes a declaration space (§3.3). It is thus a compile-time error for the name of a parameter of the anonymous function to match the name of a local variable, local constant or parameter whose scope includes the anonymous-method-expression or lambda-expression.

If an anonymous function has an explicit-anonymous-function-signature, then the set of compatible delegate types and expression tree types is restricted to those that have the same parameter types and modifiers in the same order. In contrast to method group conversions (§6.6), contra-variance of anonymous function parameter types is not supported. If an anonymous function does not have an anonymous-function-signature, then the set of compatible delegate types and expression tree types is restricted to those that have no out parameters.

Note that an anonymous-function-signature cannot include attributes or a parameter array. Nevertheless, an anonymous-function-signature may be compatible with a delegate type whose parameter list contains a parameter array.

Note also that conversion to an expression tree type, even if compatible, may still fail at compile-time (§4.6).

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