Emulating delegates with free generic type parameters in C#

Question

This is a hard question about language design, patterns and semantics. Please, don\'t down-vote just because you don\'t see the practical value.

Answer 1

This does actually not make sense under .Net's type system.

What you're describing is a type constructor – a "function" that takes one or more types and returns a concrete (parameterized, or closed) type.

The problem is that type constructors themselves are not types. You cannot have an object or variable of an open type; type constructors can only be used to generate concrete types.

In other words, there is no way to represent a reference to an open function within .Net's type system.

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The best you can do is to use reflection; a MethodInfo can describe an open generic method.
You can get a compile-time type-safe reference to an open MethodInfo by writing a generic method that takes an expression tree with a fake generic parameter:

public MethodInfo GetMethod(Expression method) {
    //Find the MethodInfo and remove all TPlaceholder parameters
}

GetMethod(() => SomeMethod(...));

The TPlaceholder parameter is necessary in case you want to reference an open generic method with a constraint on that parameter; you can pick a placeholder type that meets the constraint.