Covariance and Contravariance with Func in generics

Question

I need more information about variance in generics and delegates. The following code snippet does not compile:

Error CS1961 Invalid variance: The ty

Answer 1

remove the in and out key words:

public interface Test
{
    TOut F (Func transform);
}

you can read about the meaning of them here:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/in-generic-modifier

A type can be declared contravariant in a generic interface or delegate if it is used only as a type of method arguments and not used as a method return type

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/out-generic-modifier

The type parameter is used only as a return type of interface methods and not used as a type of method arguments.