C# Generics won't allow Delegate Type Constraints

Question

Is it possible to define a class in C# such that

class GenericCollection : SomeBaseCollection where T : Delegate

I couldn

Answer 1

Yes it's possible in C# 7.3, Constraints family increased to include Enum, Delegate and unmanaged types. You can write this code without a problem:

void M<D, E, T>(D d, E e, T* t) where D : Delegate where E : Enum where T : unmanaged
    {

    }

From Docs:

Beginning with C# 7.3, you can use the unmanaged constraint to specify that the type parameter must be a non-nullable unmanaged type. The unmanaged constraint enables you to write reusable routines to work with types that can be manipulated as blocks of memory

Useful links:

The future of C#, from Microsoft Build 2018

What's new in C# 7.3?

Answer 2

If you are willing to take a compile time dependency on an IL Weaver you can do this with Fody.

Using this addin to Fody https://github.com/Fody/ExtraConstraints

Your code can look like this

public class Sample
{
    public void MethodWithDelegateConstraint<[DelegateConstraint] T> ()
    {        
    }
    public void MethodWithEnumConstraint<[EnumConstraint] T>()
    {
    }
} 

And be compiled to this

public class Sample
{
    public void MethodWithDelegateConstraint<T>() where T: Delegate
    {
    }

    public void MethodWithEnumConstraint<T>() where T: struct, Enum
    {
    }
}