C# Generics won't allow Delegate Type Constraints

A number of classes are unavailable as generic contraints - Enum being another.

For delegates, the closest you can get is ": class", perhaps using reflection to check (for example, in the static constructor) that the T is a delegate:

static GenericCollection()
{
    if (!typeof(T).IsSubclassOf(typeof(Delegate)))
    {
        throw new InvalidOperationException(typeof(T).Name + " is not a delegate type");
    }
}

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?

Edit: Some proposed work-arounds are proposed in these articles:

http://jacobcarpenters.blogspot.com/2006/06/c-30-and-delegate-conversion.html

http://jacobcarpenters.blogspot.com/2006_11_01_archive.html

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From the C# 2.0 specification we can read (20.7, Constraints):

A class-type constraint must satisfy the following rules:

• The type must be a class type.
• The type must not be sealed.
• The type must not be one of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.
• The type must not be object. Because all types derive from object, such a constraint would have no effect if it were permitted.
• At most one constraint for a given type parameter can be a class type.

And sure enough VS2008 spits out an error:

error CS0702: Constraint cannot be special class 'System.Delegate'

For info and investigation on this issue read here.