C# Generics won't allow Delegate Type Constraints
Is it possible to define a class in C# such that
class GenericCollection<T> : SomeBaseCollection<T> where T : Delegate
I couldn't for the life of me accomplish this last night in .NET 3.5. I tried using
delegate, Delegate, Action<T> and Func<T, T>
It seems to me that this should be allowable in some way. I'm trying to implement my own EventQueue.
I ended up just doing this [primitive approximation mind you].
internal delegate void DWork();
class EventQueue {
private Queue<DWork> eventq;
}
But then I lose the ability to reuse the same definition for different types of functions.
Thoughts?
Solution 1:
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");
}
}
Solution 2:
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?
Solution 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
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.
Solution 4:
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
{
}
}