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

I came across a situation where I needed to deal with a Delegate internally but I wanted a generic constraint. Specifically, I wanted to add an event handler using reflection, but I wanted to use a generic argument for the delegate. The code below does NOT work, since "Handler" is a type variable, and the compiler won't cast Handler to Delegate:

public void AddHandler(Control c, string eventName, Handler d) {
  c.GetType().GetEvent(eventName).AddEventHandler(c, (Delegate) d);
}

However, you can pass a function that does the conversion for you. convert takes a Handler argument and returns a Delegate:

public void AddHandler(Control c, string eventName, 
                  Func convert, Handler d) {
      c.GetType().GetEvent(eventName).AddEventHandler(c, convert(d));
}

Now the compiler is happy. Calling the method is easy. For example, attaching to the KeyPress event on a Windows Forms control:

AddHandler(someControl, 
           "KeyPress", 
           (h) => (KeyEventHandler) h,
           SomeControl_KeyPress);

where SomeControl_KeyPress is the event target. The key is the converter lambda - it does no work, but it convinces the compiler you gave it a valid delegate.

(Begin 280Z28) @Justin: Why not use this?

public void AddHandler(Control c, string eventName, Handler d) { 
  c.GetType().GetEvent(eventName).AddEventHandler(c, d as Delegate); 
} 

(End 280Z28)