Co- and Contravariance bugs in .NET 4.0

Question

Some strange behavior with the C# 4.0 co- and contravariance support:

using System;

class Program {
  static void Foo(object x) { }
  static void Main() {
          


        

Answer 1

This solution was originally posted by cdhowie for my question: Delegate conversion breaks equality and unables to disconnect from event but is appears to solve the problem of covariance and contravariance in context of multicast delegates.

You first need a helper method:

public static class DelegateExtensions
{
    public static Delegate ConvertTo(this Delegate self, Type type)
    {
        if (type == null) { throw new ArgumentNullException("type"); }
        if (self == null) { return null; }

        if (self.GetType() == type)
            return self;

        return Delegate.Combine(
            self.GetInvocationList()
                .Select(i => Delegate.CreateDelegate(type, i.Target, i.Method))
                .ToArray());
    }

    public static T ConvertTo(this Delegate self)
    {
        return (T)(object)self.ConvertTo(typeof(T));
    }
}

When you have a delegate:

public delegate MyEventHandler(T arg);

You can use it while combining delegates simply by converting a delegate do desired type:

MyEventHandler handler = null;
handler += new MyEventHandler(c => Console.WriteLine(c)).ConvertTo>();
handler += new MyEventHandler