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While investigating this question I got curious about how the new covariance/contravariance features in C# 4.0 will affect it. In Beta 1, C# seems to disagree with the CLR. Back in C# 3.0, if you had: public event EventHandler<ClickEventArgs> Click; ... and then elsewhere you had: button.Click += new EventHandler<EventArgs>(button_Click); ... the compiler would barf because they're incompatible delegate types. But in C# 4.0, it compiles fine, because in CLR 4.0 the type parameter is now marked as in , so it is contravariant, and so the compiler assumes the multicast delegate += will work. Here's my test: public class ClickEventArgs : EventArgs { } public class Button { public event EventHandler<ClickEventArgs> Click; public void MouseDown() { Click(this, new ClickEventArgs()); } } class Program { static void Main(string[] args) { Button button = new Button(); button.Click += new EventHandler<ClickEventArgs>(button_Click); button.Click += new EventHandler<EventArgs>(button_Click); button.MouseDown(); } static void button_Click(object s, EventArgs e) { Console.WriteLine("Button was clicked"); } } But although it compiles, it doesn't work at runtime ( ArgumentException : Delegates must be of the same type). It's okay if you only add either one of the two delegate types. But the combination of two different types in a multicast causes the exception when the second one is added. I guess this is a bug in the CLR in beta 1 (the compiler's behaviour looks hopefully right). Update for Release Candidate: The above code no longer compiles. It must be that the contravariance of TEventAr

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