Question

I have a Windows Phone page (MainPage), and in the codebehind, I want to dynamically add an event handler – for example to the MouseEnter event. The trick is, I want this event handler to have the event argument as an object:

    private void MyEventHandler(object sender, object args)
    {

    }

(The reason for this is that this is a simplified example)

This works:

this.MouseEnter += MyEventHandler;

But this throws an ArgumentException:

var handlerType = Type.GetType("System.EventHandler`1").
                  MakeGenericType(typeof(EventArgs));
this.GetType().GetEvent("Hold").AddEventHandler(
                  this, Delegate.CreateDelegate(handlerType, this, "MyEventHandler"));

And the message of the exception is:
“Cannot bind to the target method because its signature or security transparency is not compatible with that of the delegate type.” If I change the signature of the event handler, all works, but I want to keep it as “object” or “dynamic”.

How can I add the event handler with the AddEventHandler call?

Any help is highly appreciated.

Answer 1

Keyword is delegate covariance:

class Program
{
  static void Main(string[] args)
  {
    var t = new Test();

    var e = t.GetType().GetEvent("TestEvent");
    var te = Delegate.CreateDelegate(e.EventHandlerType, new EventHandler(MyMethod).Method);

    e.AddEventHandler(t, te);

    t.RaiseTest();
  }

  static void MyMethod(object sender, object args)
  {

  }
}

public class Test
{
  public class MyEventArgs : EventArgs { }

  public void RaiseTest()
  {
    var e = TestEvent;
    if (e != null)
      e(this, new MyEventArgs());
  }

  public event EventHandler<MyEventArgs> TestEvent;
}