private static void InvalidateEventHandler(WeakEventHandler <EventHandler <EventArgs>, EventArgs> weakEventHandler)
{
// use reflection to set WeakReference.Target to null (this simulates the target object going out of scope)
FieldInfo callbackField = weakEventHandler.GetType().GetField("_target", BindingFlags.Instance | BindingFlags.NonPublic);
Assert.IsNotNull(callbackField);
WeakReference reference = callbackField.GetValue(weakEventHandler) as WeakReference;
Assert.IsNotNull(reference);
reference.Target = null;
}