Mine

		static object MouseLeftButtonDownEvent = new object ();
		public event MouseEventHandler MouseLeftButtonDown {
			add {
				if (events[MouseLeftButtonDownEvent] == null)
					NativeMethods.SetFlag (this, WANT_BUTTON_PRESS);
				events.AddHandler (MouseLeftButtonDownEvent, value);
			}
			remove {
				bool before = (events[MouseLeftButtonDownEvent] != null);
				events.RemoveHandler (MouseLeftButtonDownEvent, value);
				bool after = (events[MouseLeftButtonDownEvent] == null);
				if (before && after)
					NativeMethods.ClearFlag (this, WANT_BUTTON_PRESS);
			}
		}