internal override DragDropEffects StartDrag (IntPtr handle, object data, DragDropEffects allowed_effects) { Hwnd hwnd = Hwnd.ObjectFromHandle (handle); if (hwnd == null) throw new ArgumentException ("Attempt to begin drag from invalid window handle (" + handle.ToInt32 () + ")."); return Dnd.StartDrag (hwnd.client_window, data, allowed_effects); }