InfoPath window change event

private void ThisAddIn_Startup(object sender, System.EventArgs e)
{
			InfoPath._Application3 iApplication = (InfoPath._Application3)this.Application;
			events = (InfoPath.ApplicationEvents)iApplication.Events;
			events.WindowActivate += new InfoPath._ApplicationEvents_WindowActivateEventHandler(events_WindowActivate);
		}

		void events_WindowActivate(InfoPath._XDocument pDocument, InfoPath.Window pWindow)
		{
			throw new NotImplementedException();
		}