Cambie el evento cuando haga clic en Múltiples API de Revit

  public Result OnStartup( UIControlledApplication a )
  {
    selectionChangedWatcher = new SelectionChangedWatcher( a );

    selectionChangedWatcher.SelectionChanged
      += new EventHandler(
        selectionChangedWatcher_SelectionChanged);

    // . . .
Sore Stork