UIApplication uiapp = commandData.Application;
UIDocument uidoc = uiapp.ActiveUIDocument;
//Application app = uiapp.Application;
Document doc = uidoc.Document;
//Create Filtered Element Collector and Filter
FilteredElementCollector collector = new FilteredElementCollector(doc);
ElementCategoryFilter filter = new ElementCategoryFilter(BuiltInCategory.OST_DuctFitting);