[Transaction(TransactionMode.Manual)]
[Regeneration(RegenerationOption.Manual)]
public class cmd : IExternalCommand
{
    public Result Execute(ExternalCommandData cmdData, ref string msg, ElementSet elements)
    {
        UIDocument uiDoc = cmdData.Application.ActiveUIDocument;
        UIApplication uiApp = cmdData.Application;
        Document doc = uiDoc.Document;
        Selection selection = uiDoc.Selection;

Transaction ts = new Transaction(doc, "www");
        ts.Start();
        //找个风管类型
        FilteredElementCollector collector = new FilteredElementCollector(doc);
        collector.OfClass(typeof(Duct)).OfCategory(BuiltInCategory.OST_DuctCurves);
        Duct duct = null;
        foreach (Element el in collector)
        {
            duct = el as Duct;
            break;
        }
        //找个弯头类型
        FilteredElementCollector collectorFitting = new FilteredElementCollector(doc);
        collectorFitting.OfClass(typeof(FamilyInstance)).OfCategory(BuiltInCategory.OST_DuctFitting);
        FamilyInstance fitting = null;
        foreach (Element el in collectorFitting)
        {
            fitting = el as FamilyInstance;
            break;
        }
        //
        foreach (Parameter p in duct.DuctType.Parameters)
        {
            if (p.Definition.Name == "首选连接类型")
            {
                p.Set(1);//p.StorageType为Intger所以用此法修改
            }
            if (p.Definition.Name == "接头")
            {
                p.Set(fitting.Category.Id);//p.StorageType为ElementId
            }
        }

ts.Commit();

return Result.Succeeded;
    }
}