Revit 开启事务组回滚事务

public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            Document document = commandData.Application.ActiveUIDocument.Document;
            UIApplication uIApplication = commandData.Application;

            TransactionGroup tg = new TransactionGroup(document, "Test TransactionGroup");
            tg.Start();
            try
            {
                using (Transaction transaction = new Transaction(document, "Test"))
                {
                    transaction.Start();

                    //CreatOpening(document, uIApplication.ActiveUIDocument.Selection);

                    EllipseDimenssion(commandData);
                    transaction.Commit();
                }
            }
            catch (Exception)
            {
                tg.RollBack();
            }
            tg.Assimilate();
            return Result.Succeeded;
        }