Transaction stitching SQL statement operations on the database. Exception rollback

        PublicList<userbaseinfomodel>Getcitybyexamid (Guid examid) {varCur_user = This.            Getcurrentuser (); using(SqlConnection conn =NewSqlConnection (connectionString)) {Conn.                Open ();                SqlCommand cmd; using(SqlTransaction trans =Conn. BeginTransaction ()) {Try                    {                        stringsql =@"SELECT u.city from Exam_user_power p,dbo. User_baseinfo u WHERE [email protected] and P.userid=u.userid GROUP by U.city"; CMD=NewSqlCommand (SQL, Conn, trans); Cmd. Parameters.addwithvalue ("Examid", Examid); SqlDataAdapter SDA=NewSqlDataAdapter (CMD); DataSet DS=NewDataSet (); Sda.                        Fill (DS); List<UserBaseInfoModel> list = (list<userbaseinfomodel>) Ilistdataset.datasettoilist<userbaseinfomodel > (DS,0);                        Trans.commit (); returnlist; }                    Catch{trans.                        Rollback (); Throw; }                    finally{Conn.                    Close (); }                }            }            return NULL; }

Transaction stitching SQL statement operations on the database. Exception rollback