public static int AddGuest(Guest guest)   {   int guestId = 0;   StringBuilder addString = new StringBuilder();   addString.Append("insert into Guest (GuestID,Name,CategoryID,Sex,Mobile,RoomID,ArriveTime,LeaveTime) values(");   addString.Append("'"+ guest.GuestId + "'");   addString.Append(",'" + guest.Name + "'");   addString.Append("," + guest.CategoryId);   addString.Append(",'" + guest.Sex + "'");   addString.Append(",'" + guest.Mobile + "'");   addString.Append("," + guest.RoomId);   addString.Append(",'" + guest.ArriveTime + "'");   addString.Append(",'" + guest.LeaveTime + "')");   string sql = addString.ToString();   guestId = DBHelper.GetScalar(sql);   return guestId;   }