bool isWriteInMessage;
            int m_id = 0;
            Message m = new Message();
            m.sender = sender;
            m.message_content = message;
            m.send_time = DateTime.Now;
            db.Message.InsertOnSubmit(m);
            try
            {
                db.SubmitChanges();
                m_id = (int)(m.message_id); //先写入数据库，然后才能获得messageId的值
                isWriteInMessage = true;
            }
            catch 
            {
                isWriteInMessage = false;
            }