Mysql System.Exception: 错误：Fatal error encountered during command execution.

折磨死人的报错Fatal error encountered during command execution. 服务使用的是长连接

 private MySqlConnection _Conn;

 public MySqlHelper(String dbConnectString)
        {
            _DataBaseConnectString = dbConnectString;

            _Conn = new MySqlConnection(dbConnectString);
         
            _Conn.Open();
        }

private void ExecuteNonQuery(string connectString, CommandType cmdType, string cmdText, params MySqlParameter[] commandParameters)
        {
           
                MySqlCommand cmd = new MySqlCommand();

                PrepareCommand(cmd, _Conn, cmdType, cmdText, commandParameters);
                try
                {
                    cmd.ExecuteNonQuery();
                }
                catch (MySqlException e)
                {
                    throw new Exception(String.Format("错误：{0} 出错Sql：{1}", e.Message, cmdText));
                }
                cmd.Parameters.Clear();
           
        }

构造函数就初始化了_Conn; Mysql 服务器配置wait_timeout 时间为120S，那么在主系统休眠超过120秒之后，再执行数据操作就报上面的错误，后续就是System.NullReferenceException: 未将对象引用设置到对象的实例。

这异常提示也太难猜了