系统最近总是不定时的出现：Invalid attempt to Read when reader is closed 和 The connection is closed 这两个错误。下面是报错的代码。

public DataSet SelectDataBySQL(string sql)
        {
            OracleConnection conn = RedOralceDbPool.GetConnection();
            OracleDataAdapter da = new OracleDataAdapter(sql, conn);
            DataSet ds = new DataSet();
            try
            {
                this.Open(conn);
                da.Fill(ds);
            }
            catch (OracleException ex)
            {
                logger.Error(ex.Message);
            }
            finally
            {
                this.Close(conn);
            }
            return ds;
        }