防止查询数据返回数据行为零引起的逻辑判断重复或抛出异常

以下代码以操作ORACLE数据库为例

1.返回DataSet数据，若查询返回行数为零，返回null

        /// <summary>
        /// 执行查询语句，返回DataSet
        /// </summary>
        /// <param name="SQLString">查询语句</param>
        /// <returns>DataSet</returns>
        public static DataSet Query(string SQLString, params OracleParameter[] cmdParms)
        {
            using (OracleConnection connection = new OracleConnection(connectionString))
            {
                OracleCommand cmd = new OracleCommand();
                PrepareCommand(cmd, connection, null, SQLString, cmdParms);
                using (OracleDataAdapter da = new OracleDataAdapter(cmd))
                {
                    DataSet ds = new DataSet();
                    try
                    {
                        da.Fill(ds, "ds");
                        cmd.Parameters.Clear();
                    }
                    catch (System.Data.OracleClient.OracleException ex)
                    {
                        throw new Exception(ex.Message);
                    }
24                     //没有数据返回，返回null
25                     if (ds.Tables[0].Rows.Count <= 0)
26                     {
27                         return null;
28                     }
                    return ds;
                }
            }
        }

2.返回DataTable数据，若查询返回行数为零，返回null

        /// <summary>
        /// 执行查询语句，返回DataTable
        /// </summary>
        /// <param name="SQLString">查询语句</param>
        /// <returns>DataTable</returns>
        public static DataTable Query(string SQLString, params OracleParameter[] cmdParms)
        {
            using (OracleConnection connection = new OracleConnection(connectionString))
            {
                OracleCommand cmd = new OracleCommand();
                PrepareCommand(cmd, connection, null, SQLString, cmdParms);
                using (OracleDataAdapter da = new OracleDataAdapter(cmd))
                {
                    DataTable dt = new DataTable();
                    try
                    {
                        da.Fill(dt);
                        cmd.Parameters.Clear();
                    }
                    catch (System.Data.OracleClient.OracleException ex)
                    {
                        throw new Exception(ex.Message);
                    }
24                     //没有数据返回，返回null
25                     if (dt.Rows.Count <= 0)
26                     {
27                         return null;
28                     }
                    return dt;
                }
            }
        }

PrepareCommand的实现(1和2中均有用到)

        /// <summary>
        /// 打开数据库连接，传递参数等前置处理
        /// </summary>
        /// <param name="cmd">OracleCommand</param>
        /// <param name="conn">数据库连接</param>
        /// <param name="trans">数据库事务</param>
        /// <param name="cmdText">SQL语句</param>
        /// <param name="cmdParms">传递的参数</param>
        private static void PrepareCommand(OracleCommand cmd, OracleConnection conn,
        OracleTransaction trans, string cmdText, OracleParameter[] cmdParms)
        {
            if (conn.State != ConnectionState.Open)
                conn.Open();
            cmd.Connection = conn;
            cmd.CommandText = cmdText;
            if (trans != null)
                cmd.Transaction = trans;
            cmd.CommandType = CommandType.Text;
            if (cmdParms != null)
            {
                foreach (OracleParameter parm in cmdParms)
                {
                    if (parm.Value == null)
                    {
                        parm.Value = DBNull.Value;
                    }
                    cmd.Parameters.Add(parm);
                }
            }
        }

 应用场景

1.防止多次判断如：

if (dt == null || dt.Rows.Count <= 0) { return; }

只需写作

if (dt == null) { return; }

2.防止引用为null的数据引发异常：

this.dataGridView1.DataSource = dt;

//若dt返回数据行数为零,以下赋值会产生异常

this.dataGridView1.Rows[0].Selected = false;