在调用Read()之前试图访问字段无效，但我先调用Read()

本文关键字：Read 调用 无效 字段 访问 | 更新日期: 2023-09-27 18:04:50

我试图从MySQL数据库处理一些数据(主要是双值)。我得到这个错误信息:

Invalid attempt to access a field before calling Read()

观察窗口:

readerCompare1["Fe1"]   'readerCompare1["Fe1"]' threw an exception of type 'MySql.Data.MySqlClient.MySqlException'  object {MySql.Data.MySqlClient.MySqlException}

我以前用过这段代码，它工作得很好。

代码:

  for (int k = 1; k <= pocet; k++)
            {
                string queryCompare = " SELECT F1 AS Fe1, F2 AS Fe2, F3 AS Fe3, F4 AS Fe4, F5 AS Fe5, F6 AS Fe6, F7 AS Fe7, F8 AS Fe8, cluster FROM features WHERE ID=" + k;
                MySqlCommand cmdCompare = new MySqlCommand(queryCompare, conect);
                readerCompare1 = cmdCompare.ExecuteReader();
                readerCompare1.Read();
                //  MessageBox.Show("OK");
                double f1 = Convert.ToDouble(readerCompare1["Fe1"].ToConvertibleDouble());   //Exception THROWN HERE
                double f2 = Convert.ToDouble(readerCompare1["Fe2"].ToConvertibleDouble());
                double f3 = Convert.ToDouble(readerCompare1["Fe3"].ToConvertibleDouble());
                double f4 = Convert.ToDouble(readerCompare1["Fe4"].ToConvertibleDouble());
                double f5 = Convert.ToDouble(readerCompare1["Fe5"].ToConvertibleDouble());
                double f6 = Convert.ToDouble(readerCompare1["Fe6"].ToConvertibleDouble());
                double f7 = Convert.ToDouble(readerCompare1["Fe7"].ToConvertibleDouble());
                double f8 = Convert.ToDouble(readerCompare1["Fe8"].ToConvertibleDouble());
                // string f88 = readerCompare["Fe8"].ToString();
                //double f8 = Convert.ToDouble(f88, CultureInfo.InvariantCulture);
                int cluster = Convert.ToInt32(readerCompare1["cluster"].ToString());
                readerCompare1.Close();

在:double f1 = Convert这一行抛出异常。ToDouble (readerCompare1 [" Fe1 "]…

但是正如你所看到的，我首先调用了Read():

readerCompare1.Read();

谁能解释一下，为什么会发生这种情况?谢谢你

在调用Read()之前试图访问字段无效，但我先调用Read()

Read方法返回一个布尔值，告诉您是否有任何信息可用。如果返回false，则不能尝试从读取器读取。

你应该用if语句(或者while语句，取决于你期望得到多少个结果)来包装你的代码:

if(readerCompare1.Read()) {
    double f1 = Convert.ToDouble(readerCompare1["Fe1"].ToConvertibleDouble());
    double f2 = Convert.ToDouble(readerCompare1["Fe2"].ToConvertibleDouble());
    double f3 = Convert.ToDouble(readerCompare1["Fe3"].ToConvertibleDouble());
    double f4 = Convert.ToDouble(readerCompare1["Fe4"].ToConvertibleDouble());
    double f5 = Convert.ToDouble(readerCompare1["Fe5"].ToConvertibleDouble());
    double f6 = Convert.ToDouble(readerCompare1["Fe6"].ToConvertibleDouble());
    double f7 = Convert.ToDouble(readerCompare1["Fe7"].ToConvertibleDouble());
    double f8 = Convert.ToDouble(readerCompare1["Fe8"].ToConvertibleDouble());
}

MySqlDataReader示例

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