Question

我试图在一个简单的语句中从多个MySQL表中选择数据，但我的问题是当我尝试运行它时出现以下错误：

类型＆＃39; System.Exception＆＃39;的例外情况发生在MySql.Data.dll中但未在用户代码中处理

其他信息：数据阅读器中没有当前查询

我用来运行语句的代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication1
{
    public partial class RetrieveCars : System.Web.UI.Page
    {
        String Model;
        MySql.Data.MySqlClient.MySqlConnection conn;
        MySql.Data.MySqlClient.MySqlCommand cmd;
        MySql.Data.MySqlClient.MySqlDataReader reader;
        String queryStr;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.QueryString["model_id"] != null)
            {
                Model = Request.QueryString["model_id"];

                String connString = System.Configuration.ConfigurationManager.ConnectionStrings["WebAppConnString"].ToString();
                conn = new MySql.Data.MySqlClient.MySqlConnection(connString);
                conn.Open();

                queryStr = "";

                queryStr = "SELECT c.CarID, c.RegNumber, c.Colour , m.CarModel, m.CarMake, m.CostPerDay FROM car c INNER JOIN model m ON m.ModelID=c.ModelID WHERE c.ModelID = '" + Model + "'";

                cmd = new MySql.Data.MySqlClient.MySqlCommand(queryStr, conn);

                reader = cmd.ExecuteReader();

                reader.Close();
                conn.Close();

                CarN.Text = reader.GetString(reader.GetOrdinal("CarModel"));
            }


        }
    }
}

Answer 1

在获取任何数据之前，您正在关闭阅读器和连接。只需将.Close()次呼叫移至功能结束即可您还需要在Read()上调用reader来实际获取一行数据。

cmd = new MySql.Data.MySqlClient.MySqlCommand(queryStr, conn);
reader = cmd.ExecuteReader();

// Read() will return a bool indicating whether a new row has been read
// false signals no more data
while (reader.Read()) {
    // Do something with the row data
    CarN.Text = reader.GetString(reader.GetOrdinal("CarModel"));
}

reader.Close();
conn.Close();

从C＃中的多个表中进行选择

1 个答案: