MySQL C＃錯誤； “連接必須有效且開放”

Question

您好，我得到了我正在處理的該應用程序，但仍無法填充下拉框。 我想從下拉列表中的數據庫中獲取我的SQL數據，但是SQL表示“連接必須有效且打開”，但是我得到了另一個功能，該功能包括使用相同的代碼將文本保存到SQL數據庫中，並且可以正常工作，所以我不這樣做真的不知道怎么了。

這是我的代碼。

private void Form1_Load(object sender, EventArgs e)
    {
        try {
            //This is my connection string i have assigned the database file address path  
            string MyConnection2 = "datasource=localhost;username=root;database=game4rent";
            //This is my insert query in which i am taking input from the user through windows forms  
            string Query = "select Klantnummer,voornaam from game4rent.klanten";
            //This is  MySqlConnection here i have created the object and pass my connection string.  
            MySqlConnection conn = new MySqlConnection(MyConnection2);
            //This is command class which will handle the query and connection object.  
            MySqlCommand sc = new MySqlCommand(Query, conn);
            MySqlDataReader MyReader2;
            MyReader2 = sc.ExecuteReader();
            conn.Open();

            DataTable dt = new DataTable();
            dt.Columns.Add("Klantnummer", typeof(string));
            dt.Columns.Add("voornaam", typeof(string));
            dt.Load(MyReader2);

            cbKlantenNummers.ValueMember = "Klantnummer";
            cbKlantenNummers.DisplayMember = "Klantnummer,voornaam";
            cbKlantenNummers.DataSource = dt;

            conn.Close();
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
   }

這是其他起作用的代碼。

 private void cmdOpslaanKlanten_Click(object sender, EventArgs e)
    {
        try
        {
            //This is my connection string i have assigned the database file address path  
            string MyConnection2 = "datasource=localhost;username=root;password=";
            //This is my insert query in which i am taking input from the user through windows forms  
            string Query = "insert into game4rent.klanten(voornaam,achternaam,straat,huisnummer,woonplaats) values('" + this.txtVoornaam.Text + "','" + this.txtAchternaam.Text + "','" + this.txtStraat.Text + "','" + this.txtHuisnummer.Text + "','" + this.txtWoonplaats.Text + "');";
            //This is  MySqlConnection here i have created the object and pass my connection string.  
            MySqlConnection MyConn2 = new MySqlConnection(MyConnection2);
            //This is command class which will handle the query and connection object.  
            MySqlCommand MyCommand2 = new MySqlCommand(Query, MyConn2);
            MySqlDataReader MyReader2;
            MyConn2.Open();
            MyReader2 = MyCommand2.ExecuteReader();     // Here our query will be executed and data saved into the database.  
            MessageBox.Show("Save Data");
            while (MyReader2.Read())
            {
            }
            MyConn2.Close();
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }

Answer 1

您正在嘗試在打開連接之前執行查詢：

MyReader2 = sc.ExecuteReader();
conn.Open();

由於錯誤狀態，必須在執行查詢之前打開連接：

conn.Open();
MyReader2 = sc.ExecuteReader();