無法連接到任何指定的MySQL主機

Question

這是問題所在。 我正在嘗試在connection.Open執行查詢及其拋出和異常。 奇怪的是，在同一應用程序上，我正在執行“選擇”查詢，並且運行正常。 但是，當我執行“更新”查詢時，它將引發此“無法連接到任何指定的MySQL主機”錯誤。 永遠被困在這。 有人可以發現我要去哪里了嗎？

 private void button1_Click(object sender, EventArgs e)
    {
        if (radioButton1.Checked)
        {
            timerEnabled = 1;
        }

        connection.Open();

        //update the settings to the database table 
        MySqlCommand command = connection.CreateCommand();
        command.CommandText = "update Admin_Settings set Difficulty='" + comboBox3.Text + "'," + "NoOfQuestions='" + comboBox4.Text + "'," + "NoOfChoices='" + comboBox5.Text + "'," +
            "Subject='" + comboBox8.Text + "'," + "Timer='" + comboBox2.Text + "," + "TimerEnabled=" + timerEnabled + "," + "TimerType='" + comboBox1.Text + "'";


        command.ExecuteNonQuery();

        MessageBox.Show("Settings updated");
    }

Answer 1

我將建議您執行以下操作：

private void button1_Click(object sender, EventArgs e)
        {
            using (System.Data.SqlClient.SqlConnection connection = new System.Data.SqlClient.SqlConnection(connString))
            {
                if (radioButton1.Checked)
                {
                    timerEnabled = 1;
                }

                connection.Open();

                //update the settings to the database table 
                MySqlCommand command = connection.CreateCommand();
                command.CommandText = "update Admin_Settings set Difficulty='" + comboBox3.Text + "'," + "NoOfQuestions='" + comboBox4.Text + "'," + "NoOfChoices='" + comboBox5.Text + "'," +
                    "Subject='" + comboBox8.Text + "'," + "Timer='" + comboBox2.Text + "," + "TimerEnabled=" + timerEnabled + "," + "TimerType='" + comboBox1.Text + "'";


                command.ExecuteNonQuery();

                MessageBox.Show("Settings updated");
            }
        }

我了解您正在考慮自己，應該保持連接的便利性，以方便使用等等，但以我的經驗來看，這是浪費時間。 最終會發生您不想要或不需要的許多麻煩。 您最終沒有意識到自己在其他地方打開了連接，並且花了數小時來排查本不應該做的事情。 打開您的連接，完成后將其關閉。

如果您希望有一個連接對象，那很好，但是請使用using模式，以便每次都將其丟棄，並始終從連接開始。

注意：用yoru MySqlConnection對象替換我的連接！

Answer 2

正如Mike所說，您總是最好使用“ using”塊，因為一旦它退出了using塊，它就會處置任何連接。 我在下面使用了兩個using塊，一個用於連接，另一個用於命令對象。

嘗試這個

    private void button1_Click(object sender, EventArgs e)
    {
        using (SqlConnection connection = new SqlConnection(connString))
        {
            if (radioButton1.Checked)
            {
                timerEnabled = 1;
            }

           string queryString = "update Admin_Settings set Difficulty='" +      
            comboBox3.Text + "'," + "NoOfQuestions='" + comboBox4.Text + "'," + 
            "NoOfChoices='" + comboBox5.Text + "'," + "Subject='" + comboBox8.Text +  
           "'," + "Timer='" + comboBox2.Text + "," + "TimerEnabled=" + timerEnabled +  
              "," + "TimerType='" + comboBox1.Text + "'";

          using (SqlCommand command = new SqlCommand(queryString, connection))
          {
            //update the settings to the database table 


            command.Connection.Open();
            command.ExecuteNonQuery();
            command.Connection.Close();

            MessageBox.Show("Settings updated");
          }
        }
    }