c# - 无法连接到任何指定的 MySQL 主机

Question

问题来了。我正在尝试在 connection.Open 处执行查询及其抛出和异常。奇怪的是，在同一个应用程序上我正在执行一个“选择”查询并且它工作正常。但是当我执行“更新”查询时，它会抛出此无法连接到任何指定的 MySQL 主机错误。一直卡在这个上面。有人可以发现我哪里出错了。

 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

我建议您执行以下操作:

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");
            }
        }

我知道您在想自己应该保持连接以便于使用等等，但根据我的经验，这是白费力气。最终会发生很多你不想要或不需要的麻烦。您最终没有意识到您在其他地方打开了一个连接，并且您花费数小时来解决您不应该做的事情。打开您的连接，完成后关闭它。

如果你想要一个单一的连接对象，那很好，但是使用 using 模式，这样它每次都会被处理掉，并且总是从你的连接开始。

注意:用 yoru MySqlConnection 对象替换我的连接!