[MySQL] an exception occurs when connecting to MySQL: connection must be valid and open

Try connecting today MySQL An exception occurred during ：

 Unhandled exception :  System.InvalidOperationException: Connection must be valid and open.
    stay  MySql.Data.MySqlClient.MySqlCommand.Throw(Exception ex)
    stay  MySql.Data.MySqlClient.MySqlCommand.CheckState()
    stay  MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
    stay  MySql.Data.MySqlClient.MySqlCommand.ExecuteReader()

This exception means that the connection must be valid and open , But strange , I have already called connection.Open() ah ？

After inspection , It turns out that there is a problem with this sentence ：

MySqlCommand command = new MySqlCommand(commandString);

In fact, the constructor also needs to be passed in MySqlConnection The object of , Using the above construction method with only one parameter will cause exceptions .

The correct code is as follows ：

    string connectString = "Persist Security Info=False;Username=root;Password=123456;" +
                                       "database=MMOG;server=localhost;Connect Timeout=30";
    // string connectString = "server=localhost;port=3306;user=root;password=123456; database=MMOG;";
    // string connectString = "Database = MMOG;Data Source=127.0.0.1;port=3306;User Id=root;password=123456;";
    MySqlConnection connection = new MySqlConnection(connectString);
    connection.Open();
    string commandString = "insert into `playerdata`(`username`,`password`) values(@username,@password)";
    MySqlCommand command = new MySqlCommand(commandString,connection);
    command.Parameters.AddWithValue("username", "player1");
    command.Parameters.AddWithValue("password", "player1");
    command.ExecuteReader();