Question

ionline - class，string myname

private static void SetOnlineStatus(PacketHeader header, Connection connection, ionline message)
    {
        Console.WriteLine("Check online: " + message.myname);
        MySqlCommand mycmd = new MySqlCommand();
        mycmd.CommandText = "SELECT * FROM users WHERE username = ?user";
        mycmd.Connection = mconnection;
        mycmd.Parameters.AddWithValue("user", message.myname);
        MySqlDataReader Reader = mycmd.ExecuteReader();

        while (Reader.Read())
        {
            Console.WriteLine("Check online: " + message.myname+" "+GetDBString("username",Reader));
            MySqlCommand mycmd2 = new MySqlCommand();
            mycmd2.CommandText = "UPDATE users SET online = 0 WHERE userid = @user2";
            mycmd2.Parameters.AddWithValue("@user2", Reader.GetInt32("userid"));
            mycmd2.Connection = mconnection;
            Console.WriteLine(mycmd2.ExecuteNonQuery().ToString());
        }


    }

未执行Mysql请求“mycmd2”。我的查询中的内容不正确？

Answer 1

当DataReader打开时，其连接正忙于为读者提供服务该连接不能用于对数据库进行其他操作你应该得到一个例外。

如果您的第一个查询返回零或一行，那么您可以使用ExecuteScalar方法简化代码，并且无需使用MySqlDataReader

    Console.WriteLine("Check online: " + message.myname);
    MySqlCommand mycmd = new MySqlCommand();
    mycmd.CommandText = "SELECT userid FROM users WHERE username = ?user";
    mycmd.Connection = mconnection;
    mycmd.Parameters.AddWithValue("user", message.myname);
    object result = mycmd.ExecuteScalar();
    if(result != null)
    {
        int userID = Convert.ToInt32(result);
        MySqlCommand mycmd2 = new MySqlCommand();
        mycmd2.CommandText = "UPDATE users SET online = 0 WHERE userid = @user2";
        mycmd2.Parameters.AddWithValue("@user2", userID);
        mycmd2.Connection = mconnection;
        mycmd2.ExecuteNonQuery();
    }

MySql UPDATE错误。 C＃

1 个答案: