是否有可能使用的ExecuteReader（）两次？

Question

我编程为一个称为OTServer游戏服务器数据库管理器，以及我使用的ExecuteReader（）的第二时间有问题。此处的代码：

    private void button1_Click(object sender, EventArgs e)
    {
        Form1 f = new Form1();
        MySqlConnection conn = new MySqlConnection();
        conn.ConnectionString = "Server=" + f.GetText1().Text + ";Username=" + f.GetText2().Text + ";Pwd=" + f.GetText3().Text + ";Database=" + f.GetText4().Text + ";";
        conn.Open();
        MySqlCommand cmd = new MySqlCommand("SELECT * FROM `players` WHERE name = @Name", conn);
        cmd.Parameters.AddWithValue("@Name", textBox1.Text);

        MySqlDataReader Reader = cmd.ExecuteReader(CommandBehavior.SingleRow);
        while (Reader.Read())
        {
            label7.Text = (string)Reader[1];
            label7.Show();
            label8.Text = Reader[5].ToString();
            label8.Show();
            if ((int)Reader[6] == 1)
            {
                label9.Text = "Sorcerer (1)";
            }
            if ((int)Reader[6] == 2)
            {
                label9.Text = "Druid (2)";
            }
            if ((int)Reader[6] == 3)
            {
                label9.Text = "Paladin (3)";
            }
            if ((int)Reader[6] == 4)
            {
                label9.Text = "Knight (4)";
            }

            if ((int)Reader[6] == 0)
            {
                label9.Text = "None (0)";
            }
            label9.Show();

            if ((int)Reader[3] == 1)
            {
                label10.Text = "Player";
            }

            if ((int)Reader[3] == 2)
            {
                label10.Text = "Tutor";
            }

            if ((int)Reader[3] == 3)
            {
                label10.Text = "Senior Tutor";
            }

            if ((int)Reader[3] == 4)
            {
                label10.Text = "Gamemaster";
            }

            if ((int)Reader[3] == 5)
            {
                label10.Text = "Community Manager";
            }

            if ((int)Reader[3] == 6)
            {
                label10.Text = "God";
            }

            if ((int)Reader[3] < 1 || (int)Reader[3] > 6)
            {
                label10.Text = "Unknown";
            }

            label10.Show();

            label13.Text = "Account: " + Reader[4].ToString();
            label13.Show();
        }
        Reader.Close();

        cmd = new MySqlCommand("SELECT * FROM accounts WHERE id = @Account_ID", conn);
        cmd.Parameters.AddWithValue("@Account_ID", label13.Text);
        Reader = cmd.ExecuteReader(CommandBehavior.SingleRow);

        while (Reader.Read())
        {
            label11.Text = (string)Reader[0];
            label11.Show();
        }
        Reader.Close();
    }

Answer 1

建议解决方案：尝试把一个using块周围的DataReader，或致电Dispose就可以了：

using (DataReader Reader = cmd.ExecuteReader(CommandBehavior.SingleRow))
{
    // ...do something with your data reader... then finish by:
    Reader.Close();
}  // <-- Reader.Dispose() called automatically at the end of using block.

// ...prepare second command...

// the same again for the second command:
using (DataReader Reader = cmd.ExecuteReader(CommandBehavior.SingleRow))
{
    // ...
    Reader.Close();
}

你的问题的可能的原因：的DB连接对象可能会做一些内部簿记跟踪数据的读者。我在一个类似的情况发现，你只允许一个DataReader在同一时间。所以我相信你的代码的问题是，当你Close的Reader，您没有明确Disposed，所以连接对象认为第一数据读取器仍然是当你执行第二个使用。

---

另外... 为什么不简化这个代码：

        if ((int)Reader[6] == 1)
        {
            label9.Text = "Sorcerer (1)";
        }
        if ((int)Reader[6] == 2)
        {
            label9.Text = "Druid (2)";
        }
        ...

到switch语句：

        int x = (int)(Reader[6]);
        string label9Text = string.Empty;

        switch (x)
        {
            case 1:  label9Text = "Sorcerer (1)";  break;
            case 2:  label9Text = "Druid (2)";     break;
            ...
        }

        label9.Text = label9Text;

（这会节省你不少重复输入的。）

Answer 2

好吧，假设你的代码是正确的，你不应该在执行两个读者的问题，因为你在你的代码显示。也许你有没有因为处置命令或其他什么东西的问题。我建议这样的方法（实施例与罗斯文分贝制造）：

using (SqlConnection connection = new SqlConnection("Data Source=.\\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=SSPI;"))
        {
            connection.Open();

            using (SqlCommand command = new SqlCommand("SELECT * FROM Orders", connection))
            {
                using (SqlDataReader reader = command.ExecuteReader(System.Data.CommandBehavior.SingleRow))
                {
                    while (reader.Read())
                    {
                        Console.WriteLine(reader.GetString(2));
                    }
                }
            }

            using (SqlCommand command = new SqlCommand("SELECT * FROM Products", connection))
            {
                using (SqlDataReader reader = command.ExecuteReader(System.Data.CommandBehavior.SingleRow))
                {
                    while (reader.Read())
                    {
                        Console.WriteLine(reader.GetString(1));
                    }
                }
            }
        }

识别类型的球员时，你应该清理你的代码。创建一个枚举来代替：

public enum PlayerType
{
    None = 0,
    Sorcerer = 1,
    Druid = 2,
    Paladin = 3
}

和然后执行下列操作，而读：

PlayerType playerType = (PlayerType)reader.GetInt32(6);
label9.Text = playerType.ToString();