Question

那么，为什么这不会成为回调函数呢？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace sqlAsyncTesting {
    public partial class Form1 : Form {

    public Form1() {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e) {
        using (SqlConnection conn = new SqlConnection(@"Data Source = bigapple; Initial Catalog = master; Integrated Security = SSPI; Asynchronous Processing = true;")) {
            conn.Open();
            SqlCommand cmd = new SqlCommand(@"WAITFOR DELAY '00:03'; Select top 3 * from sysobjects;", conn);
            IAsyncResult result = cmd.BeginExecuteReader(new AsyncCallback(HandleCallback), cmd, CommandBehavior.CloseConnection);
        }
    }

    private void HandleCallback(IAsyncResult result) {
        SqlDataReader dr;
        SqlCommand _this = (SqlCommand)result.AsyncState;

        if (result.IsCompleted) {
            dr = _this.EndExecuteReader(result);
        } else dr = null;

        DataTable dt = new DataTable();
        DataSet ds = new DataSet();

        dt.Load(dr);
        ds.Tables.Add(dt);
        dr.Close();
        Complete(ds);
    }

    private void Complete(DataSet ds) {
        string output = string.Empty;
        foreach (DataColumn c in ds.Tables[0].Columns) {
            output += c.ColumnName + "\t";
        }
        output += "\r\n";
        foreach (DataRow dr in ds.Tables[0].Rows) {
            foreach (object i in dr.ItemArray) {
                output += i.ToString() + "\t";
            }
            output += "\r\n";
        }
    }
}

}

Answer 1

我认为在读者可以工作之前关闭了连接......

using (SqlConnection conn = new SqlConnection(@"Data Source = bigapple; Initial Catalog = master; Integrated Security = SSPI; Asynchronous Processing = true;"))

尝试将其更改为...

SqlConnection conn = new SqlConnection(@"Data Source = bigapple; Initial Catalog = master; Integrated Security = SSPI; Asynchronous Processing = true;");
        conn.Open();
        SqlCommand cmd = new SqlCommand(@"WAITFOR DELAY '00:03'; Select top 3 * from sysobjects;", conn);
        IAsyncResult result = cmd.BeginExecuteReader(new AsyncCallback(HandleCallback), cmd, CommandBehavior.CloseConnection);

顺便说一下这段代码等了3分钟？因为暂停3秒不应该是WAITFOR DELAY'0：0：3'？

Answer 2

我注意到的一些观点：

只有在我删除了WAITFOR DELAY后才调用回调方法   语句。

没有必要轮询result.IsCompleted，因为   回调方法只在被激活后被触发   异步处理完成。

无需在else部分中显式设置dr = null，因为默认情况下   将为空。

您应该处理InvalidOperationException和   ArgumentException中的   HandleCallback方法。

在句柄回调中，每次都调用EndExecuteReader（）   不断获得例外“The   异步操作已经存在   完成了。“所以我永远无法得到   结果在博士。

如果您遇到点no中列出的问题。 5，您可以使用通过使用异步委托而不是内置的BeginExecuteReader（）和EndExecuteReader（）实现的以下备用解决方案。在下面的解决方案中，控件将在调用委托后立即返回到下一行，就像在BeginExecuteReader（）的情况下一样。

替代解决方案：

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private delegate DataSet GetDSDelegate(string query);

    private void button1_Click(object sender, EventArgs e)
    {
        GetDSDelegate del = new GetDSDelegate(GetDataSetAsync);
        del.BeginInvoke(@"Select top 3 * from table1;", null, null);
    }

    private DataSet GetDataSetAsync(string query)
    {
        DataSet ds;
        using (SqlConnection conn = new SqlConnection(@"Data Source = mmmmm000011\sqlexpress; Initial Catalog = SOExamples; Integrated Security = SSPI; Asynchronous Processing = true;"))
        using (SqlCommand cmd = new SqlCommand(query, conn))
        {
            try
            {
                conn.Open();
                SqlDataReader dr = cmd.ExecuteReader();

                DataTable dt = new DataTable();
                ds = new DataSet();

                dt.Load(dr);
                ds.Tables.Add(dt);
                dr.Close();
                Complete(ds);
            }
            finally
            {
                if (conn.State != ConnectionState.Closed)
                    conn.Close();
            }
        }
        MessageBox.Show("Done!!!");
        return ds;
    }

    private void Complete(DataSet ds)
    {
        ...
    }
}

sql异步查询问题

2 个答案: