学步园

在MSDN看了MS的ASP.NET 2.0 中的异步页文章，其中试了一下运行效果。发觉其根本没有达到界面效果（异步功能是实现了).

http://www.microsoft.com/china/msdn/library/webservices/asp.net/issuesWickedCodetoc.mspx

数据访问代码如下.

using System;

using System.Data;

using System.Data.SqlClient;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.Configuration;

public partial class AsyncDataBind : System.Web.UI.Page

{
    private SqlConnection _connection;

    private SqlCommand _command;

    private SqlDataReader _reader;


    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            // Hook PreRenderComplete event for data binding

            this.PreRenderComplete += new EventHandler(Page_PreRenderComplete);

            AddOnPreRenderCompleteAsync(new BeginEventHandler(BeginAsyncOperation), new EndEventHandler(EndAsyncOperation)
            );
        }
    }

    IAsyncResult BeginAsyncOperation (object sender, EventArgs e,         AsyncCallback cb, object state)
    {

        string connect = WebConfigurationManager.ConnectionStrings         ["PubsConnectionString"].ConnectionString;
        

       _connection = new SqlConnection(connect);

        _connection.Open();

        _command = new SqlCommand("SELECT title_id, title, price FROM titles", _connection);
        

        return _command.BeginExecuteReader (cb, state);
    }

    void EndAsyncOperation(IAsyncResult ar)

    {

        _reader = _command.EndExecuteReader(ar);

    }

    protected void Page_PreRenderComplete(object sender, EventArgs e)

    {

        Output.DataSource = _reader;

        Output.DataBind();

    }

    public override void Dispose()

    {
        if (_connection != null) _connection.Close();

        base.Dispose();
    }
}

 原理不讲了，看MS的帮助就行了。讲一下问题，上面红色的一段在运行时会调用二次。就是因为此二次调用，造成界面数据无法显示：因为_reader中的数据在第一次调用时已全部读取了。指针到最后，第二次调用就会为null.

不明白的就是Page_PreRenderComplete 为什么会调用二次。没办法，只好变通一下。用一个标记来判断是否要进行绑定.代码改成如下就可以正常显示:

 bool _isread = false;
    protected void Page_PreRenderComplete(object sender, EventArgs e)
    {
       if(_isread==false)
       {
            this.GridView1.DataSource = _reader;
            GridView1.DataBind();
            _isread = true;
        }
    }