在asp.net页面中使用异步读取

刚亲自撰写一篇技术文章，请大家多指教。哈哈。
原文首发：http://bbs.5inet.net/topic.aspx?topicid=181

有的时候我们需要在网页里读取论坛的信息，在传统ASP的时候我们使用的是JS或者是IFRAME，这两种方式都不是很方便，而且对搜索引擎不友好。现在有了.Net，我们有了另一种方式。

要求：论坛需要提供RSS支持。

代码如下：

#region task class
//这是一个任务类，执行具体的任务
public class RssAsyncTask
{
private String _rssContent;
private AsyncTaskDelegate _dlgt;
private string rssUrl;
private bool _success;

public bool IsSuccess
{
get
{
return _success;
}
}

public RssAsyncTask(string rssUrl)
{
this.rssUrl = rssUrl;
}

// Create delegate.
protected delegate void AsyncTaskDelegate();

public String GetRssContent()
{
return _rssContent;
}
public void DoTheAsyncTask()
{
// Introduce an artificial delay to simulate a delayed
// asynchronous task. Make this greater than the
// AsyncTimeout property.
WebClient wc = new WebClient();
try
{
_rssContent = wc.DownloadString(rssUrl);
_success = true;
}
catch (Exception e)
{
_rssContent = e.Message;
}
finally
{
wc.Dispose();
}
//Thread.Sleep(TimeSpan.FromSeconds(5.0));
}

// Define the method that will get called to
// start the asynchronous task.
public IAsyncResult OnBegin(object sender, EventArgs e,
AsyncCallback cb, object extraData)
{
//_rssContent = "Beginning async task.";

_dlgt = new AsyncTaskDelegate(DoTheAsyncTask);
IAsyncResult result = _dlgt.BeginInvoke(cb, extraData);

return result;
}

// Define the method that will get called when
// the asynchronous task is ended.
public void OnEnd(IAsyncResult ar)
{
//_rssContent = "Asynchronous task completed.";
_dlgt.EndInvoke(ar);
}

// Define the method that will get called if the task
// is not completed within the asynchronous timeout interval.
public void OnTimeout(IAsyncResult ar)
{
_rssContent = "Ansynchronous task failed to complete "
"because it exceeded the AsyncTimeout parameter.";
}
}
#endregion

//一个自定义的控件，继承自另一个自定义控件。
public class RArticle
: LPanel
{
#region properties
string rssUrl;

public string RssUrl
{
get { return rssUrl; }
set { rssUrl = value; }
}

int maxRecordNumber = 6;

public int MaxRecordNumber
{
get { return maxRecordNumber; }
set { maxRecordNumber = value; }
}
#endregion

RssAsyncTask task;
protected override void OnInit(EventArgs e)
{
base.OnInit(e);
task = new RssAsyncTask(this.rssUrl);
PageAsyncTask asyncTask = new PageAsyncTask(task.OnBegin, task.OnEnd, task.OnTimeout, null);

Page.RegisterAsyncTask(asyncTask);
Page.ExecuteRegisteredAsyncTasks();
}

static Random r = new Random();
protected override void Render(System.Web.UI.HtmlTextWriter writer)
{
string rssContent = task.GetRssContent();
XmlDocument doc = null;
if (task.IsSuccess)
{
doc = new XmlDocument();
doc.LoadXml(rssContent);

this.Title = doc.SelectSingleNode("rss/channel/title").InnerText;
this.TitleNavigateUrl = doc.SelectSingleNode("rss/channel/link").InnerText;
this.ShowTitle = true;
}
base.RenderBegin(writer);

writer.WriteBeginTag("div");
writer.WriteAttribute("class", "child2");
Right(writer);
writer.WriteBeginTag("ul");
Right(writer);

if (doc != null)
{
#region success

XmlNodeList items = doc.SelectNodes("rss/channel/item");
List<XmlNode> nodes = new List<XmlNode>();
foreach (XmlNode node in items)
nodes.Add(node);

//使用范型进行日期的倒序排列

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