1    WebClient webClenet=new WebClient();
 2                     webClenet.DownloadStringAsync(new Uri("http://news.sina.com.cn/s/2011-11-25/120923524756.shtml", UriKind.RelativeOrAbsolute));     
 3            webClenet.DownloadStringCompleted += new DownloadStringCompletedEventHandler(webClenet_DownloadStringCompleted);
 4 
 5 
 6 
 7 
 8 
 9 
10 回调事件：
11 void webClenet_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
12         {
13             string s= e.Result;
14 }