WebClient 二

时间：2011-04-07 来源：AndyYu

WebClient 一种已对WebClient从网页读取数据做了简要的介绍。其中对异步读取网页数据流进一步剖析。

本文还将对WebClient 上传和下载左详细的分析。

1          // 异步调用
2             WebClient webClient = new WebClient();
3             webClient.Credentials = CredentialCache.DefaultCredentials;
4             webClient.OpenReadCompleted += new OpenReadCompletedEventHandler(OpenReadCallback);
5             webClient.OpenReadAsync(new Uri("http://www.cnblogs.cn/"));
6
7         public static void OpenReadCallback(Object sender, OpenReadCompletedEventArgs e)
8         {
9             Stream reply = null;
10             StreamReader s = null;
11
12             try
13             {
14                 reply = (Stream)e.Result;
15                 s = new StreamReader(reply);
16                 string strResult = s.ReadToEnd();
17                 AppLog.Write(strResult, AppLog.LogMessageType.Error);
18             }
19             finally
20             {
21                 if (s != null)
22                 {
23                     s.Close();
24                 }
25
26                 if (reply != null)
27                 {
28                     reply.Close();
29                 }
30             }
31         }

WebClient 方法 OpenReadAsync 访问Url 参数指定的页码数据流 Steam的事例，该方法是自动从线程池分配线程自语获取，数据流获取有效同时接到同时，出发OpenReadCompleted事件，

故必须为改时间添加去就数据流程方法。本实例中通过调用注册 OpenReadCallback时间来获取读取数据流资源。其中那个对OpenReadAsync 主要实现其功能如下：

2
3       public void OpenReadAsync(Uri address)
4         {
5             OpenReadAsync(address, null);
6         }
7
8         public void OpenReadAsync(Uri address, object userToken)
9         {
10             if (address == null)
11                 throw new ArgumentNullException("address");
12
13             lock (this)
14             {
15                 SetBusy();
16                 async = true;
17
18                 async_thread = new Thread(delegate(object state)
19                 {
20                     object[] args = (object[])state;
21                     WebRequest request = null;
22                     try
23                     {
24                         request = SetupRequest((Uri)args[0]);
25                         WebResponse response = GetWebResponse(request);
26                         Stream stream = response.GetResponseStream();
27                         OnOpenReadCompleted( new OpenReadCompletedEventArgs(stream, null, false, args[1]));
28                     }
29                     catch (ThreadInterruptedException)
30                     {
31                         if (request != null)
32                             request.Abort();
33
34                         OnOpenReadCompleted(new OpenReadCompletedEventArgs(null, null, true, args[1]));
35                     }
36                     catch (Exception e)
37                     {
38                         OnOpenReadCompleted(new OpenReadCompletedEventArgs(null, e, false, args[1]));
39                     }
40                 });
41                 object[] cb_args = new object[] { address, userToken };
42                 async_thread.Start(cb_args);
43             }
44         }
45

SetupRequest((Uri)args[0]) 方法主要功能是 WebRequest.Create(address) 还有代理验证及相关头文件，GetWebResponse(request) 方法主要功能是 request.GetResponse();OpenReadCompletedEventHandler 是此事件的委托。OpenReadCompletedEventArgs 类为事件处理程序提供事件数据。