c# - 实现 HttpWebRequest 异步调用

Question

我有这个代码

using (var stream = new StreamWriter(request.GetRequestStream(), Encoding))
   stream.Write(body.ToString());

我需要让它异步。据我了解，这意味着我需要将调用更改request.GetRequestStream()为asychronous.BeginGetRequestStream(). 我看过这个例子，但不知道如何将它应用到这个场景中。有人可以帮忙吗？

Answer 1

该文档有一个很好的例子（http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.begingetrequeststream(v=vs.100).aspx）：

using System;
using System.Net;
using System.IO;
using System.Text;
using System.Threading;

class HttpWebRequestBeginGetRequest
{
    private static ManualResetEvent allDone = new ManualResetEvent(false);
public static void Main(string[] args)
{
    // Create a new HttpWebRequest object.
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://www.contoso.com/example.aspx");

    request.ContentType = "application/x-www-form-urlencoded";

    // Set the Method property to 'POST' to post data to the URI.
    request.Method = "POST";

    // start the asynchronous operation
    request.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), request);

    // Keep the main thread from continuing while the asynchronous 
    // operation completes. A real world application 
    // could do something useful such as updating its user interface. 
    allDone.WaitOne();
}

private static void GetRequestStreamCallback(IAsyncResult asynchronousResult)
{
    HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

    // End the operation
    Stream postStream = request.EndGetRequestStream(asynchronousResult);

    Console.WriteLine("Please enter the input data to be posted:");
    string postData = Console.ReadLine();

    // Convert the string into a byte array. 
    byte[] byteArray = Encoding.UTF8.GetBytes(postData);

    // Write to the request stream.
    postStream.Write(byteArray, 0, postData.Length);
    postStream.Close();

    // Start the asynchronous operation to get the response
    request.BeginGetResponse(new AsyncCallback(GetResponseCallback), request);
}

private static void GetResponseCallback(IAsyncResult asynchronousResult)
{
    HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

    // End the operation
    HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult);
    Stream streamResponse = response.GetResponseStream();
    StreamReader streamRead = new StreamReader(streamResponse);
    string responseString = streamRead.ReadToEnd();
    Console.WriteLine(responseString);
    // Close the stream object
    streamResponse.Close();
    streamRead.Close();

    // Release the HttpWebResponse
    response.Close();
    allDone.Set();
}

Answer 2

你可以通过这段代码理解。

该程序定义了两个供自己使用的类，RequestState 类，它通过异步调用传递数据，以及 ClientGetAsync 类，它实现对 Internet 资源的异步请求。

RequestState 类在对为请求提供服务的异步方法的调用中保留请求的状态。它包含 WebRequest 和 Stream 实例，其中包含对资源的当前请求和响应中接收到的流，包含当前从 Internet 资源接收的数据的缓冲区，以及包含完整响应的 StringBuilder。当向 WebRequest.BeginGetResponse 注册 AsyncCallback 方法时，将 RequestState 作为状态参数传递。

ClientGetAsync 类实现对 Internet 资源的异步请求，并将生成的响应写入控制台。它包含以下列表中描述的方法和属性。

allDone 属性包含一个 ManualResetEvent 类的实例，该实例发出请求完成的信号。

Main() 方法读取命令行并开始对指定 Internet 资源的请求。它创建 WebRequest wreq 和 RequestState rs，调用 BeginGetResponse 开始处理请求，然后调用 allDone.WaitOne() 方法，以便应用程序在回调完成之前不会退出。从 Internet 资源读取响应后，Main() 将其写入控制台，应用程序结束。

showusage() 方法在控制台上编写示例命令行。当命令行上没有提供 URI 时，它由 Main() 调用。

RespCallBack() 方法实现了互联网请求的异步回调方法。它创建包含来自 Internet 资源的响应的 WebResponse 实例，获取响应流，然后开始异步读取流中的数据。

ReadCallBack() 方法实现了读取响应流的异步回调方法。它将从 Internet 资源接收到的数据传输到 RequestState 实例的 ResponseData 属性中，然后开始另一个异步读取响应流，直到不再返回数据。读取完所有数据后，ReadCallBack() 关闭响应流并调用 allDone.Set() 方法以指示整个响应存在于 ResponseData 中。

using System;
using System.Net;
using System.Threading;
using System.Text;
using System.IO;

// The RequestState class passes data across async calls.
public class RequestState
{
   const int BufferSize = 1024;
   public StringBuilder RequestData;
   public byte[] BufferRead;
   public WebRequest Request;
   public Stream ResponseStream;
   // Create Decoder for appropriate enconding type.
   public Decoder StreamDecode = Encoding.UTF8.GetDecoder();

   public RequestState()
   {
      BufferRead = new byte[BufferSize];
      RequestData = new StringBuilder(String.Empty);
      Request = null;
      ResponseStream = null;
   }     
}

// ClientGetAsync issues the async request.
class ClientGetAsync 
{
   public static ManualResetEvent allDone = new ManualResetEvent(false);
   const int BUFFER_SIZE = 1024;

   public static void Main(string[] args) 
   {
      if (args.Length < 1) 
      {
         showusage();
         return;
      }

      // Get the URI from the command line.
      Uri httpSite = new Uri(args[0]);

      // Create the request object.
      WebRequest wreq = WebRequest.Create(httpSite);

      // Create the state object.
      RequestState rs = new RequestState();

      // Put the request into the state object so it can be passed around.
      rs.Request = wreq;

      // Issue the async request.
      IAsyncResult r = (IAsyncResult) wreq.BeginGetResponse(
         new AsyncCallback(RespCallback), rs);

      // Wait until the ManualResetEvent is set so that the application 
      // does not exit until after the callback is called.
      allDone.WaitOne();

      Console.WriteLine(rs.RequestData.ToString());
   }

   public static void showusage() {
      Console.WriteLine("Attempts to GET a URL");
      Console.WriteLine("\r\nUsage:");
      Console.WriteLine("   ClientGetAsync URL");
      Console.WriteLine("   Example:");
      Console.WriteLine("      ClientGetAsync http://www.contoso.com/");
   }

   private static void RespCallback(IAsyncResult ar)
   {
      // Get the RequestState object from the async result.
      RequestState rs = (RequestState) ar.AsyncState;

      // Get the WebRequest from RequestState.
      WebRequest req = rs.Request;

      // Call EndGetResponse, which produces the WebResponse object
      //  that came from the request issued above.
      WebResponse resp = req.EndGetResponse(ar);         

      //  Start reading data from the response stream.
      Stream ResponseStream = resp.GetResponseStream();

      // Store the response stream in RequestState to read 
      // the stream asynchronously.
      rs.ResponseStream = ResponseStream;

      //  Pass rs.BufferRead to BeginRead. Read data into rs.BufferRead
      IAsyncResult iarRead = ResponseStream.BeginRead(rs.BufferRead, 0, 
         BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs); 
   }


   private static void ReadCallBack(IAsyncResult asyncResult)
   {
      // Get the RequestState object from AsyncResult.
      RequestState rs = (RequestState)asyncResult.AsyncState;

      // Retrieve the ResponseStream that was set in RespCallback. 
      Stream responseStream = rs.ResponseStream;

      // Read rs.BufferRead to verify that it contains data. 
      int read = responseStream.EndRead( asyncResult );
      if (read > 0)
      {
         // Prepare a Char array buffer for converting to Unicode.
         Char[] charBuffer = new Char[BUFFER_SIZE];

         // Convert byte stream to Char array and then to String.
         // len contains the number of characters converted to Unicode.
      int len = 
         rs.StreamDecode.GetChars(rs.BufferRead, 0, read, charBuffer, 0);

         String str = new String(charBuffer, 0, len);

         // Append the recently read data to the RequestData stringbuilder
         // object contained in RequestState.
         rs.RequestData.Append(
            Encoding.ASCII.GetString(rs.BufferRead, 0, read));         

         // Continue reading data until 
         // responseStream.EndRead returns –1.
         IAsyncResult ar = responseStream.BeginRead( 
            rs.BufferRead, 0, BUFFER_SIZE, 
            new AsyncCallback(ReadCallBack), rs);
      }
      else
      {
         if(rs.RequestData.Length>0)
         {
            //  Display data to the console.
            string strContent;                  
            strContent = rs.RequestData.ToString();
         }
         // Close down the response stream.
         responseStream.Close();         
         // Set the ManualResetEvent so the main thread can exit.
         allDone.Set();                           
      }
      return;
   }    
}