WebRequest.BeginGetResponse(AsyncCallback, Object) 方法

定义

命名空间:

System.Net

程序集:

netstandard.dll, System.Net.Requests.dll

Source:

WebRequest.cs

在后代类中重写时，开始对 Internet 资源的异步请求。

public:
 virtual IAsyncResult ^ BeginGetResponse(AsyncCallback ^ callback, System::Object ^ state);

public virtual IAsyncResult BeginGetResponse(AsyncCallback? callback, object? state);

abstract member BeginGetResponse : AsyncCallback * obj -> IAsyncResult
override this.BeginGetResponse : AsyncCallback * obj -> IAsyncResult

Public Overridable Function BeginGetResponse (callback As AsyncCallback, state As Object) As IAsyncResult

参数

callback

AsyncCallback

AsyncCallback 委托。

state

Object

包含此异步请求的状态信息的对象。

返回

IAsyncResult

引用异步请求的 IAsyncResult。

例外

NotImplementedException

在子代类中未重写该方法时，将尝试访问该方法。

示例

以下示例使用 BeginGetResponse 异步请求目标资源。 获取资源后，将执行指定的回调方法。

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

public static class WebRequestAPMSample
{
    private const int BufferSize = 1024;

    private class RequestState
    {
        public StringBuilder ResponseBuilder { get; }
        public byte[] ReadBuffer { get; }
        public WebRequest Request { get; }
        public WebResponse Response { get; set; }
        public Stream ResponseStream { get; set; }
        public RequestState(WebRequest request)
        {
            ReadBuffer = new byte[BufferSize];
            ResponseBuilder = new StringBuilder();
            Request = request;
        }
        public void OnResponseBytesRead(int read) => ResponseBuilder.Append(Encoding.UTF8.GetString(ReadBuffer, 0, read));
    }

    public static ManualResetEvent allDone = new ManualResetEvent(false);
    

    public static void Main()
    {
        try
        {
            // Create a WebRequest object to the desired URL.
            WebRequest webRequest = WebRequest.Create("http://www.contoso.com");
            webRequest.Timeout = 10_000; // Set 10sec timeout.

            // Create an instance of the RequestState and assign the previous myHttpWebRequest
            // object to its request field.
            RequestState requestState = new RequestState(webRequest);

            // Start the asynchronous request.
            IAsyncResult result = webRequest.BeginGetResponse(new AsyncCallback(ResponseCallback), requestState);

            // Wait for the response or for failure. The processing happens in the callback.
            allDone.WaitOne();

            // Release the WebResponse resources.
            requestState.Response?.Close();
        }
        catch (WebException e)
        {
            Console.WriteLine("\nMain(): WebException raised!");
            Console.WriteLine("\nMessage:{0}", e.Message);
            Console.WriteLine("\nStatus:{0}", e.Status);
            Console.WriteLine("Press any key to continue..........");
            Console.Read();
        }
        catch (Exception e)
        {
            Console.WriteLine("\nMain(): Exception raised!");
            Console.WriteLine("Source :{0} ", e.Source);
            Console.WriteLine("Message :{0} ", e.Message);
            Console.WriteLine("Press any key to continue..........");
            Console.Read();
        }
    }

    private static void HandleSyncResponseReadCompletion(IAsyncResult asyncResult)
    {
        RequestState requestState = (RequestState)asyncResult.AsyncState;
        Stream responseStream = requestState.ResponseStream;

        bool readComplete = false;
        while (asyncResult.CompletedSynchronously && !readComplete)
        {
            int read = responseStream.EndRead(asyncResult);
            if (read > 0)
            {
                requestState.OnResponseBytesRead(read);
                asyncResult = responseStream.BeginRead(requestState.ReadBuffer, 0, BufferSize, new AsyncCallback(ReadCallBack), requestState);
            }
            else
            {
                readComplete = true;
                HandleReadCompletion(requestState);
            }
        }
    }

    private static void ResponseCallback(IAsyncResult asynchronousResult)
    {
        try
        {
            // AsyncState is an instance of RequestState.
            RequestState requestState = (RequestState)asynchronousResult.AsyncState;
            WebRequest request = requestState.Request;
            requestState.Response = request.EndGetResponse(asynchronousResult);

            // Read the response into a Stream.
            Stream responseStream = requestState.Response.GetResponseStream();
            requestState.ResponseStream = responseStream;

            // Begin the Reading of the contents of the HTML page and print it to the console.
            IAsyncResult asynchronousReadResult = responseStream.BeginRead(requestState.ReadBuffer, 0, BufferSize, new AsyncCallback(ReadCallBack), requestState);
            HandleSyncResponseReadCompletion(asynchronousReadResult);
        }
        catch (WebException e)
        {
            Console.WriteLine("\nRespCallback(): Exception raised!");
            Console.WriteLine("\nMessage:{0}", e.Message);
            Console.WriteLine("\nStatus:{0}", e.Status);
            allDone.Set();
        }   
    }

    // Print the webpage to the standard output, close the stream and signal completion.
    private static void HandleReadCompletion(RequestState requestState)
    {
        Console.WriteLine("\nThe contents of the Html page are : ");
        if (requestState.ResponseBuilder.Length > 1)
        {
            string stringContent;
            stringContent = requestState.ResponseBuilder.ToString();
            Console.WriteLine(stringContent);
        }
        Console.WriteLine("Press any key to continue..........");
        Console.ReadLine();

        requestState.ResponseStream.Close();
        allDone.Set();
    }

    private static void ReadCallBack(IAsyncResult asyncResult)
    {
        if (asyncResult.CompletedSynchronously)
        {
            // To avoid recursive synchronous calls into ReadCallBack,
            // synchronous completion is handled at the BeginRead call-site.
            return;
        }

        try
        {
            RequestState requestState = (RequestState)asyncResult.AsyncState;
            Stream responseStream = requestState.ResponseStream;
            int read = responseStream.EndRead(asyncResult);
            // Read the HTML page and then print it to the console.
            if (read > 0)
            {
                requestState.OnResponseBytesRead(read);
                IAsyncResult asynchronousResult = responseStream.BeginRead(requestState.ReadBuffer, 0, BufferSize, new AsyncCallback(ReadCallBack), requestState);
                HandleSyncResponseReadCompletion(asynchronousResult);
            }
            else
            {
                HandleReadCompletion(requestState);
            }
        }
        catch (WebException e)
        {
            Console.WriteLine("\nReadCallBack(): Exception raised!");
            Console.WriteLine("\nMessage:{0}", e.Message);
            Console.WriteLine("\nStatus:{0}", e.Status);
            allDone.Set();
        }
    }
}

注解

谨慎

WebRequest、HttpWebRequest、ServicePoint和 WebClient 已过时，不应将其用于新开发。 请改用 HttpClient。

BeginGetResponse 方法启动响应的异步请求。 实现 AsyncCallback 委托的回调方法使用 EndGetResponse 方法从 Internet 资源返回 WebResponse。

注意

WebRequest 类是 abstract 类。 运行时 WebRequest 实例的实际行为由 WebRequest.Create 方法返回的后代类确定。 有关默认值和异常的详细信息，请参阅子代类的文档，例如 HttpWebRequest 和 FileWebRequest。

注意

如果引发 WebException，请使用异常的 Response 和 Status 属性来确定来自服务器的响应。

另请参阅

• EndGetResponse(IAsyncResult)
• GetResponse()
• 发出异步请求