HttpWebRequest::BeginGetResponse Method

Begins an asynchronous request to an Internet resource.

Namespace:  System.Net
Assembly:  System (in System.dll)

[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
public:
virtual IAsyncResult^ BeginGetResponse(
	AsyncCallback^ callback, 
	Object^ state
) override

Parameters

callback
Type: System::AsyncCallback

The AsyncCallback delegate

state
Type: System::Object

The state object for this request.

Return Value

Type: System::IAsyncResult
An IAsyncResult that references the asynchronous request for a response.

ExceptionCondition
InvalidOperationException

The stream is already in use by a previous call to BeginGetResponse

-or-

TransferEncoding is set to a value and SendChunked is false.

-or-

The thread pool is running out of threads.

ProtocolViolationException

Method is GET or HEAD, and either ContentLength is greater than zero or SendChunked is true.

-or-

KeepAlive is true, AllowWriteStreamBuffering is false, and either ContentLength is -1, SendChunked is false and Method is POST or PUT.

-or-

The HttpWebRequest has an entity body but the BeginGetResponse method is called without calling the BeginGetRequestStream method.

-or-

The ContentLength is greater than zero, but the application does not write all of the promised data.

WebException

Abort was previously called.

The BeginGetResponse method starts an asynchronous request for a response from the Internet resource. The asynchronous callback method uses the EndGetResponse method to return the actual WebResponse.

A ProtocolViolationException is thrown in several cases when the properties set on the HttpWebRequest class are conflicting. This exception occurs if an application sets the ContentLength property and the SendChunked property to true, and then sends an HTTP GET request. This exception occurs if an application tries to send chunked to a server that only supports HTTP 1.0 protocol, where this is not supported. This exception occurs if an application tries to send data without setting the ContentLength property or the SendChunked is false when buffering is disabled and on a keepalive connection (the KeepAlive property is true).

If a WebException is thrown, use the Response and Status properties of the exception to determine the response from the server.

The BeginGetResponse method requires some synchronous setup tasks to complete (DNS resolution, proxy detection, and TCP socket connection, for example) before this method becomes asynchronous. As a result, this method should never be called on a user interface (UI) thread because it might take considerable time (up to several minutes depending on network settings) to complete the initial synchronous setup tasks before an exception for an error is thrown or the method succeeds.

To learn more about the thread pool, see The Managed Thread Pool.

NoteNote

Your application cannot mix synchronous and asynchronous methods for a particular request. If you call the BeginGetRequestStream method, you must use the BeginGetResponse method to retrieve the response.

NoteNote

This member outputs trace information when you enable network tracing in your application. For more information, see Network Tracing in the .NET Framework.

NoteNote

The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The following code example uses the BeginGetResponse method to make an asynchronous request for an Internet resource.

NoteNote

In the case of asynchronous requests, it is the responsibility of the client application to implement its own time-out mechanism. The following code example shows how to do it.

#using <System.dll>

using namespace System;
using namespace System::Net;
using namespace System::IO;
using namespace System::Text;
using namespace System::Threading;
public ref class RequestState
{
private:

   // This class stores the State of the request. 
   const int BUFFER_SIZE;

public:
   StringBuilder^ requestData;
   array<Byte>^BufferRead;
   HttpWebRequest^ request;
   HttpWebResponse^ response;
   Stream^ streamResponse;
   RequestState()
      : BUFFER_SIZE( 1024 )
   {
      BufferRead = gcnew array<Byte>(BUFFER_SIZE);
      requestData = gcnew StringBuilder( "" );
      request = nullptr;
      streamResponse = nullptr;
   }

};

ref class HttpWebRequest_BeginGetResponse
{
public:
   static ManualResetEvent^ allDone = gcnew ManualResetEvent( false );
   literal int BUFFER_SIZE = 1024;
   literal int DefaultTimeOut = 120000; // 2 minute timeout  

   // Abort the request if the timer fires. 
   static void TimeoutCallback( Object^ state, bool timedOut )
   {
      if ( timedOut )
      {
         HttpWebRequest^ request = dynamic_cast<HttpWebRequest^>(state);
         if ( request != nullptr )
         {
            request->Abort();
         }
      }
   }

   static void RespCallback( IAsyncResult^ asynchronousResult )
   {
      try
      {

         // State of request is asynchronous.
         RequestState^ myRequestState = dynamic_cast<RequestState^>(asynchronousResult->AsyncState);
         HttpWebRequest^ myHttpWebRequest = myRequestState->request;
         myRequestState->response = dynamic_cast<HttpWebResponse^>(myHttpWebRequest->EndGetResponse( asynchronousResult ));

         // Read the response into a Stream object.
         Stream^ responseStream = myRequestState->response->GetResponseStream();
         myRequestState->streamResponse = responseStream;

         // Begin the Reading of the contents of the HTML page and print it to the console.
         IAsyncResult^ asynchronousInputRead = responseStream->BeginRead( myRequestState->BufferRead, 0, BUFFER_SIZE, gcnew AsyncCallback( ReadCallBack ), myRequestState );
         return;
      }
      catch ( WebException^ e ) 
      {
         Console::WriteLine( "\nRespCallback Exception raised!" );
         Console::WriteLine( "\nMessage: {0}", e->Message );
         Console::WriteLine( "\nStatus: {0}", e->Status );
      }

      allDone->Set();
   }

   static void ReadCallBack( IAsyncResult^ asyncResult )
   {
      try
      {
         RequestState^ myRequestState = dynamic_cast<RequestState^>(asyncResult->AsyncState);
         Stream^ responseStream = myRequestState->streamResponse;
         int read = responseStream->EndRead( asyncResult );

         // Read the HTML page and then print it to the console. 
         if ( read > 0 )
         {
            myRequestState->requestData->Append( Encoding::ASCII->GetString( myRequestState->BufferRead, 0, read ) );
            IAsyncResult^ asynchronousResult = responseStream->BeginRead( myRequestState->BufferRead, 0, BUFFER_SIZE, gcnew AsyncCallback( ReadCallBack ), myRequestState );
            return;
         }
         else
         {
            Console::WriteLine( "\nThe contents of the Html page are : " );
            if ( myRequestState->requestData->Length > 1 )
            {
               String^ stringContent;
               stringContent = myRequestState->requestData->ToString();
               Console::WriteLine( stringContent );
            }
            Console::WriteLine( "Press any key to continue.........." );
            Console::ReadLine();
            responseStream->Close();
         }
      }
      catch ( WebException^ e ) 
      {
         Console::WriteLine( "\nReadCallBack Exception raised!" );
         Console::WriteLine( "\nMessage: {0}", e->Message );
         Console::WriteLine( "\nStatus: {0}", e->Status );
      }

      allDone->Set();
   }

};

int main()
{
   try
   {

      // Create a HttpWebrequest object to the desired URL.
      HttpWebRequest^ myHttpWebRequest = dynamic_cast<HttpWebRequest^>(WebRequest::Create( "http://www.contoso.com" ));

      /**
            * If you are behind a firewall and you do not have your browser proxy setup
            * you need to use the following proxy creation code.

            // Create a proxy object.
            WebProxy* myProxy = new WebProxy();

            // Associate a new Uri object to the _wProxy object, using the proxy address
            // selected by the user.
            myProxy.Address = new Uri(S"http://myproxy");

            // Finally, initialize the Web request object proxy property with the _wProxy
            // object.
            myHttpWebRequest.Proxy=myProxy;
            ***/ 
      // Create an instance of the RequestState and assign the previous myHttpWebRequest 
      // object to its request field.
      RequestState^ myRequestState = gcnew RequestState;
      myRequestState->request = myHttpWebRequest;

      // Start the asynchronous request.
      IAsyncResult^ result = dynamic_cast<IAsyncResult^>(myHttpWebRequest->BeginGetResponse( gcnew AsyncCallback( HttpWebRequest_BeginGetResponse::RespCallback ), myRequestState ));

      // this line impliments the timeout, if there is a timeout, the callback fires and the request becomes aborted
      ThreadPool::RegisterWaitForSingleObject( result->AsyncWaitHandle, gcnew WaitOrTimerCallback( HttpWebRequest_BeginGetResponse::TimeoutCallback ), myHttpWebRequest, HttpWebRequest_BeginGetResponse::DefaultTimeOut, true );

      // The response came in the allowed time. The work processing will happen in the 
      // callback function.
      HttpWebRequest_BeginGetResponse::allDone->WaitOne();

      // Release the HttpWebResponse resource.
      myRequestState->response->Close();
   }
   catch ( WebException^ e ) 
   {
      Console::WriteLine( "\nMain Exception raised!" );
      Console::WriteLine( "\nMessage: {0}", e->Message );
      Console::WriteLine( "\nStatus: {0}", e->Status );
      Console::WriteLine( "Press any key to continue.........." );
   }
   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();
   }

}

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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