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HttpWebRequest.Abort Method

Cancels a request to an Internet resource.

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

'Declaration
Public Overrides Sub Abort
'Usage
Dim instance As HttpWebRequest

instance.Abort()

The Abort method cancels a request to a resource. After a request is canceled, calling the GetResponse, BeginGetResponse, EndGetResponse, GetRequestStream, BeginGetRequestStream, or EndGetRequestStream method causes a WebException with the Status property set to RequestCanceled.

NoteNote:

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

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 this.

Imports System
Imports System.Net
Imports System.IO
Imports System.Text
Imports System.Threading
Imports Microsoft.VisualBasic

Public Class RequestState
   ' This class stores the State of the request. 
   Private BUFFER_SIZE As Integer = 1024
   Public requestData As StringBuilder
   Public BufferRead() As Byte
   Public request As HttpWebRequest
   Public response As HttpWebResponse
   Public streamResponse As Stream

   Public Sub New()
      BufferRead = New Byte(BUFFER_SIZE) {}
      requestData = New StringBuilder("")
      request = Nothing
      streamResponse = Nothing 
   End Sub 'New 
End Class 'RequestState


Class HttpWebRequest_BeginGetResponse

   Public Shared allDone As New ManualResetEvent(False)
   Private BUFFER_SIZE As Integer = 1024
   Private DefaultTimeout As Integer = 2 * 60 * 1000

    ' 2 minutes timeout 
   ' Abort the request if the timer fires. 
   Private Shared Sub TimeoutCallback(state As Object, timedOut As Boolean)
      If timedOut Then 
         Dim request As HttpWebRequest = state 

         If Not (request Is Nothing) Then
            request.Abort()
         End If 
      End If 
   End Sub 'TimeoutCallback


   Shared Sub Main()

      Try 
         ' Create a HttpWebrequest object to the desired URL.  
            Dim myHttpWebRequest As HttpWebRequest = WebRequest.Create("http://www.contoso.com")

         ' Create an instance of the RequestState and assign the previous myHttpWebRequest 
         ' object to its request field.   

         Dim myRequestState As New RequestState()
         myRequestState.request = myHttpWebRequest

         Dim myResponse As New HttpWebRequest_BeginGetResponse()

         ' Start the asynchronous request. 
         Dim result As IAsyncResult = CType(myHttpWebRequest.BeginGetResponse(New AsyncCallback(AddressOf RespCallback), myRequestState), IAsyncResult)

            ' this line implements the timeout, if there is a timeout, the callback fires and the request aborts.
         ThreadPool.RegisterWaitForSingleObject(result.AsyncWaitHandle, New WaitOrTimerCallback(AddressOf TimeoutCallback), myHttpWebRequest, myResponse.DefaultTimeout, True)

         ' The response came in the allowed time. The work processing will happen in the  
         ' callback function.
         allDone.WaitOne()

         ' Release the HttpWebResponse resource.
         myRequestState.response.Close()
      Catch e As WebException
         Console.WriteLine(ControlChars.Lf + "Main Exception raised!")
         Console.WriteLine(ControlChars.Lf + "Message:{0}", e.Message)
         Console.WriteLine(ControlChars.Lf + "Status:{0}", e.Status)
         Console.WriteLine("Press any key to continue..........")
      Catch e As Exception
         Console.WriteLine(ControlChars.Lf + "Main Exception raised!")
         Console.WriteLine("Source :{0} ", e.Source)
         Console.WriteLine("Message :{0} ", e.Message)
         Console.WriteLine("Press any key to continue..........")
         Console.Read()
      End Try 
   End Sub 'Main

   Private Shared Sub RespCallback(asynchronousResult As IAsyncResult)
      Try 
         ' State of request is asynchronous. 
         Dim myRequestState As RequestState = CType(asynchronousResult.AsyncState, RequestState)
         Dim myHttpWebRequest As HttpWebRequest = myRequestState.request
         myRequestState.response = CType(myHttpWebRequest.EndGetResponse(asynchronousResult), HttpWebResponse)

         ' Read the response into a Stream object. 
         Dim responseStream As Stream = myRequestState.response.GetResponseStream()
         myRequestState.streamResponse = responseStream

         ' Begin the Reading of the contents of the HTML page and print it to the console. 
         Dim asynchronousInputRead As IAsyncResult = responseStream.BeginRead(myRequestState.BufferRead, 0, 1024, New AsyncCallback(AddressOf ReadCallBack), myRequestState)
         Return 
      Catch e As WebException
         Console.WriteLine(ControlChars.Lf + "RespCallback Exception raised!")
         Console.WriteLine(ControlChars.Lf + "Message:{0}", e.Message)
         Console.WriteLine(ControlChars.Lf + "Status:{0}", e.Status)
      End Try
      allDone.Set()
   End Sub 'RespCallback

   Private Shared Sub ReadCallBack(asyncResult As IAsyncResult)
      Try 

         Dim myRequestState As RequestState = CType(asyncResult.AsyncState, RequestState)
         Dim responseStream As Stream = myRequestState.streamResponse
         Dim read As Integer = responseStream.EndRead(asyncResult)
         ' Read the HTML page and then print it to the console. 
         If read > 0 Then
            myRequestState.requestData.Append(Encoding.ASCII.GetString(myRequestState.BufferRead, 0, read))
            Dim asynchronousResult As IAsyncResult = responseStream.BeginRead(myRequestState.BufferRead, 0, 1024, New AsyncCallback(AddressOf ReadCallBack), myRequestState)
            Return 
         Else
            Console.WriteLine(ControlChars.Lf + "The contents of the Html page are : ")
            If myRequestState.requestData.Length > 1 Then 
               Dim stringContent As String
               stringContent = myRequestState.requestData.ToString()
               Console.WriteLine(stringContent)
            End If
            Console.WriteLine("Press any key to continue..........")
            Console.ReadLine()

            responseStream.Close()
         End If 

      Catch e As WebException
         Console.WriteLine(ControlChars.Lf + "ReadCallBack Exception raised!")
         Console.WriteLine(ControlChars.Lf + "Message:{0}", e.Message)
         Console.WriteLine(ControlChars.Lf + "Status:{0}", e.Status)
      End Try
      allDone.Set()
   End Sub 'ReadCallBack 
End Class 'HttpWebRequest_BeginGetResponse
#using <mscorlib.dll>
#using <System.dll>
using namespace System;
using namespace System::Net;
using namespace System::IO;
using namespace System::Text;
using namespace System::Threading;

public __gc class RequestState {
   // This class stores the State of the request.
   const int BUFFER_SIZE;
public:
   StringBuilder* requestData;
   Byte BufferRead[];
   HttpWebRequest* request;
   HttpWebResponse* response;
   Stream* streamResponse;

   RequestState() : BUFFER_SIZE(1024){
      BufferRead = new Byte[BUFFER_SIZE];
      requestData = new StringBuilder(S"");
      request = 0;
      streamResponse = 0;
   }
};

__gc class HttpWebRequest_BeginGetResponse {
public:
   static ManualResetEvent* allDone = new ManualResetEvent(false);
   static const int BUFFER_SIZE = 1024;
   static const 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 != 0) {
            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,
            new AsyncCallback(0, ReadCallBack),
            myRequestState);
         return;
      } catch (WebException* e) {
         Console::WriteLine(S"\nRespCallback Exception raised!");
         Console::WriteLine(S"\nMessage: {0}", e->Message);
         Console::WriteLine(S"\nStatus: {0}", __box(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,
               new AsyncCallback(0, ReadCallBack),
               myRequestState);
            return;
         } else {
            Console::WriteLine(S"\nThe contents of the Html page are : ");
            if (myRequestState->requestData->Length>1) {
               String* stringContent;
               stringContent = myRequestState->requestData->ToString();
               Console::WriteLine(stringContent);
            }
            Console::WriteLine(S"Press any key to continue..........");
            Console::ReadLine();

            responseStream->Close();
         }

      } catch (WebException* e) {
         Console::WriteLine(S"\nReadCallBack Exception raised!");
         Console::WriteLine(S"\nMessage: {0}", e->Message);
         Console::WriteLine(S"\nStatus: {0}", __box(e->Status));
      }
      allDone->Set();
   }
};

int main() {

   try {
      // Create a HttpWebrequest object to the desired URL.
      HttpWebRequest * myHttpWebRequest= dynamic_cast<HttpWebRequest*>
         (WebRequest::Create(S"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 = new RequestState();
      myRequestState->request = myHttpWebRequest;

      // Start the asynchronous request.
      IAsyncResult* result= dynamic_cast<IAsyncResult*>(myHttpWebRequest->BeginGetResponse(
         new AsyncCallback(0, 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,
         new WaitOrTimerCallback(0, 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(S"\nMain Exception raised!");
      Console::WriteLine(S"\nMessage: {0}", e->Message);
      Console::WriteLine(S"\nStatus: {0}", __box(e->Status));
      Console::WriteLine(S"Press any key to continue..........");
   } catch (Exception* e) {
      Console::WriteLine(S"\nMain Exception raised!");
      Console::WriteLine(S"Source : {0} " , e->Source);
      Console::WriteLine(S"Message : {0} " , e->Message);
      Console::WriteLine(S"Press any key to continue..........");
      Console::Read();
   }
}

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

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

.NET Framework

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

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0
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