HttpWebRequest.Abort Method ()

 

Cancels a request to an Internet resource.

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

Public Overrides Sub 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.

The Abort method will synchronously execute the callback specified to the BeginGetRequestStream or BeginGetResponse methods if the Abort method is called while either of these operations are outstanding. This can lead to potential deadlock issues.

System_CAPS_noteNote

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

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 'NewNew
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

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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