中止HttpWebRequest无法正常工作

Question

I am using this method to get google suggest query:

public void GetSuggestFromGoogle(string query)
    {
        try
        {
            query = HttpUtility.UrlEncode(query);
            string targetUrl = "http://suggestqueries.google.com/complete/search?ds=yt&output=toolbar&hl=en-US&q=" + query;

            UTF8Encoding utf8 = new UTF8Encoding();
            byte[] bytes = utf8.GetBytes(targetUrl);
            targetUrl = targetUrl.ToString();

            request = HttpWebRequest.Create(new Uri(targetUrl)) as HttpWebRequest;
            request.Method = "GET";

            IsDownload = true;
            request.BeginGetResponse(new AsyncCallback(FinishWebRequest), request);
        }
        catch
        {
            IsDownload = false;
            failCallback();
        }


    }

And before i call this method i use:

public void CancelGoogleRequest()
    {
        if (IsDownload)
        {
            request.Abort();
            IsDownload = false;
        }
    }

and this is the Finish Request Method:

private void FinishWebRequest(IAsyncResult result)
    {
        try
        {
            HttpWebResponse wResponse = (result.AsyncState as HttpWebRequest).EndGetResponse(result) as HttpWebResponse;
            Stream stream = wResponse.GetResponseStream();

            StreamReader reader = new StreamReader(stream);
            string xml = reader.ReadToEnd();

            List<string> list = this.ParseXml(xml);
            finishCallback(list);
            IsDownload = false;
        }
        catch//(Exception erere)
        {
            IsDownload = false;
            failCallback();
        }
    }

And my issue is with the Abort of HttpWebRequest , even if i am aborting request before calling a new one the FinishWebRequest method is execute, It is possible to cancel it?

Answer 1

From the Remarks section in HttpWebRequest.Abort :

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.

So you can't avoid the call. But you already have your IsDownload flag and could use it this way:

private void FinishWebRequest(IAsyncResult result)
{
    if (IsDownload)
    {
        // handle response
    }
}

when you change the Cancel method like this:

public void CancelGoogleRequest()
{
    if (IsDownload)
    {
        IsDownload = false; // set before Abort
        request.Abort();
    }
}

---

UPDATE: In case you can't use the IsDownload flag you could simply rely on the following (also from the Remarks):

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

As you already have a try block around your response handling code, everything should be ok. However you shouldn't set IsDownload = false in the exception handler when a new request has already been started, at least not when the WebException Status is RequestCanceled :

private void FinishWebRequest(IAsyncResult result)
{
    try
    {
        // handle response
    }
    catch (WebException webEx)
    {
        if (webEx.Status != WebExceptionStatus.RequestCanceled)
        {
            IsDownload = false;
            failCallback();
        }
    }
    catch
    {
        IsDownload = false;
        failCallback();
    }

}

---

Another idea. As Abort calls the FinishWebRequest callback synchronously, you may use an IsAborted flag like this:

public void CancelGoogleRequest()
{
    if (IsDownload)
    {
        IsDownload = false; // set before Abort
        IsAborting = true;
        request.Abort();
        IsAborting = false;
    }
}

with a callback like this:

private void FinishWebRequest(IAsyncResult result)
{
    if (!IsAborting)
    {
        // handle response
    }
}