Async/await with CancellationToken doesn't cancel the operation

I want to use the CancellationToken to abort a file download. This is what I tried:

public async Task retrieveDocument(Document document)
{
    // do some preparation work first before retrieving the document (not shown here)
    if (cancelToken == null)
    {
        cancelToken = new CancellationTokenSource();
        try
        {
            Document documentResult = await webservice.GetDocumentAsync(document.Id, cancelToken.Token);
            // do some other stuff (checks ...)
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("abort download");
        }
        finally
        {
            cancelToken = null;
        }
    }
    else
    {
        cancelToken.Cancel();
        cancelToken = null;
    }
}

public async Task<Document> GetDocumentAsync(string documentId, CancellationToken cancelToken)
{
    Document documentResult = new Document();

    try
    {

        cancelToken.ThrowIfCancellationRequested();

        documentResult = await Task.Run(() => manager.GetDocumentById(documentId));
    }

    return documentResult;
}

The cancelToken should then be used to cancel the operation:

public override void DidReceiveMemoryWarning ()
{
    // Releases the view if it doesn't have a superview.
    base.DidReceiveMemoryWarning ();

    if (cancelToken != null) {
        Console.WriteLine ("Token cancelled");
        cancelToken.Cancel ();
    }
}

It seems that IsCancellationRequested is not updated. So the operation is not cancelled. I also tried to use this

cancelToken.ThrowIfCancellationRequested();
try{
    documentResult = await Task.Run(() => manager.GetDocumentById (documentId), cancelToken);
} catch(TaskCanceledException){
    Console.WriteLine("task canceled here");
}

but nothing changed.

What I'm doing wrong?

Edit:

Here are the missing parts like GetDocumentById:

public Document GetDocumentById(string docid)
{
    GetDocumentByIdResult res;
    try
    {
        res = ws.CallGetDocumentById(session, docid);
    }
    catch (WebException e)
    {
        throw new NoResponseFromServerException(e.Message);
    }

    return res;
}

public Document CallGetDocumentById(Session session, string parmsstring)
{
    XmlDocument soapEnvelope = Factory.GetGetDocumentById(parmsstring);
    HttpWebRequest webRequest = CreateWebRequest(session);
    webRequest = InsertEnvelope(soapEnvelope, webRequest);
    string result = WsGetResponseString(webRequest);
    return ParseDocument(result);
}

static string WsGetResponseString(WebRequest webreq)
{
    string soapResult = "";
    IAsyncResult asyncResult = webreq.BeginGetResponse(null, null);
    if (asyncResult.AsyncWaitHandle.WaitOne(50000))
    {
        using (WebResponse webResponse = webreq.EndGetResponse(asyncResult))
        {
            if (webResponse != null)
            {
                using (var rd = new StreamReader(webResponse.GetResponseStream()))
                {
                    soapResult = rd.ReadToEnd();
                }
            }
        }
    }
    else
    {
        webreq.Abort();
        throw new NoResponseFromServerException();
    }

    return soapResult;
}

Solution

I want to use the CancellationToken to abort a file download

Downloading a file is an I/O operation, for which asynchronous cancelable (I/O completion port based) functions are available on the .NET platform. Yet you seem to not be using them.

Instead you appear to be creating (a chain of) tasks using Task.Run that perform blocking I/O, where a cancelation token is not passed on to each task in your Task.Run chain.

For examples of doing async, awaitable and cancelable file downloads, refer to:

Using HttpClient: How to copy HttpContent async and cancelable?
Windows Phone: Downloading and saving a file Async in Windows Phone 8
Using WebClient: Has its own cancellation mechanism: the CancelAsync method, you can connect it to your cancellation token, using the token's Register method:
```
myToken.Register(myWebclient.CancelAsync);
```
Using the abstract WebRequest: If it was not created using an attached cancelation token, as seems to be the case for your edited example, and you are not actually downloading a file, but reading a content string, you need to use a combination of a few of the earlier mentioned methods.

You can do the following:

static async Task<string> WsGetResponseString(WebRequest webreq, CancellationToken cancelToken)`
{
    cancelToken.Register(webreq.Abort);
    using (var response = await webReq.GetResponseAsync())
    using (var stream = response.GetResponseStream())
    using (var destStream = new MemoryStream())
    {
        await stream.CopyToAsync(destStream, 4096, cancelToken);
        return Encoding.UTF8.GetString(destStream.ToArray());
    }
}