How to attach CancellationTokenSource to DownloadStringTaskAsync method and cancel the async call?

Question

I an creating a sample example to call link using WebClient using async and await method now I want to attach cancel async call functionality also. But I am not able to get CancellationTokenSource token and attach DownloadStringTaskAsync to this cancellation token. Following Is my code can anyone tell me how to accomplish this.

private async void DoWork()
        {
            this.Cursor = Cursors.WaitCursor;
            Write("DoWork started.");
            cts = new CancellationTokenSource();
            WebClient wc = new WebClient();
            string result = await wc.DownloadStringTaskAsync(new Uri("http://gyorgybalassy.wordpress.com"));

            if (result.Length < 100000)
            {
                Write("The result is too small, download started from second URL.");
                result = await wc.DownloadStringTaskAsync(new Uri("https://www.facebook.com/balassy"));
            }
            Write("Download completed. Downloaded bytes: " + result.Length.ToString());
            Write("DoWork ended.");
            this.Cursor = Cursors.Default;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            Write("Cancellation started.");
            this.cts.Cancel();
            Write("Cancellation ended.");
        }

When my Cancel button calls cts.Cancel the DownloadStringTaskAsync call is not canceled. Why cancel button is not able to cancel the Async calls?

Answer 1

The async capabilities of WebClient predate .Net 4.5, so it supports the Task-based Asynchronous Pattern only partially. That includes having its own cancellation mechanism: the CancelAsync() method, which works even with the new -TaskAsync methods. To call this method when a CancellationToken is canceled, you can use its Register() method:

cts.Token.Register(wc.CancelAsync);

As an alternative, you could use the new HttpClient, as Stephen suggested, which fully supports TAP, including CancellationTokens.

Answer 2

Extension methods based on svick's answer:

public static async Task<string> DownloadStringTaskAsync(this WebClient webClient, string url, CancellationToken cancellationToken) {
    using (cancellationToken.Register(webClient.CancelAsync)) {
        return await webClient.DownloadStringTaskAsync(url);
    }
}

public static async Task<string> DownloadStringTaskAsync(this WebClient webClient, Uri uri, CancellationToken cancellationToken) {
    using (cancellationToken.Register(webClient.CancelAsync)) {
        return await webClient.DownloadStringTaskAsync(uri);
    }
}

Answer 3

WebClient doesn't support cancellation. I recommend you use a newer type such as HttpClient:

...
cts = new CancellationTokenSource();
string result;
using (var client = new HttpClient())
using (var response = await client.GetAsync("http://gyorgybalassy.wordpress.com", cts.Token))
{
  result = await response.Content.ReadAsStringAsync();
}

if (result.Length < 100000)
...

The GetAsync method by default will not complete until it reads the entire response, so the await response.Content.ReadAsStringAsync line will actually complete synchronously.