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

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.

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.

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);
    }
}