取消 'HttpClient' POST 请求

Question

I am uploading an image with HttpClient.PostAsync() on my Windows Phone 8 app. The user has the option to cancel this upload via a UI button.

To cancel the POST request, I set a CancellationToken . But this doesn't work. After the cancellation request, I see still see the upload taking place in my proxy and it is evident that the request was ignored. My code:

using (var content = new MultipartFormDataContent())
{
    var file = new StreamContent(stream);
    file .Headers.ContentDisposition = new ContentDispositionHeaderValue("form-data")
    {
        FileName =  "filename.jpg",
    };
    file.Headers.ContentType = new MediaTypeHeaderValue("image/jpeg");
    content.Add(file);

    await httpclient.PostAsync(new Uri("myurl", UriKind.Absolute), content,
        cancellationToken);
}

Please also note that I have a CancellationTokenSource for the CancellationToken . After the user clicks on the Cancel button, tokensource.Cancel() is called. Also, the images in my testcase are from 1 to 2 MB (not that big).

So, is there a way to cancel an HttpClient POST request?

Answer 1

   try
   {
          var client = new HttpClient();

          var cts = new CancellationTokenSource();
          cts.CancelAfter(3000); // 3seconds

          var request = new HttpRequestMessage();

          await client.PostAsync(url, content, cts.Token);

  }
  catch(OperationCanceledException ex)
  {
          // timeout has been hit
  }

Answer 2

Cancelling a task doesn't terminate it instantly. You have to check manually before doing your work by checking the token's status:

if (ct.IsCancellationRequested) 
{
    ct.ThrowIfCancellationRequested();
}

// Post request here...

This article is quite useful: How to: Cancel a Task and Its Children

Answer 3

In implementations of HttpClient for Mono like in Xamarin, the get & post methods don't honor the cancellation token if the host is not reachable , and wait up to 2 minutes before failing. This is perhaps why CancellationTokenSource with a CancelAfter does not work here.

To fix this, the simplest method I found was using the Polly library https://github.com/App-vNext/Polly

var policy = Policy.TimeoutAsync(TimeSpan.FromMilliseconds(3000),
    TimeoutStrategy.Pessimistic,
    (context, timespan, task) => throw new Exception("Cannot connect to server."));

await policy.ExecuteAsync(async () =>
{
    var httpClient = new HttpClient();
    var response = await httpClient.PostAsync(...);
    response.EnsureSuccessStatusCode();
    ...
});