PostAsync HttpClient error with Web Api - System.AggregateException "A task was canceled."

Question

I'm trying to call PostAsync method using System.Net.Http.HttpClient from the Web API. I get the following error:

System.AggregateException "A task was canceled."

Task:

Id = 1, Status = System.Threading.Tasks.TaskStatus.Canceled, Method = "{null}", Result = "{Not yet computed}"

Code:

using (HttpClientHandler handler = new HttpClientHandler())
{
    handler.Credentials = new NetworkCredential("MyUsername", "p@ssw0rd");

    using (HttpClient client = new HttpClient(handler))
    {
        var postData = new List<KeyValuePair<string, string>>();
        postData.Add(new KeyValuePair<string, string>("status", "Hello world"));

        HttpContent content = new FormUrlEncodedContent(postData);

        var responseTask = client.PostAsync(url, content).ContinueWith(
            (postTask) =>
            {
                postTask.Result.EnsureSuccessStatusCode();
            });
    }

I assume the responseTask will force the method to run synchronously?

It's a WPF application, not ASP.NET.

Answer 1

In terms of debugging you could try writing an extension method to get the exception:

public static HttpResponseMessage PostAsyncSafe(this HttpClient client, string requestUri, string content)
        {
            var requestContent = new StringContent(content, Encoding.UTF8, "application/x-www-form-urlencoded");
            return PerformActionSafe(() => (client.PostAsync(requestUri, requestContent)).Result);
        }

public static HttpResponseMessage PerformActionSafe(Func<HttpResponseMessage> action)
        {
            try
            {
                return action();
            }
            catch (AggregateException aex)
            {
                Exception firstException = null;
                if (aex.InnerExceptions != null && aex.InnerExceptions.Any())
                {
                    firstException = aex.InnerExceptions.First();

                    if (firstException.InnerException != null)
                        firstException = firstException.InnerException;
                }

                var response = new HttpResponseMessage(HttpStatusCode.InternalServerError)
                {
                    Content =
                        new StringContent(firstException != null
                                            ? firstException.ToString()
                                            : "Encountered an AggreggateException without any inner exceptions")
                };

                return response;
            }
        }

Answer 2

I was getting this same error and tracked it down to my HttpClient was timing out. The default timeout is 100 seconds. I added the following to the create of the HttpClient.

HttpClient httpClient = new HttpClient();
httpClient.Timeout = TimeSpan.FromMinutes(10);

Answer 3

Not synchronously, second task will be also executed async but chained with first task, therefore only after first task executed.

Seems to be first task - PostAsync was executed with error. Try to catch TPL aggregated exceptions and find more details in inner exceptions collection from AggregateException For example like here or subscribe to TaskScheduler.UnobservedTaskException and log there all your exceptions