HttpClient - A task was cancelled?

问题

It works fine when have one or two tasks however throws an error "A task was cancelled" when we have more than one task listed.

List<Task> allTasks = new List<Task>();
allTasks.Add(....);
allTasks.Add(....);
Task.WaitAll(allTasks.ToArray(), configuration.CancellationToken);


private static Task<T> HttpClientSendAsync<T>(string url, object data, HttpMethod method, string contentType, CancellationToken token)
{
    HttpRequestMessage httpRequestMessage = new HttpRequestMessage(method, url);
    HttpClient httpClient = new HttpClient();
    httpClient.Timeout = new TimeSpan(Constants.TimeOut);

    if (data != null)
    {
        byte[] byteArray = Encoding.ASCII.GetBytes(Helper.ToJSON(data));
        MemoryStream memoryStream = new MemoryStream(byteArray);
        httpRequestMessage.Content = new StringContent(new StreamReader(memoryStream).ReadToEnd(), Encoding.UTF8, contentType);
    }

    return httpClient.SendAsync(httpRequestMessage).ContinueWith(task =>
    {
        var response = task.Result;
        return response.Content.ReadAsStringAsync().ContinueWith(stringTask =>
        {
            var json = stringTask.Result;
            return Helper.FromJSON<T>(json);
        });
    }).Unwrap();
}

回答1:

There's 2 likely reasons that a TaskCanceledException would be thrown:

Something called Cancel() on the CancellationTokenSource associated with the cancellation token before the task completed.
The request timed out, i.e. didn't complete within the timespan you specified on HttpClient.Timeout.

My guess is it was a timeout. (If it was an explicit cancellation, you probably would have figured that out.) You can be more certain by inspecting the exception:

try
{
    var response = task.Result;
}
catch (TaskCanceledException ex)
{
    // Check ex.CancellationToken.IsCancellationRequested here.
    // If false, it's pretty safe to assume it was a timeout.
}

回答2:

I ran into this issue because my Main() method wasn't waiting for the task to complete before returning, so the Task<HttpResponseMessage> myTask was being cancelled when my console program exited.

The solution was to call myTask.GetAwaiter().GetResult() in Main() (from this answer).

回答3:

Another possibility is that the result is not awaited on the client side. This can happen if any one method on the call stack does not use the await keyword to wait for the call to be completed.

回答4:

var clientHttp = new HttpClient();
clientHttp.Timeout = TimeSpan.FromMinutes(30);

The above is the best approach for waiting on a large request. You are confused about 30 minutes; it's random time and you can give any time that you want.

In other words, request will not wait for 30 minutes if they get results before 30 minutes. 30 min means request processing time is 30 min. When we occurred error "Task was cancelled", or large data request requirements.

回答5:

Another reason can be that if you are running the service (API) and put a breakpoint in the service (and your code is stuck at some breakpoint (e.g Visual Studio solution is showing Debugging instead of Running)). and then hitting the API from the client code. So if the service code a paused on some breakpoint, you just hit F5 in VS.

来源：https://stackoverflow.com/questions/29179848/httpclient-a-task-was-cancelled

标签

task-parallel-library

dotnet-httpclient