8

我想使用 aCancellationToken来取消对 的调用HttpClient.PostAsJsonAsync。但是，通过以下设置，调用会PostAsJsonAsync无限期挂起（我让它运行了几分钟）。

CancellationTokenSource source = new CancellationTokenSource();
source.Cancel();
HttpClient client = new HttpClient();

try
{
    var task = client.PostAsJsonAsync<MyObject>("http://server-address.com",
        new MyObject(), source.Token);

    task.Wait();
}
catch (Exception ex)
{
    //Never gets hit.
}

请注意，我传递了一个已经取消的CancellationTokenSource- 如果我在Task.Delay短时间内取消使用令牌，我也会遇到同样的问题。

我意识到我可以简单地在调用之前检查令牌是否已被取消，但即便如此，如果令牌在短暂延迟后被取消，我也会遇到同样的问题，即它在方法调用之前没有被取消，但在之后不久就变得如此它。

所以我的问题是，是什么原因造成的，我能做些什么来解决/修复它？

编辑

对于那些寻找解决方法的人，受@Darrel Miller 的回答启发，我想出了以下扩展方法：

public static async Task<HttpResponseMessage> PostAsJsonAsync2<T>(this HttpClient client, string requestUri, T value, CancellationToken token)
{
    var content = new ObjectContent(typeof(T), value, new JsonMediaTypeFormatter());
    await content.LoadIntoBufferAsync();

    return await client.PostAsync(requestUri, content, token);
}
4

2 回答 2

7

这似乎是您遇到的一个错误您可以通过自己构建 HttpContent/ObjectContent 对象来解决它，就像这样。

CancellationTokenSource source = new CancellationTokenSource();
source.Cancel();
HttpClient client = new HttpClient();

var content = new ObjectContent(typeof (MyObject), new MyObject(), new JsonMediaTypeFormatter());
content.LoadIntoBufferAsync().Wait();
try
{
    var task = client.PostAsync("http://server-address.com",content, source.Token);

    task.Wait();
}
catch (Exception ex)
{
    //This will get hit now with an AggregateException containing a TaskCancelledException.
}

调用content.LoadIntoBufferAsync强制反序列化在 PostAsync 之前发生，似乎可以避免死锁。

于 2014-04-15T14:09:02.850 回答
6

同意@Darrel Miller 的回答。这是一个错误。只需为错误报告添加更多详细信息。

问题是在内部TaskCompletionSource使用了 a ，但是当在这种特定情况下由于取消而引发异常时，它不会被捕获，并且永远不会设置为已完成状态之一（TaskCompletionSource因此，等待永远不会回来。 TaskCompletionSourceTask

使用 ILSpy，看着HttpClientHandler.SendAsync你可以看到TaskCompletionSource：

// System.Net.Http.HttpClientHandler
/// <summary>Creates an instance of  <see cref="T:System.Net.Http.HttpResponseMessage" /> based on the information provided in the <see cref="T:System.Net.Http.HttpRequestMessage" /> as an operation that will not block.</summary>
/// <returns>Returns <see cref="T:System.Threading.Tasks.Task`1" />.The task object representing the asynchronous operation.</returns>
/// <param name="request">The HTTP request message.</param>
/// <param name="cancellationToken">A cancellation token to cancel the operation.</param>
/// <exception cref="T:System.ArgumentNullException">The <paramref name="request" /> was null.</exception>
protected internal override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    if (request == null)
    {
        throw new ArgumentNullException("request", SR.net_http_handler_norequest);
    }
    this.CheckDisposed();
    if (Logging.On)
    {
        Logging.Enter(Logging.Http, this, "SendAsync", request);
    }
    this.SetOperationStarted();
    TaskCompletionSource<HttpResponseMessage> taskCompletionSource = new TaskCompletionSource<HttpResponseMessage>();
    HttpClientHandler.RequestState requestState = new HttpClientHandler.RequestState();
    requestState.tcs = taskCompletionSource;
    requestState.cancellationToken = cancellationToken;
    requestState.requestMessage = request;
    this.lastUsedRequestUri = request.RequestUri;
    try
    {
        HttpWebRequest httpWebRequest = this.CreateAndPrepareWebRequest(request);
        requestState.webRequest = httpWebRequest;
        cancellationToken.Register(HttpClientHandler.onCancel, httpWebRequest);
        if (ExecutionContext.IsFlowSuppressed())
        {
            IWebProxy webProxy = null;
            if (this.useProxy)
            {
                webProxy = (this.proxy ?? WebRequest.DefaultWebProxy);
            }
            if (this.UseDefaultCredentials || this.Credentials != null || (webProxy != null && webProxy.Credentials != null))
            {
                this.SafeCaptureIdenity(requestState);
            }
        }
        Task.Factory.StartNew(this.startRequest, requestState);
    }
    catch (Exception e)
    {
        this.HandleAsyncException(requestState, e);
    }
    if (Logging.On)
    {
        Logging.Exit(Logging.Http, this, "SendAsync", taskCompletionSource.Task);
    }
    return taskCompletionSource.Task;
}

稍后，通过该行Task.Factory.StartNew(this.startRequest, requestState);我们得到以下方法：

// System.Net.Http.HttpClientHandler
private void PrepareAndStartContentUpload(HttpClientHandler.RequestState state)
{
    HttpContent requestContent = state.requestMessage.Content;
    try
    {
        if (state.requestMessage.Headers.TransferEncodingChunked == true)
        {
            state.webRequest.SendChunked = true;
            this.StartGettingRequestStream(state);
        }
        else
        {
            long? contentLength = requestContent.Headers.ContentLength;
            if (contentLength.HasValue)
            {
                state.webRequest.ContentLength = contentLength.Value;
                this.StartGettingRequestStream(state);
            }
            else
            {
                if (this.maxRequestContentBufferSize == 0L)
                {
                    throw new HttpRequestException(SR.net_http_handler_nocontentlength);
                }
                requestContent.LoadIntoBufferAsync(this.maxRequestContentBufferSize).ContinueWithStandard(delegate(Task task)
                {
                    if (task.IsFaulted)
                    {
                        this.HandleAsyncException(state, task.Exception.GetBaseException());
                        return;
                    }
                    contentLength = requestContent.Headers.ContentLength;
                    state.webRequest.ContentLength = contentLength.Value;
                    this.StartGettingRequestStream(state);
                });
            }
        }
    }
    catch (Exception e)
    {
        this.HandleAsyncException(state, e);
    }
}

您会注意到调用中的委托在委托中ContinueWithStandard没有异常处理，并且没有人保留返回的任务（因此，当此任务引发异常时，它会被忽略）。调用this.StartGettingRequestStream(state);确实会引发异常：

System.Net.WebException occurred
  HResult=-2146233079
  Message=The request was aborted: The request was canceled.
  Source=System
  StackTrace:
       at System.Net.HttpWebRequest.BeginGetRequestStream(AsyncCallback callback, Object state)
  InnerException:

这是异常发生时的完整调用堆栈：

>   System.dll!System.Net.HttpWebRequest.BeginGetRequestStream(System.AsyncCallback callback, object state) Line 1370   C#
    System.Net.Http.dll!System.Net.Http.HttpClientHandler.StartGettingRequestStream(System.Net.Http.HttpClientHandler.RequestState state) + 0x82 bytes  
    System.Net.Http.dll!System.Net.Http.HttpClientHandler.PrepareAndStartContentUpload.AnonymousMethod__0(System.Threading.Tasks.Task task) + 0x92 bytes    
    mscorlib.dll!System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke() Line 59 + 0xc bytes  C#
    mscorlib.dll!System.Threading.Tasks.Task.Execute() Line 2459 + 0xb bytes    C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecutionContextCallback(object obj) Line 2815 + 0x9 bytes C#
    mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 581 + 0xd bytes  C#
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 530 + 0xd bytes  C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot) Line 2785  C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteEntry(bool bPreventDoubleExecution) Line 2728   C#
    mscorlib.dll!System.Threading.Tasks.ThreadPoolTaskScheduler.TryExecuteTaskInline(System.Threading.Tasks.Task task, bool taskWasPreviouslyQueued) Line 91 + 0xb bytes    C#
    mscorlib.dll!System.Threading.Tasks.TaskScheduler.TryRunInline(System.Threading.Tasks.Task task, bool taskWasPreviouslyQueued) Line 221 + 0x12 bytes    C#
    mscorlib.dll!System.Threading.Tasks.TaskContinuation.InlineIfPossibleOrElseQueue(System.Threading.Tasks.Task task, bool needsProtection) Line 259 + 0xe bytes   C#
    mscorlib.dll!System.Threading.Tasks.StandardTaskContinuation.Run(System.Threading.Tasks.Task completedTask, bool bCanInlineContinuationTask) Line 334 + 0xc bytes   C#
    mscorlib.dll!System.Threading.Tasks.Task.ContinueWithCore(System.Threading.Tasks.Task continuationTask, System.Threading.Tasks.TaskScheduler scheduler, System.Threading.CancellationToken cancellationToken, System.Threading.Tasks.TaskContinuationOptions options) Line 4626 + 0x12 bytes    C#
    mscorlib.dll!System.Threading.Tasks.Task.ContinueWith(System.Action<System.Threading.Tasks.Task> continuationAction, System.Threading.Tasks.TaskScheduler scheduler, System.Threading.CancellationToken cancellationToken, System.Threading.Tasks.TaskContinuationOptions continuationOptions, ref System.Threading.StackCrawlMark stackMark) Line 3840 C#
    mscorlib.dll!System.Threading.Tasks.Task.ContinueWith(System.Action<System.Threading.Tasks.Task> continuationAction, System.Threading.CancellationToken cancellationToken, System.Threading.Tasks.TaskContinuationOptions continuationOptions, System.Threading.Tasks.TaskScheduler scheduler) Line 3805 + 0x1b bytes   C#
    System.Net.Http.dll!System.Net.Http.HttpUtilities.ContinueWithStandard(System.Threading.Tasks.Task task, System.Action<System.Threading.Tasks.Task> continuation) + 0x2c bytes  
    System.Net.Http.dll!System.Net.Http.HttpClientHandler.PrepareAndStartContentUpload(System.Net.Http.HttpClientHandler.RequestState state) + 0x16b bytes  
    System.Net.Http.dll!System.Net.Http.HttpClientHandler.StartRequest(object obj) + 0x5a bytes 
    mscorlib.dll!System.Threading.Tasks.Task.InnerInvoke() Line 2835 + 0xd bytes    C#
    mscorlib.dll!System.Threading.Tasks.Task.Execute() Line 2459 + 0xb bytes    C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecutionContextCallback(object obj) Line 2815 + 0x9 bytes C#
    mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 581 + 0xd bytes  C#
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 530 + 0xd bytes  C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot) Line 2785  C#
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteEntry(bool bPreventDoubleExecution) Line 2728   C#
    mscorlib.dll!System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() Line 2664 + 0x7 bytes   C#
    mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch() Line 829   C#
    mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() Line 1170 + 0x5 bytes   C#
    [Native to Managed Transition]

我相信目的是不要忽略它，并且在发生异常的情况下调用HandleAsyncException确实将其设置TaskCompletionSource为最终状态的方法。

于 2014-04-15T15:02:31.463 回答