Question

这里的想法很简单，但实施有一些有趣的细微差别。这是我想在 .NET 4 中实现的扩展方法的签名。

public static Task<WebResponse> GetResponseAsync(this WebRequest request, CancellationToken token);

这是我的初步实施。根据我的阅读，网页请求可能需要cancelled due to a timeout。除了该页面上描述的支持外，如果通过request.Abort()请求取消，我还需要正确致电CancellationToken。

public static Task<WebResponse> GetResponseAsync(this WebRequest request, CancellationToken token)
{
    if (request == null)
        throw new ArgumentNullException("request");

    return Task.Factory.FromAsync<WebRequest, CancellationToken, WebResponse>(BeginGetResponse, request.EndGetResponse, request, token, null);
}

private static IAsyncResult BeginGetResponse(WebRequest request, CancellationToken token, AsyncCallback callback, object state)
{
    IAsyncResult asyncResult = request.BeginGetResponse(callback, state);
    if (!asyncResult.IsCompleted)
    {
        if (request.Timeout != Timeout.Infinite)
            ThreadPool.RegisterWaitForSingleObject(asyncResult.AsyncWaitHandle, WebRequestTimeoutCallback, request, request.Timeout, true);
        if (token != CancellationToken.None)
            ThreadPool.RegisterWaitForSingleObject(token.WaitHandle, WebRequestCancelledCallback, Tuple.Create(request, token), Timeout.Infinite, true);
    }

    return asyncResult;
}

private static void WebRequestTimeoutCallback(object state, bool timedOut)
{
    if (timedOut)
    {
        WebRequest request = state as WebRequest;
        if (request != null)
            request.Abort();
    }
}

private static void WebRequestCancelledCallback(object state, bool timedOut)
{
    Tuple<WebRequest, CancellationToken> data = state as Tuple<WebRequest, CancellationToken>;
    if (data != null && data.Item2.IsCancellationRequested)
    {
        data.Item1.Abort();
    }
}

我的问题既简单又具有挑战性。当与TPL一起使用时，此实现是否会按预期运行？

Answer 1

当与TPL一起使用时，此实现是否会按预期运行？

没有。

它不会将Task<T>结果标记为已取消，因此行为将不会完全符合预期。
如果超时，WebException报告的AggregateException中包含的Task.Exception状态为WebExceptionStatus.RequestCanceled。它应该是WebExceptionStatus.Timeout。

我实际上建议使用TaskCompletionSource<T>来实现这一点。这允许您编写代码而无需使用自己的APM样式方法：

public static Task<WebResponse> GetResponseAsync(this WebRequest request, CancellationToken token)
{
    if (request == null)
        throw new ArgumentNullException("request");

    bool timeout = false;
    TaskCompletionSource<WebResponse> completionSource = new TaskCompletionSource<WebResponse>();

    AsyncCallback completedCallback =
        result =>
        {
            try
            {
                completionSource.TrySetResult(request.EndGetResponse(result));
            }
            catch (WebException ex)
            {
                if (timeout)
                    completionSource.TrySetException(new WebException("No response was received during the time-out period for a request.", WebExceptionStatus.Timeout));
                else if (token.IsCancellationRequested)
                    completionSource.TrySetCanceled();
                else
                    completionSource.TrySetException(ex);
            }
            catch (Exception ex)
            {
                completionSource.TrySetException(ex);
            }
        };

    IAsyncResult asyncResult = request.BeginGetResponse(completedCallback, null);
    if (!asyncResult.IsCompleted)
    {
        if (request.Timeout != Timeout.Infinite)
        {
            WaitOrTimerCallback timedOutCallback =
                (object state, bool timedOut) =>
                {
                    if (timedOut)
                    {
                        timeout = true;
                        request.Abort();
                    }
                };

            ThreadPool.RegisterWaitForSingleObject(asyncResult.AsyncWaitHandle, timedOutCallback, null, request.Timeout, true);
        }

        if (token != CancellationToken.None)
        {
            WaitOrTimerCallback cancelledCallback =
                (object state, bool timedOut) =>
                {
                    if (token.IsCancellationRequested)
                        request.Abort();
                };

            ThreadPool.RegisterWaitForSingleObject(token.WaitHandle, cancelledCallback, null, Timeout.Infinite, true);
        }
    }

    return completionSource.Task;
}

这里的优点是您的Task<T>结果将按预期完全运行（将被标记为已取消，或者使用超时信息引发与同步版本相同的异常等）。这也避免了使用Task.Factory.FromAsync的开销，因为您已经处理了大部分相关的困难工作。

280Z28附录

这是一个单元测试，显示了上述方法的正确操作。

[TestClass]
public class AsyncWebRequestTests
{
    [TestMethod]
    public void TestAsyncWebRequest()
    {
        Uri uri = new Uri("http://google.com");
        WebRequest request = HttpWebRequest.Create(uri);
        Task<WebResponse> response = request.GetResponseAsync();
        response.Wait();
    }

    [TestMethod]
    public void TestAsyncWebRequestTimeout()
    {
        Uri uri = new Uri("http://google.com");
        WebRequest request = HttpWebRequest.Create(uri);
        request.Timeout = 0;
        Task<WebResponse> response = request.GetResponseAsync();
        try
        {
            response.Wait();
            Assert.Fail("Expected an exception");
        }
        catch (AggregateException exception)
        {
            Assert.AreEqual(TaskStatus.Faulted, response.Status);

            ReadOnlyCollection<Exception> exceptions = exception.InnerExceptions;
            Assert.AreEqual(1, exceptions.Count);
            Assert.IsInstanceOfType(exceptions[0], typeof(WebException));

            WebException webException = (WebException)exceptions[0];
            Assert.AreEqual(WebExceptionStatus.Timeout, webException.Status);
        }
    }

    [TestMethod]
    public void TestAsyncWebRequestCancellation()
    {
        Uri uri = new Uri("http://google.com");
        WebRequest request = HttpWebRequest.Create(uri);
        CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
        Task<WebResponse> response = request.GetResponseAsync(cancellationTokenSource.Token);
        cancellationTokenSource.Cancel();
        try
        {
            response.Wait();
            Assert.Fail("Expected an exception");
        }
        catch (AggregateException exception)
        {
            Assert.AreEqual(TaskStatus.Canceled, response.Status);

            ReadOnlyCollection<Exception> exceptions = exception.InnerExceptions;
            Assert.AreEqual(1, exceptions.Count);
            Assert.IsInstanceOfType(exceptions[0], typeof(OperationCanceledException));
        }
    }

    [TestMethod]
    public void TestAsyncWebRequestError()
    {
        Uri uri = new Uri("http://google.com/fail");
        WebRequest request = HttpWebRequest.Create(uri);
        Task<WebResponse> response = request.GetResponseAsync();
        try
        {
            response.Wait();
            Assert.Fail("Expected an exception");
        }
        catch (AggregateException exception)
        {
            Assert.AreEqual(TaskStatus.Faulted, response.Status);

            ReadOnlyCollection<Exception> exceptions = exception.InnerExceptions;
            Assert.AreEqual(1, exceptions.Count);
            Assert.IsInstanceOfType(exceptions[0], typeof(WebException));

            WebException webException = (WebException)exceptions[0];
            Assert.AreEqual(HttpStatusCode.NotFound, ((HttpWebResponse)webException.Response).StatusCode);
        }
    }
}

实现扩展方法WebRequest.GetResponseAsync并支持CancellationToken

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