这里的想法很简单，但实现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);
这是我的初步实现。据我所知，网络请求可能需要是 cancelled due to a timeout 。除了该页面上描述的支持之外，如果通过 CancellationToken 请求取消，我还想正确调用 request.Abort()。
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();
}
}
我的问题很简单但很有挑战性。当与 TPL 一起使用时，此实现实际上会按预期运行吗？
最佳答案
Will this implementation actually behave as expected when used with the TPL?
没有。
- 它不会标记
Task<T>结果被取消，因此行为将不完全符合预期。 - 如果超时，
WebException包含在AggregateException中报道者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创建自己的 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;
}
这里的优点是你的Task<T>结果将完全按预期工作(将被标记为已取消，或引发与同步版本相同的超时信息的异常等)。这也避免了使用 Task.Factory.FromAsync 的开销，因为您已经自己处理了其中涉及的大部分困难工作。
280Z28 的附录
这是一个单元测试，显示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);
}
}
}
关于.net - 实现扩展方法 WebRequest.GetResponseAsync 并支持 CancellationToken，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/17494502/