Question

似乎GetResponseAsync在Async / Await中不接受cancellationToken。所以问题是我如何取消以下程序，只要我需要从回复中收集Cookie：

 using (HttpWebResponse response = (HttpWebResponse) await request.GetResponseAsync())
 {
    cookies.Add(response.Cookies);
 }

也欢迎实现上述目标的替代代码。

Answer 1

这样的事情应该有效（未经测试）：

public static class Extensions
{
    public static async Task<HttpWebResponse> GetResponseAsync(this HttpWebRequest request, CancellationToken ct)
    {
        using (ct.Register(() => request.Abort(), useSynchronizationContext: false))
        {
            var response = await request.GetResponseAsync();
            ct.ThrowIfCancellationRequested();
            return (HttpWebResponse)response;
        }
    }
}

理论上，如果在ct上请求取消并且request.Abort被调用，则await request.GetResponseAsync()应该抛出WebException。尽管如此，在消费结果时明确检查取消以减轻竞争条件总是一个好主意，所以我打电话给ct.ThrowIfCancellationRequested()。

另外，我假设request.Abort是线程安全的（可以从任何线程调用），所以我使用useSynchronizationContext: false（我还没有验证过）。

[更新] ，以解决OP关于如何区分由取消引起的WebException和任何其他错误的评论。这是如何完成的，所以TaskCanceledException（派生自OperationCanceledException）将在取消时正确抛出：

public static class Extensions
{
    public static async Task<HttpWebResponse> GetResponseAsync(this HttpWebRequest request, CancellationToken ct)
    {
        using (ct.Register(() => request.Abort(), useSynchronizationContext: false))
        {
            try
            {
                var response = await request.GetResponseAsync();
                return (HttpWebResponse)response;
            }
            catch (WebException ex)
            {
                // WebException is thrown when request.Abort() is called,
                // but there may be many other reasons,
                // propagate the WebException to the caller correctly
                if (ct.IsCancellationRequested)
                {
                    // the WebException will be available as Exception.InnerException
                    throw new OperationCanceledException(ex.Message, ex, ct);
                }

                // cancellation hasn't been requested, rethrow the original WebException
                throw;
            }
        }
    }
}

Answer 2

public static async Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken, Action action, bool useSynchronizationContext = true)
{
    using (cancellationToken.Register(action, useSynchronizationContext))
    {
        try
        {
            return await task;
        }
        catch (Exception ex)
        {

            if (cancellationToken.IsCancellationRequested)
            {
                // the Exception will be available as Exception.InnerException
                throw new OperationCanceledException(ex.Message, ex, cancellationToken);
            }

            // cancellation hasn't been requested, rethrow the original Exception
            throw;
        }
    }
}

现在，您可以在任何可取消的异步方法上使用取消令牌。例如WebRequest.GetResponseAsync：

var request = (HttpWebRequest)WebRequest.Create(url);

using (var response = await request.GetResponseAsync())
{
    . . .
}

将成为：

var request = (HttpWebRequest)WebRequest.Create(url);

using (WebResponse response = await request.GetResponseAsync().WithCancellation(CancellationToken.None, request.Abort, true))
{
    . . .
}

参见示例http://pastebin.com/KauKE0rW

GetResponseAsync不接受cancellationToken

2 个答案: