Task.WhenAny 取消未完成的任务和超时

Question

在我的应用程序中，我正在创建一些并发 Web 请求，当其中任何一个完成时我都很满意，所以我使用方法 Task.WhenAny:

var urls = new string[] {
    "https://whosebug.com",
    "https://superuser.com",
    "https://www.reddit.com/r/chess",
};
var tasks = urls.Select(async url =>
{
    using (var webClient = new WebClient())
    {
        return (Url: url, Data: await webClient.DownloadStringTaskAsync(url));
    }
}).ToArray();
var firstTask = await Task.WhenAny(tasks);
Console.WriteLine($"First Completed Url: {firstTask.Result.Url}");
Console.WriteLine($"Data: {firstTask.Result.Data.Length:#,0} chars");

First Completed Url: https://superuser.com
Data: 121.954 chars

我不喜欢这个实现的是未完成的任务继续下载我不再需要的数据，并且浪费带宽我宁愿为我的下一批请求保留。所以我正在考虑取消其他任务，但我不确定该怎么做。我找到了如何使用 CancellationToken 来取消特定的 Web 请求：

public static async Task<(string Url, string Data)> DownloadUrl(
    string url, CancellationToken cancellationToken)
{
    try
    {
        using (var webClient = new WebClient())
        {
            cancellationToken.Register(webClient.CancelAsync);
            return (url, await webClient.DownloadStringTaskAsync(url));
        }
    }
    catch (WebException ex) when (ex.Status == WebExceptionStatus.RequestCanceled)
    {
        cancellationToken.ThrowIfCancellationRequested();
        throw;
    }
}

现在我需要一个 Task.WhenAny 的实现，它将接受一个 url 数组，并将使用我的 DownloadUrl 函数来获取响应最快的站点的数据，并将处理取消逻辑较慢的任务。如果它有一个 timeout 参数就更好了，以防止永无止境的任务。所以我需要这样的东西：

public static Task<Task<TResult>> WhenAnyEx<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, CancellationToken, Task<TResult>> taskFactory,
    int timeout)
{
    // What to do here?
}

有什么想法吗？

Answer 1

更新：更好的解决方案基于Stephen Cleary's answer and MSDN and svick's answer：

CancellationTokenSource source = new CancellationTokenSource();
source.CancelAfter(TimeSpan.FromSeconds(1));

var tasks = urls.Select(url => Task.Run( async () => 
{
    using (var webClient = new WebClient())
    {
        token.Register(webClient.CancelAsync);
        var result = (Url: url, Data: await webClient.DownloadStringTaskAsync(url));
        token.ThrowIfCancellationRequested();
        return result.Url;
    }
}, token)).ToArray();

string url;
try
{
    // (A canceled task will raise an exception when awaited).
    var firstTask = await Task.WhenAny(tasks);
    url = (await firstTask).Url;
}   
catch (AggregateException ae) {
   foreach (Exception e in ae.InnerExceptions) {
      if (e is TaskCanceledException)
         Console.WriteLine("Timeout: {0}", 
                           ((TaskCanceledException) e).Message);
      else
         Console.WriteLine("Exception: " + e.GetType().Name);
   }
}

non-optimal解法

可以通过添加一个在给定时间后等待并完成的任务来解决超时问题。然后你查看哪个任务先完成，如果是等待任务，则有效超时。

Task timeout = Task.Delay(10000);
var firstTask = await Task.WhenAny(tasks.Concat(new Task[] {timeout}));
if(firstTask == timeout) { ... } //timed out
source.Cancel();

Answer 2

只需将相同的取消令牌传递给您的所有任务，如下所示：

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken ct = cts.Token;
// here you specify how long you want to wait for task to finish before cancelling
int timeout = 5000;
cts.CancelAfter(timeout);
// pass ct to all your tasks and start them
await Task.WhenAny(/* your tasks here */);
// cancel all tasks
cts.Cancel();

此外，您需要阅读此主题以了解如何正确使用 CancellationToken：

Task.WhenAny 取消未完成的任务和超时

Task.WhenAny with cancellation of the non completed tasks and timeout

.net

c#

task-parallel-library

async-await