Question

我想在设置超时时触发几项任务。我们的想法是收集击败时钟的任务的结果，并取消（甚至忽略）其他任务。

我尝试使用扩展方法WithCancellation，如here所述，但抛出异常导致WhenAll返回并且没有提供任何结果。

这是我尝试过的，但我也向其他方向开放（注意我需要使用await而不是Task.Run，因为我需要在Tasks中使用httpContext）：

var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3));

IEnumerable<Task<MyResults>> tasks = 
            from url in urls
            select taskAsync(url).WithCancellation(cts.Token);

Task<MyResults>[] excutedTasks = null;

MyResults[] res = null;
try
{
    // Execute the query and start the searches:
    excutedTasks = tasks.ToArray();

    res = await Task.WhenAll(excutedTasks);
}
catch (Exception exc)
{
    if (excutedTasks != null)
    {
        foreach (Task<MyResults> faulted in excutedTasks.Where(t => t.IsFaulted))
        {
            // work with faulted and faulted.Exception
        }
    }
}

// work with res

修改按照@ Servy的回答，这是我采用的实现：

var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3));

IEnumerable<Task<MyResults>> tasks = 
            from url in urls
            select taskAsync(url).WithCancellation(cts.Token);

// Execute the query and start the searches:
Task<MyResults>[] excutedTasks = tasks.ToArray();

try
{
    await Task.WhenAll(excutedTasks);
}
    catch (OperationCanceledException)
{
    // Do nothing - we expect this if a timeout has occurred
}

IEnumerable<Task<MyResults>> completedTasks = excutedTasks.Where(t => t.Status == TaskStatus.RanToCompletion);

var results = new List<MyResults>();
completedTasks.ForEach(async t => results.Add(await t));

Answer 1

如果任何任务未能完成，那么WhenAll不会返回任何完成的结果是正确的，它只包含所有失败的聚合异常。幸运的是，您拥有原始的任务集合，因此您可以从那里获得成功完成的结果。

var completedTasks = excutedTasks.Where(t => t.Status == TaskStatus.RanToCompletion);

只需使用它而不是res。

Answer 2

我试过你的代码并且它运行得很好，除了取消的任务不处于Faulted状态，而是处于Canceled状态。因此，如果您要处理已取消的任务，请改用t.IsCanceled。未取消的任务已完成。这是我使用的代码：

    public static async Task MainAsync()
    {
        var urls = new List<string> {"url1", "url2", "url3", "url4", "url5", "url6"};

        var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3));

        IEnumerable<Task<MyResults>> tasks =
            from url in urls
            select taskAsync(url).WithCancellation(cts.Token);

        Task<MyResults>[] excutedTasks = null;

        MyResults[] res = null;
        try
        {
            // Execute the query and start the searches:
            excutedTasks = tasks.ToArray();

            res = await Task.WhenAll(excutedTasks);
        }
        catch (Exception exc)
        {
            if (excutedTasks != null)
            {
                foreach (Task<MyResults> faulted in excutedTasks.Where(t => t.IsFaulted))
                {
                    // work with faulted and faulted.Exception
                }
            }
        }

    }

    public static async Task<MyResults> taskAsync(string url)
    {
        Console.WriteLine("Start " + url);
        var random = new Random();
        var delay = random.Next(10);
        await Task.Delay(TimeSpan.FromSeconds(delay));

        Console.WriteLine("End " + url);

        return new MyResults();
    }

    private static void Main(string[] args)
    {
        MainAsync().Wait();
    }

如何 - 具有超时和取消的多个异步任务

2 个答案: