Task.ContinueWith() 在哪个调度程序上运行？

Question

考虑以下代码：

// MyKickAssTaskScheduler is a TaskScheduler, IDisposable
using (var scheduler = new MyKickAssTaskScheduler())
{
    Task foo = new Task(() => DoSomething());
    foo.ContinueWith((_) => DoSomethingElse());
    foo.Start(scheduler);
    foo.Wait();
}

ContinueWith() 任务是否保证运行在我的调度程序上？如果不是，它将使用哪个调度程序？

Answer 1

Is the ContinueWith() Task guaranteed to run on my scheduler? If not, which scheduler will it use?

不，它将使用传递给原始 Task 的调度程序。 ContinueWith 将默认使用 TaskScheduler.Current，在这种情况下是默认的线程池任务调度程序。您提供的 task.Start 上下文与 continuation

中使用的上下文之间没有传播

From the source:

public Task ContinueWith(Action<Task> continuationAction)
{
    StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
    return ContinueWith(continuationAction, 
                        TaskScheduler.Current, 
                        default(CancellationToken),
                        TaskContinuationOptions.None, ref stackMark);
}

Answer 2

StartNew、ContinueWith 将默认为 TaskScheduler.Current，Current 将 return 默认调度程序，当未从任务 (MSDN) 中调用时。

为避免默认调度程序问题，您应该始终将显式 TaskScheduler 传递给 Task.ContinueWith 和 Task.Factory.StartNew。

ContinueWith is Dangerous

Answer 3

@Noseratio - read it, but still skeptic about the validity of this behavior - I ran the first task on a non-default scheduler for a reason. Why did TPL decide the continuation, which is always sequential to my task, should run on another?

我同意 - 这不是最好的设计 - 但我认为默认为 TaskScheduler.Current 是为了 ContinueWith 与 Task.Factory.StartNew 一致，默认为 [=11] =] 也是，首先。 Stephen Toub does explain 后一个设计决定：

In many situations, that’s the right behavior. For example, let’s say you’re implementing a recursive divide-and-conquer problem, where you have a task that’s supposed to process some chunk of work, and it in turn subdivides its work and schedules tasks to process those chunks. If that task was running on a scheduler representing a particular pool of threads, or if it was running on a scheduler that had a concurrency limit, and so on, you’d typically want those tasks it then created to also run on the same scheduler.

因此，ContinueWith 使用您调用 ContinueWith 时当前正在执行的任何任务的当前（环境）TaskScheduler.Current，而不是先前任务之一。如果这对您来说是一个问题并且您不能明确指定任务计划程序，则有一个解决方法。您可以使您的自定义任务调度程序成为特定范围的环境调度程序，如下所示：

using (var scheduler = new MyKickAssTaskScheduler())
{
    Task<Task> outer = new Task(() => 
    {
       Task foo = new Task(() => DoSomething());
       foo.ContinueWith((_) => DoSomethingElse());
       foo.Start(); // don't have to specify scheduler here
       return foo;
    }

    outer.RunSynchronously(scheduler);
    outer.Unwrap().Wait();
}

请注意 outer 是 Task<Task>，因此有 outer.Unwrap()。您也可以使用 outer.Result.Wait()，但存在一些语义差异，特别是如果您使用 outer.Start(scheduler) 而不是 outer.RunSynchronously(scheduler)。

Task.ContinueWith() 在哪个调度程序上运行？

On which scheduler Task.ContinueWith() runs?

c#

task

task-parallel-library