任务构造函数中的取消标记：为什么？

Question

某些System.Threading.Tasks.Task 构造函数将CancellationToken 作为参数：

CancellationTokenSource source = new CancellationTokenSource();
Task t = new Task (/* method */, source.Token);

对此我感到困惑的是，没有办法从方法体内部实际获取传入的令牌（例如，没有像Task.CurrentTask.CancellationToken 这样的东西）。令牌必须通过其他机制提供，例如状态对象或在 lambda 中捕获。

那么在构造函数中提供取消令牌的目的是什么？

Answer 1

将CancellationToken 传递给Task 构造函数会将其与任务相关联。

引用Stephen Toub's answer from MSDN:

这有两个主要好处：

1. 如果令牌在Task 开始执行之前请求取消，Task 将不会执行。而不是过渡到 Running，它将立即转换为 Canceled。这避免了 运行任务的成本（如果它在运行时被取消） 无论如何。
2. 如果任务主体也在监视取消令牌并抛出包含该令牌的OperationCanceledException （这是ThrowIfCancellationRequested 所做的），然后当任务 看到 OperationCanceledException，它检查 OperationCanceledException 的令牌是否与任务的 令牌。如果是，则该异常被视为对 合作取消和Task 转换为Canceled 状态（而不是 Faulted 状态）。

Answer 2

构造函数在内部使用令牌进行取消处理。如果您的代码想要访问令牌，您有责任将其传递给自己。我强烈推荐阅读Parallel Programming with Microsoft .NET book at CodePlex。

书中CTS的用法示例：

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;

Task myTask = Task.Factory.StartNew(() =>
{
    for (...)
    {
        token.ThrowIfCancellationRequested();

        // Body of for loop.
    }
}, token);

// ... elsewhere ...
cts.Cancel();

Answer 3

取消并不像许多人想象的那样简单。 msdn 上的这篇博文解释了其中的一些微妙之处：

例如：

在并行扩展和其他系统中的某些情况下，它 出于非到期原因需要唤醒阻塞的方法 由用户明确取消。例如，如果一个线程是 由于集合为空，在 blockingCollection.Take() 上被阻止 另一个线程随后调用 blockingCollection.CompleteAdding()，那么第一个电话应该叫醒 向上并抛出一个InvalidOperationException 表示不正确 用法。

Cancellation in Parallel Extensions

Answer 4

这是一个代码示例，演示了accepted answer by Max Galkin 中的两点：

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("*********************************************************************");
        Console.WriteLine("* Start canceled task, don't pass token to constructor");
        Console.WriteLine("*********************************************************************");
        StartCanceledTaskTest(false);
        Console.WriteLine();

        Console.WriteLine("*********************************************************************");
        Console.WriteLine("* Start canceled task, pass token to constructor");
        Console.WriteLine("*********************************************************************");
        StartCanceledTaskTest(true);
        Console.WriteLine();

        Console.WriteLine("*********************************************************************");
        Console.WriteLine("* Throw if cancellation requested, don't pass token to constructor");
        Console.WriteLine("*********************************************************************");
        ThrowIfCancellationRequestedTest(false);
        Console.WriteLine();

        Console.WriteLine("*********************************************************************");
        Console.WriteLine("* Throw if cancellation requested, pass token to constructor");
        Console.WriteLine("*********************************************************************");
        ThrowIfCancellationRequestedTest(true);
        Console.WriteLine();

        Console.WriteLine();
        Console.WriteLine("Test Completed!!!");
        Console.ReadKey();
    }

    static void StartCanceledTaskTest(bool passTokenToConstructor)
    {
        Console.WriteLine("Creating task");
        CancellationTokenSource tokenSource = new CancellationTokenSource();
        Task task = null;
        if (passTokenToConstructor)
        {
            task = new Task(() => TaskWork(tokenSource.Token, false), tokenSource.Token);

        }
        else
        {
            task = new Task(() => TaskWork(tokenSource.Token, false));
        }

        Console.WriteLine("Canceling task");
        tokenSource.Cancel();

        try
        {
            Console.WriteLine("Starting task");
            task.Start();
            task.Wait();
        }
        catch (Exception ex)
        {
            Console.WriteLine("Exception: {0}", ex.Message);
            if (ex.InnerException != null)
            {
                Console.WriteLine("InnerException: {0}", ex.InnerException.Message);
            }
        }

        Console.WriteLine("Task.Status: {0}", task.Status);
    }

    static void ThrowIfCancellationRequestedTest(bool passTokenToConstructor)
    {
        Console.WriteLine("Creating task");
        CancellationTokenSource tokenSource = new CancellationTokenSource();
        Task task = null;
        if (passTokenToConstructor)
        {
            task = new Task(() => TaskWork(tokenSource.Token, true), tokenSource.Token);

        }
        else
        {
            task = new Task(() => TaskWork(tokenSource.Token, true));
        }

        try
        {
            Console.WriteLine("Starting task");
            task.Start();
            Thread.Sleep(100);

            Console.WriteLine("Canceling task");
            tokenSource.Cancel();
            task.Wait();
        }
        catch (Exception ex)
        {
            Console.WriteLine("Exception: {0}", ex.Message);
            if (ex.InnerException != null)
            {
                Console.WriteLine("InnerException: {0}", ex.InnerException.Message);
            }
        }

        Console.WriteLine("Task.Status: {0}", task.Status);
    }

    static void TaskWork(CancellationToken token, bool throwException)
    {
        int loopCount = 0;

        while (true)
        {
            loopCount++;
            Console.WriteLine("Task: loop count {0}", loopCount);

            token.WaitHandle.WaitOne(50);
            if (token.IsCancellationRequested)
            {
                Console.WriteLine("Task: cancellation requested");
                if (throwException)
                {
                    token.ThrowIfCancellationRequested();
                }

                break;
            }
        }
    }
}

输出：

*********************************************************************
* Start canceled task, don't pass token to constructor
*********************************************************************
Creating task
Canceling task
Starting task
Task: loop count 1
Task: cancellation requested
Task.Status: RanToCompletion

*********************************************************************
* Start canceled task, pass token to constructor
*********************************************************************
Creating task
Canceling task
Starting task
Exception: Start may not be called on a task that has completed.
Task.Status: Canceled

*********************************************************************
* Throw if cancellation requested, don't pass token to constructor
*********************************************************************
Creating task
Starting task
Task: loop count 1
Task: loop count 2
Canceling task
Task: cancellation requested
Exception: One or more errors occurred.
InnerException: The operation was canceled.
Task.Status: Faulted

*********************************************************************
* Throw if cancellation requested, pass token to constructor
*********************************************************************
Creating task
Starting task
Task: loop count 1
Task: loop count 2
Canceling task
Task: cancellation requested
Exception: One or more errors occurred.
InnerException: A task was canceled.
Task.Status: Canceled


Test Completed!!!