任务构造器中的取消令牌：为什么？

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

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

令我感到困惑的是，没有办法从方法主体内部真正获取传入的令牌（例如，没有类似的东西Task.CurrentTask.CancellationToken）。令牌必须通过某种其他机制提供，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有什么目的呢？

传递CancellationToken到Task构造函数与任务关联起来。

引用来自MSDN的Stephen Toub的答案：

这有两个主要好处：

1. 如果令牌在Task开始执行之前已请求取消，Task则不会执行。而不是过渡到 Running，它会立即过渡到Canceled。这样就避免了在无论如何运行时都将其取消的任务。
2. 如果任务的主体还在监视取消令牌，并抛出OperationCanceledException包含该令牌的令牌（ThrowIfCancellationRequested确实如此），那么当任务看到时OperationCanceledException，它将检查OperationCanceledException的令牌是否与任务的令牌匹配。如果确实如此，则将该异常视为对协作取消的确认，并Task转换为Canceled 状态（而不是Faulted状态）。

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，则您有责任将其传递给自己。我强烈建议阅读CodePlex上的《与Microsoft .NET并行编程》一书。

本书中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();

取消并非像许多人想象的那样简单。在msdn上的此博客文章中解释了一些细微之处：

例如：

在Parallel Extensions和其他系统中的某些情况下，有必要唤醒阻塞的方法，其原因并非由用户明确取消引起。例如，如果一个线程blockingCollection.Take()由于该集合为空而被阻塞，而另一个线程随后调用 blockingCollection.CompleteAdding()，则第一个调用应唤醒并抛出，InvalidOperationException以表示不正确的用法。

并行扩展中的取消

这是一个示例，展示了Max Galkin的答案中的两点：

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 Done!!!");
        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 Done!!!