某些System.Threading.Tasks.Task构造函数将 aCancellationToken作为参数：
CancellationTokenSource source = new CancellationTokenSource();
Task t = new Task (/* method */, source.Token);
令我感到困惑的是，无法从方法主体内部实际获取传入的令牌（例如，没有类似的Task.CurrentTask.CancellationToken）。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？
将 a 传递给CancellationToken构造Task函数会将其与任务相关联。
从 MSDN引用Stephen Toub 的回答：
这有两个主要好处：
Task如果令牌在开始执行之前已请求取消，Task则不会执行。它不会过渡到Running，而是立即过渡到Canceled。这避免了运行任务的成本，如果它只是在运行时被取消的话。- 如果任务的主体也在监视取消标记并抛出一个
OperationCanceledException包含该标记的标记（这是什么ThrowIfCancellationRequested），那么当任务看到 时OperationCanceledException，它会检查 的标记是否OperationCanceledException与任务的标记匹配。如果是这样，则该异常被视为对合作取消和Task转换到Canceled状态（而不是Faulted状态）的确认。
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CodePlex 上的 Parallel Programming with Microsoft .NET 一书。
书中 CTS 的示例用法：
CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
Task myTask = Task.Factory.StartNew(() =>
{
for (...)
{
token.ThrowIfCancellationRequested();
// Body of for loop.
}
}, token);
// ... elsewhere ...
cts.Cancel();
取消并不像许多人想象的那样简单。在 msdn 上的这篇博客文章中解释了一些微妙之处：
例如：
在 Parallel Extensions 和其他系统中的某些情况下，由于非用户明确取消的原因，有必要唤醒阻塞的方法。例如，如果一个线程
blockingCollection.Take()由于集合为空而被阻塞，而另一个线程随后调用blockingCollection.CompleteAdding()，则第一个调用应该唤醒并抛出一个InvalidOperationException以表示不正确的用法。
这是一个代码示例，演示了Max Galkin接受的答案中的两点：
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 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!!!