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);
让我困惑的是，在方法体内部没有办法实际获取传入的令牌(例如，没有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呢?
当前回答
将CancellationToken传递给Task构造函数，将其与任务关联起来。
引用来自MSDN的Stephen Toub的回答:
This has two primary benefits: If the token has cancellation requested prior to the Task starting to execute, the Task won't execute. Rather than transitioning to Running, it'll immediately transition to Canceled. This avoids the costs of running the task if it would just be canceled while running anyway. If the body of the task is also monitoring the cancellation token and throws an OperationCanceledException containing that token (which is what ThrowIfCancellationRequested does), then when the task sees that OperationCanceledException, it checks whether the OperationCanceledException's token matches the Task's token. If it does, that exception is viewed as an acknowledgement of cooperative cancellation and the Task transitions to the Canceled state (rather than the Faulted state).
其他回答
将CancellationToken传递给Task构造函数，将其与任务关联起来。
引用来自MSDN的Stephen Toub的回答:
This has two primary benefits: If the token has cancellation requested prior to the Task starting to execute, the Task won't execute. Rather than transitioning to Running, it'll immediately transition to Canceled. This avoids the costs of running the task if it would just be canceled while running anyway. If the body of the task is also monitoring the cancellation token and throws an OperationCanceledException containing that token (which is what ThrowIfCancellationRequested does), then when the task sees that OperationCanceledException, it checks whether the OperationCanceledException's token matches the Task's token. If it does, that exception is viewed as an acknowledgement of cooperative cancellation and the Task transitions to the Canceled state (rather than the Faulted state).
下面是一个代码示例，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!!!
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，则您负责将其传递给自己。我强烈推荐大家阅读CodePlex上的《Microsoft . net并行编程》一书。
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();
取消并不像许多人想象的那么简单。msdn上的这篇博客文章解释了一些微妙之处:
例如:
在并行扩展和其他系统的某些情况下，它 是否有必要唤醒一个阻塞的方法的原因不是应有的 用户显式取消。例如，如果一个线程是 由于集合为空，在blockingCollection.Take()上被阻塞 另一个线程随后调用 blockingCollection.CompleteAdding()，那么第一个调用应该被唤醒 ，并抛出InvalidOperationException来表示错误 使用。
并行扩展中的取消
