等待 CancellationToken 取消请求

Question

如何暂停执行直到请求取消？

var cts = new CancellationTokenSource();

Task.Run(() =>
{
    // Wait for the Cancel...

    Console.WriteLine("Canceled!");
});

Console.ReadKey();

cts.Cancel();

Console.ReadKey();

Answer 1

可以使用WaitHandle同步等待：

static void Main()
{
    var cts = new CancellationTokenSource();

    Task.Run(() =>
    {
        // Wait for the Cancel...

        cts.Token.WaitHandle.WaitOne();

        Console.WriteLine("Canceled!");
    });

    Console.ReadKey();

    cts.Cancel();

    Console.ReadKey();
}

也就是说，"blocking a thread until something is cancelled" 是一种非常罕见的情况，因此您可能为这项工作使用了错误的工具。如果您需要等待某事（不是特别是取消），您可以使用 TaskCompletionSource 代替。如果您需要对取消做出反应，可以使用 CancellationToken.Register 附加回调（从而避免阻塞线程）。

Answer 2

CancellationTokenSource在内部使用ManualResetEvent，您可以等待暴露的WaitHandle暂停执行，直到它被设置。

var cts = new CancellationTokenSource();

Task.Run(() =>
{
    WaitHandle.WaitAny(new[] { cts.Token.WaitHandle });

    Console.WriteLine("Canceled!");
});

Console.ReadKey();

cts.Cancel();

Console.ReadKey();

这是CancellationTokenSource中定义的WaitHandle:

ManualResetEvent mre = new ManualResetEvent(false);
if (Interlocked.CompareExchange(ref m_kernelEvent, mre, null) != null)
{    
    ((IDisposable)mre).Dispose();
}

// There is a ---- between checking IsCancellationRequested and setting the event.
// However, at this point, the kernel object definitely exists and the cases are:
//   1. if IsCancellationRequested = true, then we will call Set()
//   2. if IsCancellationRequested = false, then NotifyCancellation will see that the event exists, and will call Set().
if (IsCancellationRequested)
    m_kernelEvent.Set();

return m_kernelEvent;

而 Token 只是 returns 来自源的句柄（有一个引用它的内部变量）。

另一种选择是注册 Token 回调并使用您自己的 ManualResetEvent:

var cts = new CancellationTokenSource();

Task.Run(() =>
{
    var mre = new ManualResetEvent(false);

    var registration = cts.Token.Register(() => mre.Set());

    using (registration)
    {
        mre.WaitOne();

        Console.WriteLine("Canceled!");
    }
});

Console.ReadKey();

cts.Cancel();

Console.ReadKey();

示例：https://blogs.msdn.microsoft.com/pfxteam/2009/05/22/net-4-cancellation-framework/

Answer 3

此代码可用于在不阻塞线程的情况下等待取消事件。

.NET6版本可以很简单，可以取消await

public async Task Run(CancellationToken cancellationToken)
{
    // Simplification for the sake of example
    var cts = new CancellationTokenSource();
    
    var waitForStop = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
    // IHostApplicationLifetime event can be used instead of `cts.Token`
    CancellationTokenRegistration registration = cts.Token.Register(() => waitForStop.SetResult());
    await using var _ = registration.ConfigureAwait(false);
    
    await waitForStop.Task.WaitAsync(cancellationToken).ConfigureAwait(false);
}

不会因为 cts.Cancel() 调用而抛出 TaskCanceledException。它将允许在离开方法之前做一些事情（例如正常关闭）。但是，如果为 cancellationToken 请求取消，它将按预期抛出 TaskCanceledException。

如果开发者需要 return 结果：

var cts = new CancellationTokenSource();

var waitForStop = new TaskCompletionSource<bool?>(TaskCreationOptions.RunContinuationsAsynchronously);
cts.Token.Register(obj =>
{
    var tcs = (TaskCompletionSource<bool?>)obj!;
    tcs.TrySetResult(true);
}, waitForStop);

var result = await waitForStop.Task.ConfigureAwait(false);

Answer 4

最简洁的方式大概是这样：

try { await Task.Delay(Timeout.Infinite, cts.Token); } catch { }

此方法依赖于捕获异常，并且 exceptions are expensive. This should not be a problem if you are awaiting tokens sporadically, but in case you are awaiting tokens in tight loops, you may want to include in your project a cheap awaiter for CancellationTokens, as described in this article。它将允许您这样做：

await cts.Token;

等待 CancellationToken 取消请求

Wait for request of CancellationToken cancellation

c#

task-parallel-library

cancellationtokensource

cancellation-token