Wait for request of CancellationToken cancellation

Question

How can I pause the executing until the cancellation is requested?

var cts = new CancellationTokenSource();

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

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

Console.ReadKey();

cts.Cancel();

Console.ReadKey();

Answer 1

You can use the WaitHandle to wait synchronously:

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();
}

That said, "blocking a thread until something is cancelled" is a very uncommon scenario, so it's possible that you're using the wrong tool for this job. If you need to wait on something (not specifically a cancellation), you can use a TaskCompletionSource instead. If you need to react to a cancellation, you can use CancellationToken.Register to attach a callback (and therefore avoid blocking a thread).

Answer 2

The CancellationTokenSource uses a ManualResetEvent internally and you can just wait for the exposed WaitHandle to pause the execution until it is set.

var cts = new CancellationTokenSource();

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

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

Console.ReadKey();

cts.Cancel();

Console.ReadKey();

This is the WaitHandle defined in the CancellationTokenSource:

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;

And the Token just returns the handle from the source (has an internal variable referencing it).

Another option is to register the Token callback and use your own 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();

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