Pass Cancellation Token to child thread

Question

I have scenario where I create a cancellation token that passed to parent thread, I need to pass same cancellation token to child thread. But when I pass cancellation token to child thread, threads are not cancelled when parent invokes Cancel().

public void main()
{
   CancellationTokenSource cts = new CancellationTokenSource();
   CancellationToken token = cts.Token;
   ChildClass instance = new ChildClass();

   Task.Factory.StartNew(() => 
                Task.Factory.StartNew(() => instance.start(token), token), token);
}

public void cancel()
{
    cts.Cancel();
}

public class ChildClass()
{
    public void start(CancellationToken token)
    {
        ParallelOptions po = new ParallelOptions();
        po.CancellationToken = token;

        Parallel.For(0, 10, i => 
              {"do some processing"}, po, i);
    } 
}

I have created a cancellation token in parent class and passing it to child thread through parallel options, but threads created by Parallel.For are not terminated when parent invokes Cancel().

Is there a concept of passing CancellationToken as reference?

Answer 1

There's nothing inherently wrong with how you're using CancellationTokenSource. You just don't know that the task is cancelled because you're not using Wait to handle the TaskCanceledException . When Main is this:

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
ChildClass instance = new ChildClass();

var task = Task.Factory.StartNew(() =>
                Task.Factory.StartNew(() => instance.start(token), token), token);
cts.Cancel();
task.Wait();

You would see the exceptions.

Here's a full example of Parallel.For being canceled:

internal class Program
{
    private static void Main()
    {
        var cts = new CancellationTokenSource();
        var instance = new ChildClass();

        var task = Task.Factory.StartNew(() =>
                Task.Factory.StartNew(() => instance.start(cts.Token)).Wait());
        cts.Cancel();
        task.Wait();
    }
}

public class ChildClass
{
    public void start(CancellationToken token)
    {
        var parallelOptions = new ParallelOptions {CancellationToken = token};
        try
        {
            Parallel.For(0, 100000, parallelOptions, i =>
            {
                // Do something
            });
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("canceled");
        }
    }
}

Output:

canceled