Check if CancellationToken has been cancelled

Question

I've created a little demo project to help me understand how I can use Cancellation Tokens. I understand that you cancel a token and check if a cancellation has been requested, but is there a way that I can check whether the cancellation has been realised? In my example below, I don't want to run Work() again until DoWork() has finished running.

public class Program
{
    public static CancellationTokenSource tokenSource;

    private static void Main(string[] args)
    {
        while (true)
        {
            Work();
        }
    }

    public static async void Work()
    {
        tokenSource = new CancellationTokenSource();
        Console.WriteLine("Press any key to START doing work");
        Console.ReadLine();
        Console.WriteLine("Press any key to STOP doing work");
        DoWork(tokenSource.Token);
        Console.ReadLine();
        Console.WriteLine("Stopping...");
        tokenSource.Cancel();
    }

    public static async void DoWork(CancellationToken cancelToken)
    {
        while (true)
        {
            Console.WriteLine("Working...");
            await Task.Run(() =>
            {
                Thread.Sleep(1500);
            });

            if (cancelToken.IsCancellationRequested)
            {
                Console.WriteLine("Work Cancelled!");

                return;
            }
        }
    }
}

Answer 1

You typically don't want to make your DoWork function async void -- make it async Task instead. That way you can see when it's completed (or been cancelled).

You also probably want to use cancelToken.ThrowIfCancellationRequested(). This throws an OperationCanceledException, which you can catch.

public class Program
{
    public static CancellationTokenSource tokenSource;

    private static async Task Main(string[] args)
    {
        while (true)
        {
            await Work();
        }
    }

    public static async Task Work()
    {
        tokenSource = new CancellationTokenSource();
        Console.WriteLine("Press any key to START doing work");
        Console.ReadLine();
        Console.WriteLine("Press any key to STOP doing work");

        var task = DoWork(tokenSource.Token);

        Console.ReadLine();
        Console.WriteLine("Stopping...");
        tokenSource.Cancel();

        try
        {
            await task;
        }
        catch (OperationCanceledException)
        {
            // Task was cancelled
        }
    }

    public static async Task DoWork(CancellationToken cancelToken)
    {
        while (true)
        {
            Console.WriteLine("Working...");
            await Task.Run(() =>
            {
                Thread.Sleep(1500);
            });

            cancelToken.ThrowIfCancellationRequested();
        }
    }
}

This code relies on "async main", which was introduced in C# 7. If you don't have this, you can write your Main method as:

private static void Main(string[] args)
{
    while (true)
    {
        Work().Wait();
    }
}