Cancellation with CancellationToken

Long-running async work should be cancellable, a user closes a dialog, navigates away, or a timeout expires. CancellationToken is the standard way to signal that.

using System;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        using var cts = new CancellationTokenSource();
        cts.CancelAfter(TimeSpan.FromSeconds(3));

        try
        {
            await CountAsync(cts.Token);
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("Cancelled!");
        }
    }

    static async Task CountAsync(CancellationToken token)
    {
        for (int i = 1; i <= 10; i++)
        {
            token.ThrowIfCancellationRequested();
            Console.WriteLine(i);
            await Task.Delay(1000, token);
        }
    }
}

The pieces

• CancellationTokenSource is the object that can trigger cancellation, cts.Cancel() cancels immediately, cts.CancelAfter(...) schedules it.
• CancellationToken (from cts.Token) is the read-only signal you pass down into async methods, they check it, they can't cancel anything themselves.
• token.ThrowIfCancellationRequested() throws an OperationCanceledException if cancellation was requested, stopping the method's work at a clean point instead of continuing pointlessly.
• Passing the token into Task.Delay(1000, token) lets the delay itself respond to cancellation immediately, instead of finishing its full second first.

Convention

Any async method that might run for a while should accept an optional CancellationToken token = default parameter and pass it through to everything it awaits, this makes cancellation flow all the way down a chain of async calls instead of only stopping the top-level one.