Cancel multiple HttpClient calls using CancellationToken and Ctrl+C

Question

I am trying to cancel multiple asynchronous web requests (GET) issued via a shared HttpClient using CancellationTokens, in a command-line application, full framework, .net 4.7.1, C# 7.3, VS 2017.

My example runs a couple of parallel tasks, each task is constantly downloading some data over Http, using an async GetAsync() and ReadAsStringAsync() but I get the same result using for example ReadAsByteArrayAsync().

Termination is done by hooking into Console.CancelKeyPress and cancelling my CancellationTokenSource.

Though this seems straight-forward for some reason I cannot get my head around it and come up with a solution that produces a reliable result. Sometimes everything shuts down as expected, i.e. all tasks complete (cancelled) but more often that not shutdown simply seemingly hangs. Running without debugger (with/without DEBUG) terminates the application but not in the way I expected. Fewer tasks means a more likely clean shutdown.

Pausing all threads in debug when in a "hang"-state seems to indicate some threads are stuck in GetAsync() but it is a bit hard for to see exactly what goes on.

In reality it does not matter that much how the application exits but I would like to understand this and be able to produce a clean and controlled shutdown consistently, which to me seems possible using this construct, but I have most likely missed some details.

using System;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace HttpClientAsyncTest
{
    class Program
    {
        static async Task<int> Main()
        {
            using (var cancellationTokenSource = new CancellationTokenSource())
            {
                Console.CancelKeyPress += (sender, a) =>
                {
                    Console.WriteLine("Stopped by user");
                    cancellationTokenSource.Cancel();
                };

                var task = RunAsync(cancellationTokenSource);
                Console.WriteLine("Running - Press CTRL+C to stop");
                await task;
            }

            Console.WriteLine("All done, exiting");

            return 0;
        }

        private static async Task RunAsync(CancellationTokenSource cts)
        {
            using (var client = new HttpClient())
            {
                try
                {
                    var tasks = Enumerable.Range(1, 10).Select(id => ProcessBatchAsync(client, id, cts.Token));

                    await Task.WhenAll(tasks);
                }
                catch (OperationCanceledException)
                {
                    Console.WriteLine("Operation cancelled somehow");
                }
            }
        }

        private static async Task ProcessBatchAsync(HttpClient client, int id, CancellationToken cancellationToken)
        {
            while (true)
                await ProcessNextBatchAsync(client, id, cancellationToken);
        }

        private static async Task ProcessNextBatchAsync(HttpClient client, int id, CancellationToken cancellationToken)
        {
            using (var response = await client.GetAsync("http://some.payload.to.request", cancellationToken))
            {
                if (response.StatusCode == HttpStatusCode.NotFound)
                    return;

                var data = await response.Content.ReadAsStringAsync();
                Console.WriteLine($"Id: {id} downloaded {data.Length} chars");
            }
        }
    }
}

I expect to see something like this when pressing Ctrl-C:

Running - Press CTRL+C to stop
Id: 6 downloaded 475357 chars
Id: 2 downloaded 475141 chars
Id: 3 downloaded 474927 chars
Id: 5 downloaded 474457 chars
Id: 8 downloaded 474524 chars
Id: 4 downloaded 474643 chars
Id: 7 downloaded 475133 chars
Id: 9 downloaded 475316 chars
Stopped by user
Operation cancelled somehow
All done, exiting
Press any key to continue . . .

But mostly I get something like:

Running - Press CTRL+C to stop
Id: 3 downloaded 474927 chars
Id: 8 downloaded 474524 chars
Id: 5 downloaded 474457 chars
Id: 9 downloaded 475316 chars
Id: 6 downloaded 475357 chars
Id: 7 downloaded 474952 chars
Id: 2 downloaded 475513 chars
Stopped by user
Id: 4 downloaded 475457 chars
Id: 7 downloaded 475133 chars
^CPress any key to continue . . .

Answer 1

Well, rubber ducking myself via StackOverflow worked yet again.

Ctrl+C terminates the application if not canceled via args.Cancel = true, so it has absolulutely zero to do with HttpClient or CancellationToken.

The fix is simply:

Console.CancelKeyPress += (sender, args) =>
{
    args.Cancel = true;
    Console.WriteLine("Stopped by user");
    cancellationTokenSource.Cancel(true);
};