using System;

using System.Collections.Generic;

using System.Threading;

using System.Threading.Tasks;

using System.Runtime.CompilerServices;

public class Program

{

    public static async Task Main()

    {

        await foreach (var v in Wrap(Sequence()))

            ;

    }

​

​

    static IAsyncEnumerable<int> Sequence()

    {

        return Core();

​

        static async IAsyncEnumerable<int> Core([EnumeratorCancellation] CancellationToken ct = default)

        {

            yield return 123;

            await Task.Delay(TimeSpan.FromSeconds(1), ct);

            yield return 456;

        }

    }

​

    static IAsyncEnumerable<int> Wrap(IAsyncEnumerable<int> source)

    {

        return Core(source);

​

        static async IAsyncEnumerable<int> Core(IAsyncEnumerable<int> source, [EnumeratorCancellation] CancellationToken ct = default)

        {

            Test.Context.Value = "Hello world";

            await foreach (var value in source.WithCancellation(ct))

            {

                var before = Test.Context.Value; // Surely this should always be "Hello world", right?

                yield return value;

                var after = Test.Context.Value; // .. and surely this too?

                Console.WriteLine($"Before={before}, after={after}");

            }

        }

    }

}

static class Test

{

    public static readonly AsyncLocal<string> Context = new();

}