async-local-fun.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;

static class P
{
    static Task Main() => A();
    static async Task A()
    {
        Show();
        Set();
        Show();
        await B();
        Show();
    }
    static async Task B()
    {
        Show();
        Set();
        await Task.Delay(100);

        // this one rolls back because of the await
        Show(); // B
        await C();
        Show(); // B

        // this one doesn't roll back because there is no await
        Show(); // B
        D();
        Show(); // D
    }
    static async Task C()
    {
        Show();
        Set();
        Show();
    }
    static void D()
    {
        Show();
        Set();
        Show();
    }
    static void Set([CallerMemberName] string value = default)
        => s.Value = value;
    static void Show([CallerMemberName] string caller = default, [CallerLineNumber] int line = default)
        => Console.WriteLine($"[{caller}#{line}]: {s.Value}");
    static readonly AsyncLocal<string> s = new();
}