AsyncLocal Demo

Program.cs

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

namespace AsyncLocalPlayground
{
    class Program
    {
        //private static string ThreadId; // too wide! per AppDomain
        //private static ThreadLocal<string> ThreadId = new ThreadLocal<string>(); // too narrow! per thread, await involves different threads.
        private static AsyncLocal<string> ThreadId = new AsyncLocal<string>(); // per async flow

        public static void Main()
        {
            var threads = new[] { "api-request1", "api-request2", "api-request3" }
                .Select(threadId =>
                {
                    // pretend this is a webserver e.g. IIS
                    var t = new Thread(async () =>
                    {
                        ThreadId.Value = threadId;
                        // pretend this is our executing asp.net code
                        var tasks = new[] { "a", "b", "c" }.Select(taskId => DoWork(taskId));
                        await Task.WhenAll(tasks);
                    });
                    t.Start();
                    return t;
                })
                .ToList();

            Console.ReadKey();
        }

        private static async Task DoWork(string taskId)
        {
            await Task.Delay(10);
            Console.WriteLine(ThreadId.Value + ":" + taskId);
        }
    }
}