ThreadRingTask.cs

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

namespace ThreadRingTask
{
    class Agent
    {
        public Agent Next;

        private int id;
        private TaskCompletionSource<int> taskCompletionSource;
        private TaskCompletionSource<int> done;

        public Agent(int id, TaskCompletionSource<int> done)
        {
            this.id = id;
            this.done = done;
        }

        [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.NoInlining)]
        public Task<int> Receive()
        {
            taskCompletionSource = new TaskCompletionSource<int>();
            return taskCompletionSource.Task;
        }

        [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.NoInlining)]
        public void Post(int value)
        {
            taskCompletionSource.SetResult(value);
        }

        public void Start()
        {
            Action<Task<int>> handler = null;
            handler = t =>
            {
                var r = t.Result;
                if (r == 0)
                    done.SetResult(id);
                else
                {
                    Receive().ContinueWith(handler, TaskContinuationOptions.ExecuteSynchronously);
                    
                    Next.Post(t.Result - 1);
                }
            };

            Receive().ContinueWith(handler, TaskContinuationOptions.ExecuteSynchronously);
        }
    }

    class Program
    {
        static void Main()
        {
            var tcs = new TaskCompletionSource<int>();

            var agents = new Agent[503];
            for (var i = 0; i < agents.Length; i++)
                agents[i] = new Agent(i, tcs);

            for (var i = 0; i < agents.Length - 1; i++)
                agents[i].Next = agents[i + 1];
            agents[agents.Length - 1].Next = agents[0];

            foreach (var agent in agents)
                agent.Start();

            var sw = Stopwatch.StartNew();
            
            agents[0].Post(5000000);

            var r = tcs.Task.Result;
            sw.Stop();

            Console.WriteLine(r);

            Console.WriteLine(sw.ElapsedMilliseconds);
        }
    }
}