using System.Threading.Tasks;

using System.Threading;

using System;

using System.Linq;

​

public sealed class Program

{

    private readonly ThreadLocal<Random> ThreadLocalRandom = new(() => new Random());

    private readonly SemaphoreSlim _lock = new(1);

    private int _counter = 0;

    public async Task EnsureRunThingsExchange()

    {

        // Give some delay to simulate contention

        await Task.Delay(TimeSpan.FromMilliseconds(ThreadLocalRandom.Value.Next(0, 10)));

        await _lock.WaitAsync();

        try 

        {   

            ++_counter;

            if (_counter == 1) 

            {

                Console.WriteLine($"{DateTime.Now.ToString("HH:mm:ss.fff")} Initialized.");

            } 

            else 

            {

                Console.WriteLine($"{DateTime.Now.ToString("HH:mm:ss.fff")} Truly serialized: {_counter - 1}.");

            }

        } 

        finally 

        {

            _lock.Release();

        }

    }

​

    public static async Task Main()

    {

        var r = new Program();

        await Task.WhenAll(Enumerable.Range(0, 10_000).Select(_ => r.EnsureRunThingsExchange()));

    }

}