LazyThreadSafeCookieCounter

using System.Diagnostics;

namespace ConsoleTestApp;

public class LazyThreadSafeCookieCounter
{
    private readonly object _cookiesLock = new();
    private int _cookies = 0;
    public int Cookies { get => _cookies; private set => _cookies = value; }

    private static readonly object _instanceLock = new();
    private static LazyThreadSafeCookieCounter? Instance { get; set; }

    private LazyThreadSafeCookieCounter() { }

    public static LazyThreadSafeCookieCounter GetInstance()
    {
        lock (_instanceLock)
        {
            Instance ??= new LazyThreadSafeCookieCounter();
        }

        return Instance;
    }

    public void IncrementCookie()
    {
        Interlocked.Increment(ref _cookies);
    }

    public void ClearCookies()
    {
        Cookies = 0;
    }
}

public static class Program
{
    public async static Task Main()
    {
        var cookieCounter = LazyThreadSafeCookieCounter.GetInstance();

        IncrementTenMillionTimes(cookieCounter).RunSynchronously();
        cookieCounter.ClearCookies();
        GC.Collect();

        Console.WriteLine("Cookies amount before: " + cookieCounter.Cookies);

        var stopwatch = new Stopwatch();

        var task = IncrementTenMillionTimes(cookieCounter);

        stopwatch.Start();
        task.RunSynchronously();
        stopwatch.Stop();

        Console.WriteLine("Cookies amount after: " + cookieCounter.Cookies);
        Console.WriteLine("Time spent: " + stopwatch.Elapsed.TotalMilliseconds + " ms");
        Console.WriteLine("Done.");
    }

    public static Task IncrementTenMillionTimes(LazyThreadSafeCookieCounter cookieCounter)
    {
        void incrementHundredThousandTimes()
        {
            for (var i = 0; i < 100_000; i++)
                cookieCounter.IncrementCookie();
        }

        return new Task(() =>
        {
            Parallel.For(fromInclusive: 0,
                        toExclusive: 100,
                        (int iteration, ParallelLoopState loopState) => incrementHundredThousandTimes());
        });
    }
}