using System;

using System.Threading;

using System.Collections.Concurrent;

public class Program

{

    public static int counter = 0;

    public static string SlowHello(string ignored)

    {

        Interlocked.Increment(ref counter);

        string s = $"Hello{counter}";

        Console.WriteLine($"generated {s} to add to dict");

        System.Threading.Thread.Sleep(2000 - (counter) * 1000);

        return s;

    }

​

    public static ConcurrentDictionary<string,string> dict = new();

    public static void DoGetOrAdd()

    {

        string s = dict.GetOrAdd("test", SlowHello);

        Console.WriteLine($"got {s} from dict");

    }

    public static void Main()

    {

        var t1 = new Thread(DoGetOrAdd);

        var t2 = new Thread(DoGetOrAdd);

        t1.Start();

        System.Threading.Thread.Sleep(100);

        t2.Start();

        t1.Join();

        t2.Join();

        Console.WriteLine($"Delegate calls: {counter}");

        Console.WriteLine($"Items added to dict: {dict.Count}");

    }

}