using System;
using System.Linq;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Reflection;
using System.Linq.Expressions;
using System.Threading;
using System.Threading.Tasks;

var values = new ConcurrentDictionary<string, int>(StringComparer.Ordinal);
using var factories = new Barrier(2);
int calls = 0;
int[] results = new int[2];
int Factory(string key) { int candidate = Interlocked.Increment(ref calls); if (!factories.SignalAndWait(TimeSpan.FromSeconds(5))) throw new TimeoutException(); return candidate; }
OwnedThreads.Run(() => results[0] = values.GetOrAdd("a", Factory), () => results[1] = values.GetOrAdd("a", Factory));
Console.WriteLine($"shared:{results[0] == results[1] && results[0] == values["a"]}");
Console.WriteLine($"entries:{values.Count}");
Console.WriteLine($"factory-calls:{calls}");

public static class OwnedThreads
{
    public static void Run(params Action[] actions)
    {
        var errors = new ConcurrentQueue<Exception>();
        var threads = actions.Select(action => new Thread(() => { try { action(); } catch (Exception error) { errors.Enqueue(error); } }) { IsBackground = true }).ToArray();
        foreach (var thread in threads) thread.Start();
        foreach (var thread in threads) if (!thread.Join(TimeSpan.FromSeconds(8))) throw new TimeoutException("Owned worker did not finish");
        if (!errors.IsEmpty) throw new AggregateException(errors);
    }
}