using System;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

using Microsoft.VisualStudio.Threading;

using System.Collections.Concurrent;

​

namespace ConsoleApp52

{

    class Program

    {

        static int counter = 0;

        const int MaxTasks = 100000; 

        const int MaxSubscription = 50;

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

        static void Main(string[] args)

        {

            var tasks = new Task<int>[MaxTasks];

            for (var i=1; i<=MaxTasks; i++)

            {

                var r = i;

                tasks[i-1] = Task.Run(async () => await GetValue((r % MaxSubscription).ToString()));

            }

            Task.WaitAll(tasks);            

            var maxValue = tasks.Select(t => t.Result).Max();

            Console.WriteLine("MaxValue = " + maxValue);

            // Check that maxValue = MaxSubscription

            // If Interlocked.Increment in AsyncLazy called many times for one key we have more that MaxSubscription value  

            /*

            if (maxValue == MaxSubscription)

            {

               Console.WriteLine("Checked = true");

            } 

            else

            {

               Console.WriteLine("Error");

            } 

            */

        }

​

        static async Task<int> GetValue(string val)

        {

           var res = dict.GetOrAdd(val, new AsyncLazy<int>(

                                        (async () => 

                                         {

                                             await Task.Delay(10); 

                                             var res = Interlocked.Increment(ref counter);

                                             return res;

                                         })));

            var resVal = await res.GetValueAsync().ConfigureAwait(false);           

            return resVal;

        }

    }

}