using System;

using System.Collections.Concurrent;

using System.Threading;

using System.Threading.Tasks;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var strings = new[] { "1", "1", "1", "2", "2", "3" };

        var test = new Test();

        var tasks = strings.Select(test.ProcessSomeThing);

        Task.WhenAll(tasks).Wait();

        foreach (var task in tasks)

        {

            Console.WriteLine(task.Result);

        }

        Console.WriteLine("Hello World");       

    }

    public class Test

    {

        private ConcurrentDictionary<string, Lazy<Task<string>>> dict = 

            new ConcurrentDictionary<string, Lazy<Task<string>>>();

        private int counter = 0;

        public Task<string> ProcessSomeThing(string input)

        {

            return 

                dict.AddOrUpdate(

                    input, 

                    key => new Lazy<Task<string>>(() => GetSomeValueFromAsyncMethod(key), LazyThreadSafetyMode.ExecutionAndPublication),

                   (key, currentValue) => new Lazy<Task<string>>(() => GetSomeValueFromAsyncMethod(key), LazyThreadSafetyMode.ExecutionAndPublication)).Value;

                   //(key, currentValue) => currentValue).Value; // Comment out the line above and uncomment this line to compare results

        }

        private async Task<string> GetSomeValueFromAsyncMethod(string input)

        {

            try

            {

                if (counter % 2 == 1)  

                {

                    ++ counter;

                    throw new Exception("Every other time we throw...");

                }               

                await Task.Delay(50); // Emulate latency...

                return "A value number: " + (++ counter);

            }

            catch (Exception ex)

            {

                Console.WriteLine(ex.Message);

            }

            return "An error was actually handled above...";

        }

    }

}