using System;

using System.Collections.Concurrent;

using System.Threading;

using System.Threading.Tasks;

using System.Net;

​

namespace Rextester

{

    public class Program

    {

        public class SimpleLazy<T> where T : class

        {

            private readonly Func<T> valueFactory;

            private T instance;

            private readonly object locker = new object();

​

            public SimpleLazy(Func<T> valueFactory)

            {

                this.valueFactory = valueFactory;

                this.instance = null;

            }

​

            public T Value

            {

                get

                {

                    lock (locker)

                        return instance ?? (instance = valueFactory());

                }

            }

        }

​

        public class LightsaberProvider

        {

            private static int _firstTime = 1;

​

            public LightsaberProvider()

            {

                Console.WriteLine("LightsaberProvider ctor");

            }

​

            public string GetFor(string jedi)

            {

                Console.WriteLine("LightsaberProvider.GetFor jedi: {0}", jedi);

​

                Thread.Sleep(100);

                if (jedi == "2" && 1 == Interlocked.Exchange(ref _firstTime, 0))

                {

                    throw new Exception("Dark side happened...");

                }

​

                Thread.Sleep(100);

                return string.Format("Lightsaver for: {0}", jedi);

            }

        }

​

        public class LightsabersCache

        {

            private readonly LightsaberProvider _lightsaberProvider;

            private readonly ConcurrentDictionary<string, SimpleLazy<string>> _producedLightsabers;

​

            public LightsabersCache(LightsaberProvider lightsaberProvider)

            {

                _lightsaberProvider = lightsaberProvider;

                _producedLightsabers = new ConcurrentDictionary<string, SimpleLazy<string>>();

            }

​

            public string GetLightsaber(string jedi)

            {

                SimpleLazy<string> result;

                if (!_producedLightsabers.TryGetValue(jedi, out result))

                {

                    result = _producedLightsabers.GetOrAdd(jedi, key => new SimpleLazy<string>(() =>

                    {

                        Console.WriteLine("Lazy Enter");

                        var light = _lightsaberProvider.GetFor(jedi);

                        Console.WriteLine("Lightsaber produced");

                        return light;

                    }));

                }

                return result.Value;

            }

        }

​

        public static void Main(string[] args)

        {

            Test();

            Console.WriteLine("Maximum 1 'Dark side happened...' strings on the console there should be. No more, no less.");

            Console.WriteLine("Maximum 5 lightsabers produced should be. No more, no less.");

        }

​

        private static void Test()

        {

            var cache = new LightsabersCache(new LightsaberProvider());

​

            Parallel.For(0, 15, t =>

            {

                for (int i = 0; i < 10; i++)

                {

                    try

                    {

                        var result = cache.GetLightsaber((t % 5).ToString());

                    }

                    catch (Exception e)

                    {

                        Console.WriteLine(e.Message);

                    }

                }

            });

        }

    }

}