using System;

using System.Collections.Concurrent;

using System.Threading;

using System.Threading.Tasks;

​

public class Program

{

    private static readonly ConcurrentDictionary<string, string> ConcurrentDictionary = new ConcurrentDictionary<string, string>();

    private static readonly LazyConcurrentDictionary<string, string> LazyConcurrentDictionary = new LazyConcurrentDictionary<string, string>();

    public static void Main(string[] args)

    {

        Parallel.For(

            0,

            3,

            i =>

            {

                var value = ConcurrentDictionary.GetOrAdd("key1", k => RunOnceMethod(k, nameof(ConcurrentDictionary)));

                Console.WriteLine(value);

            });

        Console.WriteLine();

        Parallel.For(

            0,

            3,

            i =>

            {

                var value = LazyConcurrentDictionary.GetOrAdd("key2", k => RunOnceMethod(k, nameof(LazyConcurrentDictionary)));

                Console.WriteLine(value);

            });

    }

    private static string RunOnceMethod(string key, string caller)

    {

        Console.WriteLine($"[{caller}] I should only run once");

        return $"{key}_{Guid.NewGuid()}";

    }

}

​

public class LazyConcurrentDictionary<TKey, TValue>

{

    private readonly ConcurrentDictionary<TKey, Lazy<TValue>> concurrentDictionary;

    public LazyConcurrentDictionary()

    {

        this.concurrentDictionary = new ConcurrentDictionary<TKey, Lazy<TValue>>();

    }

    public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)

    {

        var lazyResult = this.concurrentDictionary.GetOrAdd(key, k => new Lazy<TValue>(() => valueFactory(k), LazyThreadSafetyMode.ExecutionAndPublication));

        return lazyResult.Value;

    }

}