using System;

using System.Collections.Concurrent;

using System.Collections.Generic;

using System.Threading.Tasks;

​

public class Program

{

    private static ConcurrentDictionary<string, Lazy<Task<DateTime>>> _cache = new();

    public static async Task Main()

    {

        Console.WriteLine("Hello World");

​

        await DoWork();

​

        Console.WriteLine("----------------------------------------");

        // Now wait 2 seconds.

        await Task.Delay(2000);

        // Lets run this all again .. and see if we get a different result? we should get the same cached value.

        await DoWork();

    }

    public static async Task DoWork()

    {

        List<Task<DateTime>> tasks = new();

        // Just so we can compare this time to the results times.

        var start = DateTime.UtcNow;

        Console.WriteLine("The Start: " + start + " / " + start.Ticks);

        // Setup all the tasks (which are not ran/executed yet).

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

        {

            tasks.Add(GetDateFromCache());

        }

        // Await for all the tasks to run and complete.

        await Task.WhenAll(tasks);

        // Display all the results from the tasks.

        foreach(var task in tasks)

        {

            Console.WriteLine(task.Result + " / " + task.Result.Ticks);

        }

        var end = DateTime.UtcNow;

        Console.WriteLine("The End: " + end + " / " + end.Ticks);

    }

    public static async Task<DateTime> GetDateFromCache()

    {

        var result = await _cache.GetOrAdd("someDateTime", new Lazy<Task<DateTime>>(async () => 

        {

            var someData = await GetDataFromSomeExternalDependency();

            return DateTime.UtcNow;

        })).Value;

        return result;

    }

    public static async Task<int> GetDataFromSomeExternalDependency()

    {

        await Task.Delay(2000);

        return 1;

    }

}