using System;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    private static async Task<int> ProcessItem(int item) => await Task.Run(() =>

    {

        var threadNum = Thread.GetCurrentProcessorId();

        Console.WriteLine($"Task {item} is executed on thread #{threadNum}");

        Thread.Sleep(1000);

        return 2 * item;

    });

    public static async Task MainAsync()

    {

        List<int> domain = new() { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

        var tasks = domain.ToDictionary((it) => it, (it) => ProcessItem(it));

        Task.WaitAll(tasks.Values.ToArray());

​

        foreach (var i in domain)

        {

            Console.WriteLine($"2 * {i} = {await tasks[i]}");

        }

    }

    public static void Main()

    {

        var start = DateTime.Now;

        MainAsync().Wait();

        var end = DateTime.Now;

        var diff = (end - start).TotalMilliseconds;

        Console.WriteLine($"10 second operation took {diff}ms");

    }

}