using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Threading.Tasks;

using System.Linq;

public class Program

{

    public static async Task Main()

    {

        long startTime = Stopwatch.GetTimestamp();

        for (int i = 0; i <= 3; i++)

        {

            // Version 1

            //DoWorkAsync($"Task {i}", i);  

            // Version 2

            //await DoWorkAsync($"Task {i}", i);

        }

        // Version 3

        var indexes = new List<int> { 0, 1, 2, 3 };

        var tasks = indexes.Select(async key => await DoWorkAsync($"Task {key}", key));

        await Task.WhenAll(tasks);

        var elapsedTime = Stopwatch.GetElapsedTime(startTime);

        Console.WriteLine();

        Console.WriteLine($"Time taken: {elapsedTime.ToString(@"m\:ss\.fff")}");

        Console.WriteLine("Main ended");

    }

    public static async Task DoWorkAsync(string nameOfTask, int secondsToDelay)

    {

        if (secondsToDelay < 0)

        {

            throw new Exception("Seconds to delay cannot less than 0.");    

        }

        Console.WriteLine($"{nameOfTask} has started.");

        await Task.Delay(secondsToDelay * 1000);

        Console.WriteLine($"{nameOfTask} has finished.");

    }

}