using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Threading.Tasks;

using System.Linq;

using System.Text.Json;

public class Program

{

    public static async Task Main()

    {

        long startTime = Stopwatch.GetTimestamp();

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

        var tasks = indexes.Select(async key => 

        {

            Console.WriteLine($"Looping through Select()... key = {key}");

            try

            {

                var result = await DoWorkAsync($"Task {key}", key);

                return (key, result);

            }

            catch (Exception ex)

            {

                Console.WriteLine("From try catch...");

                Console.WriteLine(ex.Message);

                return (key, -999);

            }

        })

        .ToList();

        Console.WriteLine("!!! Before calling WhenAll()... !!!\n");

        try

        {

            await Task.WhenAll(tasks);

        }

        catch (OperationCanceledException)

        {

            Console.WriteLine("Cancelled"); 

        }

        catch (AggregateException aggregateException)

        {

            Console.WriteLine("2 or more exceptions were thrown");

        }

        catch (Exception exception)

        {

            Console.WriteLine($"A single exception was thrown: ${exception.Message}");

        }

        var elapsedTime = Stopwatch.GetElapsedTime(startTime);

        Console.WriteLine();

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

        Console.WriteLine();

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

        var successfulResults = tasks

            .Where(t => t.IsCompletedSuccessfully)

            .Select(t => new 

            {

                Key = t.Result.Item1,

                Value = t.Result.Item2

            })

            .ToList();

        Console.WriteLine(JsonSerializer.Serialize(successfulResults));

        Console.WriteLine();

        /*

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

        var allResults = tasks

            .Select(t => t.Result)

            .ToList();

        Console.WriteLine(JsonSerializer.Serialize(allResults));

        */

        Console.WriteLine("Main ended");

    }

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

    {

        if (secondsToDelay < 0)

        {

            throw new Exception($"Seconds with value {secondsToDelay} to delay cannot less than 0.");   

        }

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

        await Task.Delay(secondsToDelay * 1000);

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

        // Just to make things up

        return secondsToDelay * 3;

    }

}