using System;

using System.Linq;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        TestSuccess().GetAwaiter().GetResult();

        TestFail().GetAwaiter().GetResult();

        TestCanceled().GetAwaiter().GetResult();

    }

    private static async Task TestSuccess() 

    {

        try {

            await Task

                .WhenAll(DoLongThingAsyncEx1(false), DoLongThingAsyncEx2(false))

                .ContinueWith(t => t.IsFaulted ? throw t.Exception!.Flatten() : t);

        }

        catch (AggregateException ex) 

        {

            Console.WriteLine(string.Join(",\n", ex.InnerExceptions.Select(x => x.Message)));

        }

        catch (Exception ex) 

        {

            Console.WriteLine("TEST SUCCESS UNEXPECTED EXCEPTION: " + ex.GetType().FullName);

        }

        Console.WriteLine("TEST SUCCESS: OK");

    }

    private static async Task TestFail() 

    {

        try {

            await Task

                .WhenAll(DoLongThingAsyncEx1(true), DoLongThingAsyncEx2(true))

                .ContinueWith(t => t.IsFaulted ? throw t.Exception!.Flatten() : t);

        }

        catch (AggregateException ex) 

        {

            Console.WriteLine(string.Join(",\n", ex.InnerExceptions.Select(x => x.Message)));

        }

        catch (Exception ex) 

        {

            Console.WriteLine("TEST FAIL UNEXPECTED EXCEPTION: " + ex.GetType().FullName);

        }

        Console.WriteLine("TEST FAIL: OK");

    }

    private static async Task TestCanceled() 

    {

        try {

            await Task

                .WhenAll(DoLongThingAsyncEx1(false), DoLongThingAsyncEx2(false))

                .ContinueWith(t => throw t.Exception!.Flatten(), TaskContinuationOptions.OnlyOnFaulted);

        }

        catch (AggregateException ex) 

        {

            Console.WriteLine(string.Join(",\n", ex.InnerExceptions.Select(x => x.Message)));

        }

        catch (Exception ex) 

        {

            Console.WriteLine("TEST CANCELED UNEXPECTED EXCEPTION: " + ex.GetType().FullName);

        }

        Console.WriteLine("TEST CANCELED: NOK");

    }

​

    private static Task DoLongThingAsyncEx1(bool error) 

    {

        if (error) return Task.Run(() => { throw new InvalidTimeZoneException(); });

        return Task.FromResult<bool>(true);

    }

​

    private static Task DoLongThingAsyncEx2(bool error) 

    {

        if (error) return Task.Run(() => { throw new InvalidOperationException();});

        return Task.FromResult<bool>(true);

    }

}