AsyncFunc.cs

using System;
using System.Linq;
using System.Threading.Tasks;

namespace EventActionFunc
{
    public class Program
    {
        static async Task Main(string[] args)
        {
            var test = new Testclass();
            await test.AsyncTestMethod();
        }
    }

    public class Testclass
    {
        Action<bool> MyAction { get; set; }
        Func<Task<bool>> MyAwaitedAction { get; set; }

        public async Task AsyncTestMethod()
        {
            Log("Setup");
            for (int index = 0; index < 3; index++)
            {
                var capturedIndex = index;
                MyAction += (result) =>
                {
                    Log($"{result} ({capturedIndex})");
                };
            }

            MyAwaitedAction += async () =>
            {
                Log("Action 1 starting");
                await Task.Delay(3000);
                Log($"Action 1 finished");
                return true;
            };

            MyAwaitedAction += async () => {
                Log("Action 2 starting");
                await Task.Delay(5000);
                Log("Action 2 finished");
                return false;
            };

            MyAwaitedAction += () => {
                Log("Action 3 starting");
                Log("Action 3 finished");
                return Task.FromResult(false);
            };

            MyAwaitedAction += DoSomethingInteresting;

            Log("Starting...");

            var results = await MyAwaitedAction.WhenAll();

            foreach (var result in results)
            {
                MyAction(result);
            }

            Log("Done!");
        }

        public void Log(string message)
        {
            Log(DateTime.Now, message);
        }

        public void Log(DateTime when, string message)
        {
            Console.WriteLine($"{when:s} {message}");
        }

        private async Task<bool> DoSomethingInteresting()
        {
            Log("Something interesting starting");
            await Task.Delay(2500);
            Log("Something interesting finished");
            return true;
        }
    }

    public static class FuncTaskTExtensions
    {
        public static Task<T[]> WhenAll<T>(this Func<Task<T>> func)
        {
            var tasks = func.GetInvocationList()
                .OfType<Func<Task<T>>>()
                .Select(d => d());
            return Task.WhenAll(tasks);
        }
    }
}

/*
2020-05-04T14:44:08 Setup
2020-05-04T14:44:08 Starting...
2020-05-04T14:44:08 Action 1 starting
2020-05-04T14:44:08 Action 2 starting
2020-05-04T14:44:08 Action 3 starting
2020-05-04T14:44:08 Action 3 finished
2020-05-04T14:44:08 Something interesting starting
2020-05-04T14:44:11 Something interesting finished
2020-05-04T14:44:11 Action 1 finished
2020-05-04T14:44:13 Action 2 finished
2020-05-04T14:44:13 True (0)
2020-05-04T14:44:13 True (1)
2020-05-04T14:44:13 True (2)
2020-05-04T14:44:13 False (0)
2020-05-04T14:44:13 False (1)
2020-05-04T14:44:13 False (2)
2020-05-04T14:44:13 False (0)
2020-05-04T14:44:13 False (1)
2020-05-04T14:44:13 False (2)
2020-05-04T14:44:13 True (0)
2020-05-04T14:44:13 True (1)
2020-05-04T14:44:13 True (2)
2020-05-04T14:44:13 Done!*/