using System;

using System.Threading;

using System.Threading.Tasks;

using System.Linq;

public class Program

{

    public delegate void SimpleDelegate();

    public static SimpleDelegate OnDelegateInvoke;

    public delegate Task SimpleAsyncDelegate();

    public static SimpleAsyncDelegate OnAsyncDelegateInvoke;

    private static Random _rng = new Random();

    public static async Task Main()

    {

        OnDelegateInvoke += DelayedMessage;

        OnDelegateInvoke += DelayedMessage;

        OnDelegateInvoke += DelayedMessage;

        OnDelegateInvoke += DelayedMessage;

        OnDelegateInvoke += DelayedMessage;

        Console.WriteLine("=========================");

        Console.WriteLine("Start without await");

        Console.WriteLine("=========================");

        OnDelegateInvoke.Invoke();

        Console.WriteLine("Done invoking non-async");

        Thread.Sleep(5000); // Make sure all delegates are finished

        OnAsyncDelegateInvoke += DelayedMessageAsync;

        OnAsyncDelegateInvoke += DelayedMessageAsync;

        OnAsyncDelegateInvoke += DelayedMessageAsync;

        OnAsyncDelegateInvoke += DelayedMessageAsync;

        OnAsyncDelegateInvoke += DelayedMessageAsync;

        Console.WriteLine("=========================");

        Console.WriteLine("Start with await");

        Console.WriteLine("=========================");

        var delegateTasks = OnAsyncDelegateInvoke.GetInvocationList()

            .Cast<SimpleAsyncDelegate>()

            .Select(del => del.Invoke());

        await Task.WhenAll(delegateTasks);

        Console.WriteLine("Done invoking async");

    }

    private static async void DelayedMessage() // Still async, but not awaitable (void return type)

    {

        await Task.Delay(_rng.Next(1000));

        Console.WriteLine("I'm done!");

    }

    private static async Task DelayedMessageAsync()

    {

        await Task.Delay(_rng.Next(1000));

        Console.WriteLine("I'm done!");

    }

}