https://github.com/dotnet/docs/issues/4973

HomeController.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using coremvcapp.Models;
using System.Net;
using System.Runtime.CompilerServices;

namespace coremvcapp.Controllers
{
    public class HomeController : Controller
    {
        public IActionResult Index()
        {
            return View();
        }

        public async Task<int> AsyncTask()
        {
            var client = new WebClient();
            var content = await client.DownloadStringTaskAsync("http://google.com");

            return content.Length;
        }

        public CustomAwaitable<int> CustomAwaitable()
        {
            var result = new CustomAwaitable<int>();

            var client = new WebClient();
            client.DownloadStringCompleted += (s, e) => result.SetResult(e.Result.Length);
            client.DownloadStringAsync(new Uri("http://google.com"));

            return result;
        }
    }

    public class CustomAwaitable<T>
    {
        private bool isCompleted;
        private T result;

        private readonly List<Action> completedActions = new List<Action>();

        public Awaiter GetAwaiter() => new Awaiter(this);

        public void SetResult(T result)
        {
            this.result = result;
            isCompleted = true;

            foreach (var action in completedActions)
            {
                action();
            }
        }

        public class Awaiter : INotifyCompletion
        {
            private CustomAwaitable<T> awaitable;

            public Awaiter(CustomAwaitable<T> awaitable) => this.awaitable = awaitable;

            public bool IsCompleted => awaitable.isCompleted;

            public void OnCompleted(Action continuation) => awaitable.completedActions.Add(continuation);

            public T GetResult() => awaitable.result;
        }
    }
}