C# awaitおもしろコード

Yielder.cs

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace ConsoleApp2
{
    public static class EnumerableEx
    {
        public static IEnumerable<T> Create<T>(Action<IYielder<T>> create)
        {
            if (create == null) throw new ArgumentNullException("create");

            foreach (var x in new Yielder<T>(create))
            {
                yield return x;
            }
        }
    }

    public interface IYielder<in T>
    {
        IAwaitable Return(T value);
        IAwaitable Break();
    }

    public interface IAwaitable
    {
        IAwaiter GetAwaiter();
    }

    public interface IAwaiter : ICriticalNotifyCompletion
    {
        bool IsCompleted { get; }
        void GetResult();
    }

    public class Yielder<T> : IYielder<T>, IAwaitable, IAwaiter, ICriticalNotifyCompletion
    {
        private readonly Action<Yielder<T>> _create;
        private bool _running;
        private bool _hasValue;
        private T _value;
        private bool _stopped;
        private Action _continuation;

        public Yielder(Action<Yielder<T>> create)
        {
            _create = create;
        }

        public IAwaitable Return(T value)
        {
            _hasValue = true;
            _value = value;
            return this;
        }

        public IAwaitable Break()
        {
            _stopped = true;
            return this;
        }

        public Yielder<T> GetEnumerator()
        {
            return this;
        }

        public bool MoveNext()
        {
            if (!_running)
            {
                _running = true;
                _create(this);
            }
            else
            {
                _hasValue = false;
                _continuation();
            }

            return !_stopped && _hasValue;
        }

        public T Current
        {
            get
            {
                return _value;
            }
        }

        public IAwaiter GetAwaiter()
        {
            return this;
        }

        public bool IsCompleted
        {
            get { return false; }
        }

        public void GetResult() { }


        public void OnCompleted(Action continuation)
        {
            _continuation = continuation;
        }

        public void UnsafeOnCompleted(Action continuation)
        {
            _continuation = continuation;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var hoge = "あいうえお";

            var seq = EnumerableEx.Create<int>(async Yield =>
            {
                await Yield.Return(10);
                await Yield.Return(100);

                hoge = "ふがふが"; // インラインで書けるのでお外への副作用が可能

                await Yield.Return(1000);
            });


            foreach (var item in seq)
            {
                Console.WriteLine(item); // 10, 100, 1000
            }

            Console.WriteLine(hoge); // ふがふが
        }
    }
}