ref return awaitable.cs

using System;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;

namespace refreturnawaitable
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            new C().M().Wait();
        }
    }

    public class C {

        public A _a;

        public async Task M() {
            _a = new A(this);
            await DoAsync();
            Console.WriteLine(_a._i);
        }

        public ref A DoAsync()
        {
            return ref _a;
        }
    }

    public struct A: ICriticalNotifyCompletion
    {
        private C _c;
        public int _i;

        public A(C c)
        {
            _c = c;
            _i = 0;
        }

        public ref A GetAwaiter(){
            return ref _c._a;
        }

        public void UnsafeOnCompleted(Action continuation)
        {
            OnCompleted(continuation);
        }

        public void OnCompleted(Action continuation)
        {
            _i ++;
            continuation();
        }

        public bool IsCompleted => false;

        public void GetResult()
        {
            _i ++;
        }
    }
}