Program.cs

namespace AwaitPattern;

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

    static async Task UseCustomAsync()
    {
        string result = await CustomAsync();
        Console.WriteLine(result);
    }

    public static MyAwaitableType CustomAsync()
    {
        return new MyAwaitableType();
    }

    class Approximate
    {

        static void ManualUseCustomAsync()
        {
            var awaiter = CustomAsync().GetAwaiter();
            if (awaiter.IsCompleted)
            {
                TheRest(awaiter);
            }
            else
            {
                awaiter.OnCompleted(() => TheRest(awaiter));
            }
        }

        private static void TheRest(MyAwaitableType.MinimalAwaiter awaiter)
        {
            string result = awaiter.GetResult();
            Console.WriteLine(result);
        }
    }

    private class ManualUseCustomAsyncState
    {
        private int state;
        private MyAwaitableType.MinimalAwaiter? awaiter;

        public void MoveNext()
        {
            if (state == 0)
            {
                awaiter = CustomAsync().GetAwaiter();
                if (!awaiter.IsCompleted)
                {
                    state = 1;
                    awaiter.OnCompleted(MoveNext);
                    return;
                }
            }
            string result = awaiter!.GetResult();
            Console.WriteLine(result);
        }
    }

    static void ManualUseCustomAsync()
    {
        var s = new ManualUseCustomAsyncState();
        s.MoveNext();
    }
}