using System;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        Console.WriteLine("begin");

        // なんかどこかで誰かがこんな Post してる気配がなくもない
        Post(() =>
        {
            X().Wait();

            Console.WriteLine("end");
        });

        _lastTask.Wait();
    }

    static Task _lastTask = Task.FromResult<object>(null);

    static void Post(Action continueation)
    {
        var t = _lastTask.ContinueWith(_ => continueation());
        _lastTask = t;
    }

    static Task X()
    {
        Console.WriteLine("X1");
        var tcs = new TaskCompletionSource<object>();

        /* ↓みたいなコードだったとして
         * await Task.CompletedTask;
         * await Task.Delay(10);
         */
        Task.CompletedTask.ContinueWith(_1 =>
        {
            Post(() =>
            {
                Console.WriteLine("X2");

                Task.Delay(10).ContinueWith(_2 =>
                {
                    Post(() =>
                    {
                        Console.WriteLine("X3");
                        tcs.TrySetResult(null);
                    });
                });
            });
        });

        return tcs.Task;
    }
}