first_attempt_to_use_rx.cs

using System;
using System.Collections.Generic;

namespace TryRx
{
    class Program
    {
        static void Main(string[] args)
        {
            Order.Paid.Subscribe(_ => Console.WriteLine("Paid"));

            var order = new Order();            
            
            order.MarkPaid(DateTime.Now);
            order.MarkPaid(DateTime.Now);
        }

        public enum OrderStatus
        {   
            Pending,
            Paid
        }        

        public class Order        
        {
            private OrderStatus _status;
            private DateTime? _paidDate;

            private static readonly Subject<Order> PaidSubj = new Subject<Order>();
            public static IObservable<Order> Paid { get { return PaidSubj; } }

            public void MarkPaid(DateTime paidDate)
            {
                _paidDate = paidDate;
                _status = OrderStatus.Paid;                
                PaidSubj.OnNext(this);
            }
        }
    }
}