VeronaLike.cs

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace VeronaLike
{
    public delegate void WhenCallback<T>(ref T value);

    public class Cown<T>
    {
        private T _value;
        private readonly Queue<WhenCallback<T>> _queue = new Queue<WhenCallback<T>>();
        private Task _task;

        public Cown(T value)
        {
            this._value = value;
        }

        public void When(WhenCallback<T> callback)
        {
            lock (this._queue)
            {
                this._queue.Enqueue(callback);

                if (this._task == null)
                {
                    this._task = Task.Run(() =>
                    {
                        while (true)
                        {
                            WhenCallback<T> callback;
                            lock (this._queue)
                            {
                                if (!this._queue.TryDequeue(out callback))
                                {
                                    this._task = null;
                                    return;
                                }
                            }

                            callback(ref this._value);
                        }
                    });
                }
            }
        }

        public ref readonly T Get() => ref this._value;
    }

    public struct BankAccount
    {
        public ulong Balance;

        public static Cown<BankAccount> Create(ulong openingBalance)
        {
            var account = new BankAccount();
            account.Balance = openingBalance;
            return new Cown<BankAccount>(account);
        }

        public static void Add(Cown<BankAccount> account, ulong amount)
        {
            account.When((ref BankAccount self) =>
            {
                self.Balance += amount;
                Console.WriteLine("new balance: {0}", self.Balance);
            });
        }
    }

    public static class Program
    {
        public static void Example(ulong start)
        {
            var s = BankAccount.Create(start);
            BankAccount.Add(s, 1);
            BankAccount.Add(s, 2);
            BankAccount.Add(s, 3);
            BankAccount.Add(s, 4);
            BankAccount.Add(s, 5);
            BankAccount.Add(s, 6);
        }

        public static void Main(string[] args)
        {
            Example(100);
            Example(200);
            Example(300);
            Example(400);
            Console.WriteLine("done");

            // Wait for all tasks
            Thread.Sleep(1000);
        }
    }
}