Example of a Sale inside a POS system using event sourcing (Linqpad)

Sale - Event Sourcing.cs

void Main()
{
	var p1 = new StockItem { PartNo = "P100", Description = "Coca Cola", Qty = 1, Cost = 1.00m };
	var p2 = new StockItem { PartNo = "P101", Description = "Pepsi Max", Qty = 2, Cost = 1.00m };
	var p3 = new StockItem { PartNo = "P102", Description = "Pint San Miguel", Qty = 1, Cost = 3.50m };

	var cfg = new SaleConfig { AddOnTax = false, TaxRate = 20.0m };
	var sut = new Sale(Guid.NewGuid(), cfg);
	sut.AddItem(p1);
	sut.AddItem(p2);
	sut.AddItem(p3);
	
	var events = sut.GetUncommittedChanges();
	
	var copy = new Sale(events);
	copy.RemoveItem(p2);
	copy.Discount(new Discount { Percent = 10, Reason = "Father's Day" });

	copy.GetUncommittedChanges();

	//copy.Discount(new Discount { Percent = 10, Reason = "Father's Day" });		
}

public class AggregateRoot
{
	public IEnumerable<Event> GetUncommittedChanges()
	{
		return _changes;
	}
	
	public void MarkAsCommitted()
	{
		_changes.Clear();
	}
	
	public void LoadHistoryFromEvents(IEnumerable<Event> events)
	{
		foreach (var e in events)
		{
			ApplyChange(e, false);
		}
	}

	protected void ApplyChange(Event @event, bool isNew)
	{
		@event.Version = ++Version;
		
		Action<Event> handler;
		if (_handlers.TryGetValue(@event.GetType(), out handler))
			handler(@event);
		
		if (isNew) _changes.Add(@event);
	}

	protected void ApplyChange(Event @event)
	{
		ApplyChange(@event, true);
	}
	
	protected void Handles<T>(Action<T> handler) where T : Event
	{
		_handlers.Add(typeof(T), e => handler((T)e));
	}

	protected Guid Id { get; set; }

	protected int Version { get; set; }
	
	readonly Dictionary<Type, Action<Event>> _handlers = new Dictionary<Type, Action<Event>>();
	readonly List<Event> _changes = new List<Event>();
}

public class Event
{
	
	public Guid Id;
	public int Version;
}

// domain

public class Sale : AggregateRoot
{
	public Sale()
	{
		Handles<SaleWasCreated>(Apply);
		Handles<StockItemWasAdded>(Apply);
		Handles<StockItemWasRemoved>(Apply);
		Handles<SaleWasDiscounted>(Apply);
		Handles<TaxWasCalculated>(Apply);
	}
	
	public Sale(IEnumerable<Event> events) : this()
	{
		LoadHistoryFromEvents(events);
	}
	
	public Sale(Guid userId, SaleConfig config) : this()
	{
		ApplyChange(new SaleWasCreated
		{
			Id = Guid.NewGuid(),
			UserId = userId,
			Total = 0,
			UsesAddOnTax = config.AddOnTax,
			TaxRate = config.TaxRate
		});
	}
	
	public void AddItem(StockItem item)
	{
		ApplyChange(new StockItemWasAdded {
			Id = _id,
			Part = item.PartNo,
			Description = item.Description,
			Qty = item.Qty,
			Cost = item.Cost,
			LineTotal = item.Qty * item.Cost,
			Total = _currentTotal.Value + item.Qty * item.Cost
		});

		ApplyChange(new TaxWasCalculated
		{
			Id = _id,
			Percent = _taxRate,
			Amount = GetTax().Value,
			AddOn = _addOnTax,
			SubTotal = _addOnTax ? _currentTotal.Value : _currentTotal.Value - GetTax().Value,
			Total = _addOnTax ? _currentTotal.Value + GetTax().Value : _currentTotal.Value
		});
	}

	public void RemoveItem(StockItem item)
	{
		ApplyChange(new StockItemWasRemoved
		{
			Id = _id,
			Part = item.PartNo,
			Description = item.Description,
			Qty = item.Qty,
			Cost = item.Cost,
			LineTotal = item.Qty * item.Cost,
			Total = _currentTotal.Value - (item.Qty * item.Cost)
		});

		ApplyChange(new TaxWasCalculated
		{
			Id = _id,
			Percent = _taxRate,
			Amount = GetTax().Value,
			AddOn = _addOnTax,
			SubTotal = _addOnTax ? _currentTotal.Value : _currentTotal.Value - GetTax().Value,
			Total = _addOnTax ? _currentTotal.Value + GetTax().Value : _currentTotal.Value
		});
	}

	public void Discount(Discount discount)
	{
		if (_discounted) throw new InvalidOperationException("Discount cannot be applied more than once");
		
		decimal disc = _currentTotal.Value / 100 * discount.Percent;
		
		ApplyChange(new SaleWasDiscounted {
			Id = _id,
			Percent = discount.Percent,
			Amount = Money.WithValue(disc).Value,
			Reason = discount.Reason,
			Total = _currentTotal.Value - Money.WithValue(disc).Value
		});

		ApplyChange(new TaxWasCalculated
		{
			Id = _id,
			Percent = _taxRate,
			Amount = GetTax().Value,
			AddOn = _addOnTax,
			SubTotal = _addOnTax ? _currentTotal.Value : _currentTotal.Value - GetTax().Value,
			Total = _addOnTax ? _currentTotal.Value + GetTax().Value : _currentTotal.Value
		});
	}

	void Apply(SaleWasCreated @event)
	{
		_id = @event.Id;
		_currentTotal = Money.WithValue(@event.Total);
		_addOnTax = @event.UsesAddOnTax;
		_taxRate = @event.TaxRate;
	}
	
	void Apply(StockItemWasAdded @event)
	{
		_currentTotal += Money.WithValue(@event.LineTotal);
	}

	void Apply(StockItemWasRemoved @event)
	{
		_currentTotal -= Money.WithValue(@event.LineTotal);
	}

	void Apply(SaleWasDiscounted @event)
	{
		_discounted = true;
		_currentTotal = Money.WithValue(@event.Total);
	}

	void Apply(TaxWasCalculated @event)
	{
		_currentTax = Money.WithValue(@event.Amount);
	}
	
	Money GetTax()
	{
		decimal amt = _addOnTax ? _currentTotal.Value * _taxRate / 100 : _currentTotal.Value * _taxRate / (100 + _taxRate);
		return Money.WithValue(amt);
	}
	
	Guid _id;
	bool _addOnTax;
	bool _discounted;
	decimal _taxRate;
	Money _currentTotal = Money.Default;
	Money _currentTax = Money.Default;
}

public class StockItem
{
	public string PartNo { get; set; }
	public string Description { get; set; }
	public int Qty { get; set; }
	public decimal Cost { get; set; }
}

// value objects

public class SaleConfig
{
	public bool AddOnTax { get; set; }
	public decimal TaxRate { get; set; }
}

public class Discount
{
	public int Percent { get; set; }
	public string Reason { get; set; }

	public override bool Equals(object obj)
	{
		if (obj.GetType() != GetType()) return false;
		var d = (Discount)obj;
		return Percent.Equals(d.Percent) && Reason.Equals(d.Reason);
	}

	public override int GetHashCode()
	{
		int hash = 997;
		hash = hash * 23 + Percent.GetHashCode();
		hash = hash * 23 + Reason.GetHashCode();
		return hash;
	}

}

public class Money
{
	public Money(decimal value)
	{
		Value = Math.Round(value, 2, MidpointRounding.AwayFromZero); ;
	}

	public decimal Value { get; private set; }
	
	public virtual Money Negate()
	{
		return new Money(-Value);
	}

	// The addition (+), subtraction (-), multiplication (*) and division (/) operators
	// are overloaded to allow us to provide a more natural means of calculating totals
	// between two IMoney instances. For instance, Instead of having to add up the
	// Value properties of two instances which are of type decimal, we can add now add
	// up two IMoney instances.

	public static Money operator +(Money left, Money right)
	{
		return new Money(left.Value + right.Value);
	}

	public static Money operator -(Money left, Money right)
	{
		return new Money(left.Value - right.Value);
	}

	public static Money operator *(Money left, Money right)
	{
		return new Money(left.Value * right.Value);
	}

	public static Money operator /(Money left, Money right)
	{
		return new Money(left.Value / right.Value);
	}
	
	public override bool Equals(object obj)
	{
		if (obj.GetType() != GetType()) return false;
		var m = (Money)obj;
		return Value.Equals(m.Value);
	}

	public override int GetHashCode()
	{
		int hash = 997;
		hash = hash * 23 + Value.GetHashCode();
		return hash;
	}

	public static Money Default
	{
		get { return new Money(0); }
	}

	public static Money WithValue(decimal value)
	{
		return new Money(value);
	}
}

// events

public class SaleWasCreated : Event
{
	public Guid UserId { get; set; }
	public decimal Total { get; set; }
	public bool UsesAddOnTax { get; set; }
	public decimal TaxRate { get; set; }
}

public class StockItemWasAdded : Event
{
	public string Part { get; set; }
	public string Description { get; set; }
	public int Qty { get; set; }
	public decimal Cost { get; set; }
	public decimal LineTotal { get; set; }
	public decimal Total { get; set; }
}

public class StockItemWasRemoved : Event
{
	public string Part { get; set; }
	public string Description { get; set; }
	public int Qty { get; set; }
	public decimal Cost { get; set; }
	public decimal LineTotal { get; set; }
	public decimal Total { get; set; }
}

public class SaleWasDiscounted : Event
{
	public int Percent { get; set; }
	public decimal Amount { get; set; }
	public decimal Total { get; set; }
	public string Reason { get; set; }
}

public class TaxWasCalculated : Event
{
	public decimal Percent { get; set; }
	public decimal Amount { get; set; }
	public bool AddOn { get; set; }
	public decimal SubTotal { get; set; }
	public decimal Total { get; set; }
}