Can Value Object have behaviour?

Question

I have a Value Objects - Money and ExchangeRatio. I want to convert one Money to another using ExchangeRatio. So is it good to build a convert behavior on Value Object ExchangeRatio like so:

ExchangeRatio.Convert(Money) returns Money.

Or should I delegate it to some Domain Service instead? In other words can I build a behaviour on Value Object that doesnt change its state but has some logic, mathematic or other different object creation(based on its state) in it?

Yenta

Answer 1

What you are doing sounds perfectly reasonable to me. Eric Evans uses an example of a Paint object in his book that does the same thing. The mixIn method takes another Paint object as input and returns a new Paint object.

With the Paint sample he demonstrates side-effect free functions in the book.

Abigailabigale

Ok, becuase Paint only knows about Paint instances in his behaviour. Here ExchangeRatio knows about Money, so its different case. – Yenta

I would put the convert method on the money object though as you are working with money, not with ExchangeRatio. Thus Money.ConvertTo(ExchangeRatio) – Trillium

Well, Radek, further on the Paint is further refactored to include a Pigment. So the Paint knows about the pigment and uses a ratio internally. So as long as the closure of operations business holds we should be good. Anyway, as SneekyPeet has mentioned you could swop things around a bit as your example actually looks more like a domain service already ;) --- the answer still holds, though: you can have behaviour on your VO. – Abigailabigale