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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?
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Being immutable, Value Objects cannot be changed once they are created. Modifying one is conceptually the same as discarding the old one and creating a new one. Frequently, the Value Object can define helper methods (or extensions methods) that assist with such operations. The built-in string object in the .
Yes, you can have value objects inside other value objects. I think the simplest example of this would be the class Money, which contains an amount and a Currency, which is another VO.
Value objects are not only containers of data - they can also contain business logic. The fact that the value objects are also immutable makes the business operations both thread-safe and side-effect free.
In computer science, a value object is a small object that represents a simple entity whose equality is not based on identity: i.e. two value objects are equal when they have the same value, not necessarily being the same object. Examples of value objects are objects representing an amount of money or a date range.
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.
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