How do I convert Money based on FX rates in a domain object without Service Location

Question

This link describes a function on a Money object for adding funds together of different currencies.

First I do not want to use the Service Locator pattern. I currently use a pattern of not having the Money object do the conversion itself. It just throws if you try and add different currencies together.

I have the object which owns Money objects (i.e. MultiCurrencyAccount) handle the math operations on Money. My problem is that I am currently constructor injecting a IMoneyConverter into the MultiCurrencyAccount domain object. This feels dirty to me since the IMoneyConverter is really a service which in turn uses a IForeignExchangeService (this one can hit the DB). I know I am not supposed to care about the implementation, since I am dealing with an abstraction, but it feels weird.

I am not sure if I am off base and injecting a service into my domain object is "correct" or if there is a more elegant way to do this.

Answer 1

If you treat Money as Value Objects (which makes tons of sense), it's very tempting to design them with built-in conversion functionality.

However, since you refer to a link about Domain-Driven Design, you should ask yourself if that's actually the correct way to model Money?

In most cases, a bundle of US Dollars aren't just magically going to turn themselves into their equivalent value in Euros. You'd have to take them to some exchange station to trade your Dollars into Euros. Once you've done that, now you have Euros, and no Dollars. If the exchange rate changes afterwards, it's not going to affect the amount of Euros you now have.

I'm not saying that it's always going to be like that. In the end, it depends on your business domain, but I'd think it highly likely that if you ask your business experts you'll find that the business rules for currency trading are decoupled from the money itself. If this is the case, it makes more sense to model currency exchange as a Service separate from the Value Object.