Question

Right now, I have a domain entity named StyleBundle. This StyleBundle takes a list of Styles:

public class StyleBundle
{
    public StyleBundle(List<Style> styles)
    {
        this.Styles = styles;
    }

    public IEnumerable<Style> Styles { get; private set;}
}

So, in my original design, a StyleBundle should never be created with an empty Style list. This was a rule that the domain experts basically said was good.

I wrote this using a guard clause in the constructor:

if (styles.Count() == 0)
   throw new Exception("You must have at least one Style in a StyleBundle.");

which made sure I could not create StyleBundle in an invalid state. I thought an exception made sense here b/c a StyleBundle being created without at least one Style was exceptional in the system.

Of course, change came down the road during the rest of the project, and now it should be possible for a user to create a StyleBundle without Styles, but they should not be allowed to PERSIST a StyleBundle without Styles.

So now I’m looking at my guard clause and realizing that I can’t have the exception thrown from the constructor anymore.

Moving forward, I have a Service/Application layer that my code-behinds interact with when they’re working with StyleBundles. In my Service Layer, I have a StyleBundleService class, and that class exposes basic functionality to the UI… among them is “CreateStyleBundle”.

It seems as if I’ll have to have my Service Layer check to see if the StyleBundle does or does not have any Styles before it’s persisted to the database, but something about this decision feels “wrong” to me.

Anyone run into a similar thing? Basically, the different between the state of an object being valid when “new’ed up” vs. the state of the same object when it comes to persistence?

Thanks!
Mike

Answer 1

I would add an IsValid method to your entity. This would check if the entity is currently in a valid state (in your case, check if there are styles).

This method can be called from your Repository to check if an entity may be persisted. You can add more rules to the IsValid method for specific entities and you can implement something like a collection of Validation errors is you want to throw a meaningful exception.