Question

I have a specification that validates codes. It looks like the following:

public ClassificationSpecification : ISpecification<Classification> {
    HashSet<string> codes;

    // constructor elided

    public IsSatisfiedBy(Classification classification) {
        return codes.Contains(classification.Code);
    }
}

The valid codes come out of a Classification table in a database. My question is, which is the better constructor for dependency injection?

public CodeSpecification(IEnumerable<string> codes) {
    this.codes = new HashSet<string>(codes);
}

or

public CodeSpecification(IRepository<Classification> repository) {
    this.codes = new HashSet<string>(repository.Select(x => x.Code));
}

And the all important question: why?

Answer 1

Your second constructor does real work (via the repository) and this is a bad idea. See http://misko.hevery.com/code-reviewers-guide/flaw-constructor-does-real-work/

You need to decide if it’s valid to load all the values (pass in the values you need to the constructor), or you want to look them up on each call (pass in the repository in the constructor and store it).