Question

How do you deal with validation on complex aggregates in a domain driven design? Are you consolidating your business rules/validation logic?

I understand argument validation and I understand property validation which can be attached to the models themselves and do things like check that an email address or zipcode is valid or that a first name has a minimum and maximum length.

But what about complex validation that involves multiple models? Where do you typically place these rules & methods within your architecture? And what patterns if any do you use to implement them?

Answer 1

I like Jimmy Bogard’s solution to this problem. He has a post on his blog titled “Entity validation with visitors and extension methods” in which he presents a very elegant approach to entity validation that suggest the implementation of a separate class to store validation code.

public interface IValidator<T>
{
    bool IsValid(T entity);
    IEnumerable<string> BrokenRules(T entity);
}

public class OrderPersistenceValidator : IValidator<Order>
{
    public bool IsValid(Order entity)
    {
        return BrokenRules(entity).Count() == 0;
    }

    public IEnumerable<string> BrokenRules(Order entity)
    {
        if (entity.Id < 0)
            yield return "Id cannot be less than 0.";

        if (string.IsNullOrEmpty(entity.Customer))
            yield return "Must include a customer.";

        yield break;
    }
}