How to share validation between Forms and Value Objects in Domain Driven Design?

Question

#1. Validate EmailAddress on the Form

I have a backend form class with an emailAddress property that has validation logic so that I can

Answer 1

Encapsulate the validation logic into a reusable class. These classes are usually called specifications, validators or rules and are part of the domain.

There are multiple ways of doing this, here is an approach that I use:

1. Define an interface Specification that provides a bool IsSatisifed() method.
2. Implement this interface for a specific value object, e.g. EmailWellformedSpec.
3. Enforce the business rule within the domain by using the spec as precondition (i.e. violation is always a programming error).
4. Use the spec for input input validation in the service layer (i.e. violation is a user error).

If you want to combine multiple specs to a larger one, the Specification Pattern is a good approach. Note that you need to pass in the data through the constructor if you use that pattern, but this is not a problem because the specification classes are usually simple.