Domain Validation in a CQRS architecture

Question

Danger ... Danger Dr. Smith... Philosophical post ahead

The purpose of this post is to determine if placing the validation logic outside of my domain entiti

Answer 1

From my OO experience (I am not a DDD expert) moving your code from the entity to a higher abstraction level (into a command handler) will cause code duplication. This is because every time a command handler gets an email address, it has to instantiate email validation rules. This kind of code will rot after a while, and it will smell very badly. In the current example it might not, if you don't have another command which changes the email address, but in other situations it surely will...

If you don't want to move the rules back to a lower abstraction level, like the entity or an email value object, then I strongly suggest you to reduce the pain by grouping the rules. So in your email example the following 3 rules:

  if(string.IsNullOrWhitespace(email))   throw new DomainException(“...”);
  if(!email.IsEmail())  throw new DomainException();
  if(email.Contains(“@mailinator.com”))  throw new DomainException();

can be part of an EmailValidationRule group which you can reuse easier.

From my point of view there is no explicit answer to the question where to put the validation logic. It can be part of every object depending on the abstraction level. In you current case the formal checking of the email address can be part of an EmailValueObject and the mailinator rule can be part of a higher abstraction level concept in which you state that your user cannot have an email address pointing on that domain. So for example if somebody wants to contact with your user without registration, then you can check her email against formal validation, but you don't have to check her email against the mailinator rule. And so on...

So I completely agree with @pjvds who claimed that this kind of awkward placed validation is a sign of a bad design. I don't think you will have any gain by breaking encapsulation, but it's your choice and it will be your pain.