| > If you get a well-formed XML file that has -7 as a social security number, that is not something that your domain has to deal with. Agreed. That's a type/format problem. But if, for whatever reason, the domain should process social security numbers that starts with 9 differently that should not be outside of the domain by all means. That is your business rule, you should "trap it" in the domain. > where correctness is at a premium The correctness of both are the same. The programming effort and the performance between those differ, but it's never a correctness trade-off. --- But instead of arguing back and forth, let me give you a problem that I had to deal with before: Suppose that to have an entity with a state X there must already exist in the system an entity with state Y. Also, to have an entity with state Y there must already exist in the system an entity with state X. How do you solve this deadlock "the chicken or the egg" problem if you never allow invalid entities to exist? If you do allow invalid entities it's pretty simple: you instantiate both and handles both, together, to the domain. |