Question

Where would you place validation code for a domain entity? Inside the class itself in a method like Validate() or outside in the context where the object is being used? Or both?

Answer 1

If the validation only has to consult objects inside the entity then put the validation inside the entity. For a Person entity

• Checking if age > 0 or name is not empty should be within the entity in a validate() method
• Checking if a course is covered by a certain policy (from a list of all policies) will involve quering/reading other course/policy objects which may be not related to the entity itself and should be done by a context/service/validator outside the entity