What does the single responsibility principle mean for validation

Question

Does the single responsibility principle mean that your validation rules should be external to the entity?

If so do you use one class per validation rule?

Answer 1

I would normally interpret this to mean that en "entity" and the validation of an entity should be separate concerns. I would normally use a single class that can validate an entire entity, but I would see no reason to constrain its implementation by not letting that class use other classes. But I would not split validation of an entity into multiple classes just because the entity has multiple attributes; I would define the responsibility of the validator as "validate entity X". Sometimes single responsibility just boils down to defining a responsibility in a clever way, and it's really about you making the rules.

Sometimes you can come across entities that have multiple valid states that may be at a different phase of a process; an order may have separate validators for separate phases, but I consider that to be a different responsibility for each validator.

Answer 2

Depends on your definition of entity. You can, for instance, validate input in every service layer but this validation might be handled by separate classes.