I mostly agree with you except: > that class should be doing the validation, not the Rectangle class itself. I partially disagree. You should have a layer aft

I mostly agree with you except: > that class should be doing the validation, not the Rectangle class itself. I partially disagree. You should have a layer after which all (request) parameters are surely and clearly validated. This avoids repeated validations. This layer should be separated from any logic layer. Additionally a class like Rectangle should have a sanity check in the constructor, and throw a fatal exception if the passed in values are invalid. > you don't always know what different consumers of your type consider to be valid or not I try to make things in such a way that an object of a type is either valid or not. If two consumer have different notions of validity about the ~~same~~ similar (!) type of data, I'd rather create a new type. EDIT: For the exact reason that this allows to use static analysis and avoid silly checks for 0-sized x sides. What does a 0-sized side mean? This type of data can only come from bad user-input, and it should stop at the first line of the controller, and not a step further.