C# / Object oriented design - maintaining valid object state

Question

When designing a class, should logic to maintain valid state be incorporated in the class or outside of it ? That is, should properties throw exceptions on invalid states (i.e. value out of range, etc.), or should this validation be performed when the instance of the class is being constructed/modified ?

Answer 1

It belongs in the class. Nothing but the class itself (and any helpers it delegates to) should know, or be concerned with, the rules that determine valid or invalid state.

Answer 2

Yes, properties should check on valid/invalid values when being set. That's what it's for.