> Classes aren't a great abstraction when enforcing program invariants like "this object must at least have fields a and b." Can you elaborate on the functiona

> Classes aren't a great abstraction when enforcing program invariants like "this object must at least have fields a and b." Can you elaborate on the functional difference between - an enhanced dictionary where certain keys are guaranteed as part of the type, and - a record type (class, struct, whatever) with named fields?