The open-closed principle is the essential principle of keeping code maintainable and reusable. And the key to the open-closed principle is abstraction with polymorphism. Behaviors like implementing interfaces, or extending classes make polymorphic shapes, but that might not be enough.
The Liskov substitution principle declares that derived classes must be substitutable for their base classes. Or in the words of Barbara Liskov, who raised this principle:
What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.
Never mind. Let's try another one: any foreseeable usage of the instance of a class should be working with the instances of its derived classes.
