Object-Oriented Programming really varies from language to language, but the core concepts are the same

Smalltalk was the first language to be called “Object Oriented” (Trivia: Simula was first language to implement the concepts)

OOP is about objects collaborating, not functions or tasks.  Objects send and receive messages

C# is a class-based language, classes are the blueprints for the objects

The object, then, is an instance of a class, and it exists at runtime.

Encapsulation – writing code that is self contained.  Consumers don’t need to know HOW, just that communication is possible.  Expressed well through interfaces and implementation of those interfaces

Interface + Implementation leads to decoupling.  If a class is a blueprint, then an interface is the description of that class.

Inheritance - (.NET is single inheritance) is-a versus has-a –> Is this something that HAS something else, or is this something that IS something else?

Keep hierarchies shallow and take advantage of abstract classes for reuse

If you have multiple sub-classes and something higher up the “chain” changes, it changes everything below it.  Rather than sub-classing, use interfaces and always use the is-a test.

Polymorphism – I am one thing with many forms (Single state with many behaviors)
- Parametric Polymorphism – Generics

Single Responsibility Principle – Class has ONE thing to do and that’s it!  Make sure classes only do ONE thing.  Self-documenting

High cohesion/low coupling – Cohesion is a system that communicates well but doesn’t rely on other things