enums
struct
visibility (private field with accessor)
static members
static methods need static fields
constructor; and struct to class

CS0568: Structs cannot contain explicit parameterless constructors
constructors - init
constructors - reuse
validation in constructor
interface
implementing interface; multiple interfaces
ch 4.9 - inheritance
overriding; and marking virtual
overriding; property access
overriding; using base method
protecting class from inheritance
constructor chaining (using base) - and solving inheritance (hiding) problems
abstract method - making whole class abstract
fixes - abstract class instance; abstract method implementation
creating ToString for the class