Information Technology Reference
In-Depth Information
scope of the classes, you have limited the code you need to change to
update and extend the entire system.
Yo u c o u l d a l s o c r e a t e a p u b l i c a b s t r a c t b a s e c l a s s f o r P h o n e Va l i d a t o r, w h i c h
could contain common implementation algorithms. The consumers could
access the public functionality through the accessible base class. In this
example, I prefer the implementation using public interfaces because there
is little, if any, shared functionality. Other uses would be better served with
public abstract base classes. Either way you implement it, fewer classes are
publicly accessible.
In addition, fewer public types will create a smaller public surface area that
will facilitate unit testing coverage. If there are fewer public types, there
are fewer publicly accessible methods for which you need to create tests.
Also, if more of the public APIs are exposed through interfaces, you have
automatically created a system whereby you can replace those types using
some kind of stubs for unit test purposes.
Those classes and interfaces that you expose publicly to the outside world
are your contract: You must live up to them. The more cluttered that inter-
face is, the more constrained your future direction is. The fewer public
types you expose, the more options you have to extend and modify any
implementation in the future.
Inheritance
Abstract base classes provide a common ancestor for a class hierarchy. An
interface describes one atomic piece of functionality that can be imple-
mented by a type. Each has its place, but it is a different place. Interfaces
are a way to design by contract: A type that implements an interface must
supply an implementation for expected methods. Abstract base classes
provide a common abstraction for a set of related types. It's a cliché, but
it's one that works: Inheritance means “is a,” and interfaces means
“behaves like.” These clichés have lived so long because they provide a
means to describe the differences in both constructs: Base classes describe
what an object is; interfaces describe one way in which it behaves.
Interfaces describe a set of functionality, or a contract. You can create
placeholders for anything in an interface: methods, properties, indexers,
and events. Any type that implements the interface must supply concrete
