Advanced OOPLaws Ÿ Principles Ÿ Idioms

Clint EdmonsonSenior Consultant

Polaris Solutionsclinted@polarissoutions.com

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what is oop?

abstractionencapsulation

inheritancepolymorphism

structural

domain driven design

All software can be composed of only four elements:

- Value objects - Entities- Services- Modules

separation of concerns

Every class and method should have a single responsibility.

All its functionality should be narrowly aligned with that responsibility.

the DRY principle

Don’t repeat yourself.Don’t repeat yourself.Don’t repeat yourself.

theory of one right place

There should be only one right place for a piece of nontrivial of code,

and one right place to make a likely maintenance change.

unit of workDefine entity families around transactional boundaries and aggregate root entities.

the open-closed principle

Software entities (classes, methods, and modules) should be

open for extension

but closed for modification.

design by contractFortify your methods through

preconditions, post-conditions, and invariant assertions.

creational

declarative programming

Use attributes to describe what you want to happen and leverage a framework will take care of the how.

the provider model

Define a public service façade that uses an private implementation to perform all of it’s work.

the provider model

code example

inversion of control

Hide implementation details by letting subsystems automatically create their objects as they are needed.

inversion of control

code example

dependency injection

Declaratively describe dependencies between classes and an IOC framework can automatically instantiate all of them.

dependency injection

code example

object:relational mapping

Leverage IOC and dependency injection to automatically load entities from your database.

behavioral

principle of scenario driven design

All functionality should be driven by usage scenarios.

occam’s razor

The simplest solution is usually the best.

the pareto principle

For many phenomena, 80% of the consequences stem from 20% of the causes

the law of demeter“Don’t talk to your neighbor’s neighbor!”

An object should only call methods and properties belonging to:

- Itself- Any parameters passed in- Objects it creates- Child components

principle of least resource usage

The minimal amount of computational resources should be used to solve a particular need.

principle of least privilege

Provide the minimal level of access necessary for consumers to do their job.

Combined with the previous principle…

Classes and methods should be as stateless and private as possible.

the liskov substitution principle

A derived class should be completely and transparently substitutable for it’s base class.

idempotents

Transactional systems should allow thesame information to be received multiple times without being reprocessed.

cyclomatic complexity

The depth of nested logic should be kept to a minimum.

What are your principles?

Books– Code Complete - McConnell– Domain Driven Design – Evans– Effective C# - Wagner– Framework Design Guidelines – Cwalina & Abrams– Writing Solid Code – Maguire

Links– Application Architecture for .NET– OO Design Principles– Principles of Object Oriented Design– SOLID Principles (Wikipedia)

References

Clint Edmonson

Email: clinted@polarissolutions.com

Blog: http://www.notsotrivial.net

Twitter: @clinted