The Solid Principles

The SOLID Principles
Reading Geek Night

My digital identity
Blog: http://blog.lukesmith.net
Twitter: @lukesmith

What the?
An acronym of acronyms
Collection of best-practices
loose-coupling
Maintainability
Principles – not Rules
Focus on dependency management

Dependency management
Poor dependency management
Hard to change
Fragile
Non-reusable
Good dependency management
Flexible
Robust
Reusable

Single Responsibility Principle
A class should have one, and only one, reason to change

Single Responsibility Principle
Benefits
More readable code
More robust
More maintainable
Smaller functions

Open Closed Principle
Software entities should be open for extension, but closed for modification

Open Closed Principle
Change to an entity cascades changes to dependent modules
Fragile
Unpredictable
Modules should never change
Extend behaviour – add new code
Don’t change existing working code

Liskov Substitution Principle
Derived classes must be substitutable for their base classes

Important for any program conforming to OCP
Caller should not be surprised by substituting base type for subtype
Virtual members must exist in derived classes
Must do useful + expected work

Useful work
Must be implemented
Expected work
Should keep behaviour of base class

Interface Segregation Principle
Make fine grained interfaces that are client specific

Interface Segregation Principle
Deals with disadvantage of “fat” interfaces
Clients forced to depend on interfaces they don’t use subjected to changes in that interface
MembershipProvider – forces 27 methods/properties
Benefits
Lower coupling – better maintainability
Higher cohesion – better robustness

Dependency Inversion Principle
Depend on abstractions, not concretions

Modules should not depend on modules implementing details
Depend on abstractions

Further info
http://www.butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod

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The Solid Principles

by Luke Smith on Dec 09, 2009

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