Some Cool Design Patterns, Practices, and Principles

What Are Design Patterns?
 Well-understood code patterns
 Solve common problems
 Create common pattern vocabulary
 Speed up design and development
 Encourage creating design standards

Useful Design Principles
 Prefer Composition over Inheritance
 Program to abstractions instead of
concrete implementations
 Design loosely coupled components
 Encapsulate what varies
 Hide the likely-to-change code behind an
interface

Time-Tested Design Principles
 S – Single Responsibility Principle
 One reason to change per class
 O – Open-Closed Principle
 Classes open for extension, closed for modification
 L – Liskov Substitution Principle
 Classes can be substituted if designed to be
referenced via their public contracts
 I – Interface Segregation Principle
 Interfaces should not be bloated
 D – Dependency Inversion Principle
 Classes should not be responsible for creating their
dependencies

Anti-Patterns
 Commonly used
 Often seem harmless
 Degrade overall design
 Avoid like a plague!

Example Anti Patterns
 The Arrowhead
 Constructor Over-Injection
 Gold Plating
 Interface Bloat
 God Object
 Magic Literals
 Reinventing The Wheel

The Gang Of Four
 Authors of the canonical book Design
Patterns: Elements of Reusable Object-
Oriented Software
 Erich Gamma, Richard Helm, Ralph
Johnson, John Vlissides
 Main theme 1: Program to an interface
instead of an implementation
 Main theme 2: Favor composition over
inheritance

GoF Design Patterns
 Creational: Abstract Factory, Builder,
Factory Method, Prototype, Singleton
 Structural: Adapter, Bridge, Composite,
Decorator, Façade, Flyweight, Proxy
 Behavioral: Chain of Responsibility,
Command, Interpreter, Iterator,
Mediator, Memento, Observer, State,
Strategy, Template, Visitor

Enterprise Design Patterns
 Patterns found in enterprise applications
 Major proponent: Martin Fowler of
Thoughtworks
 Enterprise patterns go beyond GoF
patterns
 Often specific to technology stacks

Pattern: The Command
Source: dofactory.com

What Makes It Useful?
 Repeatable pattern to execute tasks as
objects
 Allows parameterizing command
invokers with different commands
 Allows undo operations
 Meta-Command Pattern allows creating
composite commands
 Decouples worker objects from client
objects

Practical Uses
 Background processing agents
 Allow third-party vendors to create and
distribute custom commands
 Supports queuing requests
 Facilitates logging requests
 By serializing/deserializing objects to/from log
files
 Log contains history of all command objects
 Allows implementing transaction semantics

Pattern: The Decorator
Source: dofactory.com

What Makes It Useful?
 Facilitates implementing the open-
closed principle
 Allows adding new functionality without
modifying existing classes
 Promotes composition over inheritance
 Promotes iterative development

Practical Uses
 Dynamically creating specialized objects
 Provide flexible API with object
decorators
 Example uses in the .Net framework
 BufferedStream decorates MemoryStream

Pattern: Lazy Load
 Avoid performing time-consuming
operations unless unavoidable
 Load data from database or an external
service as late as possible
 Cache fetched data for as long as
feasible
 Reduce database and/or network load

Code Sample
Private Shared ReadOnly Property
DTSearchIndexLocation() As String
Get
If String.IsNullOrEmpty(_dtSearchIndexLocation)
Then
' fetch from database and assign to
_dtSearchIndexLocation
End If
Return _dtSearchIndexLocation
End Get
End Property

Design Patterns In .Net
 Observer
 Iterator
 Decorator
 Adapter
 Factory
 Strategy
 Composite
 Template Method
 Intercepting Filter
 Page Controller

Further Study
 Writings of The Godfather of Enterprise
Design Patterns a.k.a. Martin Fowler
 Martinfowler.com
 Head-First Design Patterns by O’Reilly
 Writings of Uncle Bob a.k.a. Robert C.
Martin
 ButUncleBob.com, blog.objectmentor.com
 Presentations by Jeremy Miller
 codebetter.com/blogs/jeremy.miller

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