An Introduction to Design Patterns

An Introduction to
Design Patterns
(C#, WPF, ASP.NET, AJAX, PATTERNS)

Formatted By-
Ashish Tripathi
Email: ak.tripathi@yahoo.com

Note: All the contents present into the book is extracted from the site
http://www.dofactory.com/Patterns/Patterns.aspx without any textual change only
formatting and presentation of the text is changed. For any genuine change in the book
you can ask for the editable copy of the book by sending a mail to
ak.tripathi@yahoo.com with subject line ‘Design Patterns’

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 An Introduction to Design Patterns

Design Patterns...........................................................................................................3
1. Introduction...................................................................................................3
2. Type of Design Patterns ..................................................................................4
2.1 Creational Patterns .............................................................................4
2.2 Structural Patterns .............................................................................4
2.3 Behavioral Patterns.............................................................................4
3. Creational Design Patterns .............................................................................5
3.1 Abstract Factory Design Pattern ..........................................................5
3.2 Builder Design Pattern ......................................................................11
3.3 Factory Method Design Pattern..........................................................18
3.4 Prototype Design Pattern ...................................................................23
3.5 Singleton Design Pattern ...................................................................28
4. Structural Design Patterns ...........................................................................32
4.1 Adapter Design Pattern .....................................................................32
4.2 Bridge Design Pattern .......................................................................37
4.3 Composite Design Pattern .................................................................43
4.4 Decorator Design Pattern ..................................................................49
4.5 Facade Design Pattern ......................................................................55
4.6 Flyweight Design Pattern...................................................................60
4.7 Proxy Design Pattern.........................................................................66
5. Behavioral Design Patterns...........................................................................70
5.1 Chain of Responsibility Design pattern ..............................................70
5.2 Command Design Pattern..................................................................76
5.3 Interpreter Design Pattern .................................................................82
5.4 Iterator Design pattern ......................................................................87
5.5 Mediator Design Pattern....................................................................94
5.6 Memento Design Pattern .................................................................100
5.7 Observer Design Pattern..................................................................106
5.8 State Design Pattern .......................................................................112
5.9 Strategy Design Pattern...................................................................120
5.10 Template Method Design Pattern ...................................................125
5.11 Visitor Design Pattern ...................................................................130

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 An Introduction to Design Patterns

Design Patterns
1. Introduction
Design patterns are recurring solutions to software design problems
you find again and again in real-world application development.
Patterns are about design and interaction of objects, as well as
providing a communication platform concerning elegant, reusable
solutions to commonly encountered programming challenges.

The Gang of Four (GoF) patterns are generally considered the foundation for all other
patterns. They are categorized in three groups: Creational, Structural, and Behavioral.
Here you will find information on these important patterns.

To give you a head start, the C# source code is provided in 2 forms: 'structural' and
'real-world'. Structural code uses type names as defined in the pattern definition and
UML diagrams. Real-world code provides real-world programming situations where you
may use these patterns.

A third form, '.NET optimized' demonstrates design patterns that exploit built-in .NET
2.0 features, such as, generics, attributes, delegates, and reflection. These and much
more are available in our Design Pattern Framework 2.0TM. See our Singleton page for a
.NET 2.0 Optimized code sample.

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 An Introduction to Design Patterns

2. Type of Design Patterns

2.1 Creational Patterns

Abstract Factory Creates an instance of several families of classes
Builder Separates object construction from its representation
Factory Method Creates an instance of several derived classes
Prototype A fully initialized instance to be copied or cloned
Singleton A class of which only a single instance can exist

2.2 Structural Patterns

Adapter Match interfaces of different classes
Bridge Separates an object’s interface from its implementation
Composite A tree structure of simple and composite objects
Decorator Add responsibilities to objects dynamically
Facade A single class that represents an entire subsystem
Flyweight A fine-grained instance used for efficient sharing
Proxy An object representing another object

2.3 Behavioral Patterns

Chain of Resp. A way of passing a request between a chain of objects
Command Encapsulate a command request as an object
Interpreter A way to include language elements in a program
Iterator Sequentially access the elements of a collection
Mediator Defines simplified communication between classes
Memento Capture and restore an object's internal state
Observer A way of notifying change to a number of classes
State Alter an object's behavior when its state changes
Strategy Encapsulates an algorithm inside a class
Template Method Defer the exact steps of an algorithm to a subclass
Visitor Defines a new operation to a class without change

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 An Introduction to Design Patterns

3. Creational Design Patterns

3.1 Abstract Factory Design Pattern

3.1.1 Definition
Provide an interface for creating families of related or dependent objects
without specifying their concrete classes.

3.1.2 UML class diagram

The classes and/or objects participating in this pattern are:

AbstractFactory (ContinentFactory)
Declares an interface for operations that create abstract products
ConcreteFactory (AfricaFactory, AmericaFactory)
Implements the operations to create concrete product objects
AbstractProduct (Herbivore, Carnivore)
Declares an interface for a type of product object
Product (Wildebeest, Lion, Bison, Wolf)
Defines a product object to be created by the corresponding concrete factory

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 An Introduction to Design Patterns

Implements the AbstractProduct interface

Client (AnimalWorld)
Uses interfaces declared by AbstractFactory and AbstractProduct classes

3.1.3 Sample code in C#

This structural code demonstrates the Abstract Factory pattern creating parallel
hierarchies of objects. Object creation has been abstracted and there is no need for
hard-coded class names in the client code.

// Abstract Factory pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Abstract.Structural
{
// MainApp test application

class MainApp
{
public static void Main()
{
// Abstract factory #1
AbstractFactory factory1 = new ConcreteFactory1();
Client c1 = new Client(factory1);
c1.Run();

// Abstract factory #2
AbstractFactory factory2 = new ConcreteFactory2();
Client c2 = new Client(factory2);
c2.Run();

// Wait for user input

Console.Read();
}
}

// "AbstractFactory"

abstract class AbstractFactory

{
public abstract AbstractProductA CreateProductA();
public abstract AbstractProductB CreateProductB();
}

// "ConcreteFactory1"

class ConcreteFactory1 : AbstractFactory

{
public override AbstractProductA CreateProductA()
{
return new ProductA1();
}
public override AbstractProductB CreateProductB()

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 An Introduction to Design Patterns

{
return new ProductB1();
}
}

// "ConcreteFactory2"

class ConcreteFactory2 : AbstractFactory

{
public override AbstractProductA CreateProductA()
{
return new ProductA2();
}
public override AbstractProductB CreateProductB()
{
return new ProductB2();
}
}

// "AbstractProductA"

abstract class AbstractProductA

{
}

// "AbstractProductB"

abstract class AbstractProductB

{
public abstract void Interact(AbstractProductA a);
}

// "ProductA1"

class ProductA1 : AbstractProductA

{
}

// "ProductB1"

class ProductB1 : AbstractProductB

{
public override void Interact(AbstractProductA a)
{
Console.WriteLine(this.GetType().Name +
" interacts with " + a.GetType().Name);
}
}

// "ProductA2"

class ProductA2 : AbstractProductA

{
}

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 An Introduction to Design Patterns

// "ProductB2"

class ProductB2 : AbstractProductB

{
public override void Interact(AbstractProductA a)
{
Console.WriteLine(this.GetType().Name +
" interacts with " + a.GetType().Name);
}
}

// "Client" - the interaction environment of the products

class Client
{
private AbstractProductA AbstractProductA;
private AbstractProductB AbstractProductB;

// Constructor
public Client(AbstractFactory factory)
{
AbstractProductB = factory.CreateProductB();
AbstractProductA = factory.CreateProductA();
}

public void Run()

{
AbstractProductB.Interact(AbstractProductA);
}
}
}

Output
ProductB1 interacts with ProductA1
ProductB2 interacts with ProductA2

This real-world code demonstrates the creation of different animal worlds for a
computer game using different factories. Although the animals created by the Continent
factories are different, the interactions among the animals remain the same.

// Abstract Factory pattern -- Real World example

using System;

namespace DoFactory.GangOfFour.Abstract.RealWorld
{
// MainApp test application

class MainApp
{
public static void Main()
{

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 An Introduction to Design Patterns

// Create and run the Africa animal world

ContinentFactory africa = new AfricaFactory();
AnimalWorld world = new AnimalWorld(africa);
world.RunFoodChain();

// Create and run the America animal world

ContinentFactory america = new AmericaFactory();
world = new AnimalWorld(america);
world.RunFoodChain();

// Wait for user input

Console.Read();
}
}

// "AbstractFactory"

abstract class ContinentFactory

{
public abstract Herbivore CreateHerbivore();
public abstract Carnivore CreateCarnivore();
}

// "ConcreteFactory1"

class AfricaFactory : ContinentFactory

{
public override Herbivore CreateHerbivore()
{
return new Wildebeest();
}
public override Carnivore CreateCarnivore()
{
return new Lion();
}
}

// "ConcreteFactory2"

class AmericaFactory : ContinentFactory

{
public override Herbivore CreateHerbivore()
{
return new Bison();
}
public override Carnivore CreateCarnivore()
{
return new Wolf();
}
}

// "AbstractProductA"

abstract class Herbivore

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{
}

// "AbstractProductB"

abstract class Carnivore

{
public abstract void Eat(Herbivore h);
}

// "ProductA1"

class Wildebeest : Herbivore

{
}

// "ProductB1"

class Lion : Carnivore

{
public override void Eat(Herbivore h)
{
// Eat Wildebeest
Console.WriteLine(this.GetType().Name +
" eats " + h.GetType().Name);
}
}

// "ProductA2"

class Bison : Herbivore

{
}

// "ProductB2"

class Wolf : Carnivore

{
public override void Eat(Herbivore h)
{
// Eat Bison
Console.WriteLine(this.GetType().Name +
" eats " + h.GetType().Name);
}
}

// "Client"

class AnimalWorld
{
private Herbivore herbivore;
private Carnivore carnivore;

// Constructor

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public AnimalWorld(ContinentFactory factory)

{
carnivore = factory.CreateCarnivore();
herbivore = factory.CreateHerbivore();
}

public void RunFoodChain()

{
carnivore.Eat(herbivore);
}
}
}

3.2 Builder Design Pattern

3.2.1 Definition
Separate the construction of a complex object from its representation so
that the same construction process can create different representations.

3.2.2 UML Class diagram

The classes and/or objects participating in this pattern are:

Builder (VehicleBuilder)
Specifies an abstract interface for creating parts of a Product object
ConcreteBuilder (MotorCycleBuilder, CarBuilder, ScooterBuilder)
Constructs and assembles parts of the product by implementing the Builder interface
defines and keeps track of the representation it creates provides an interface for
retrieving the product
Director (Shop)
Constructs an object using the Builder interface
Product (Vehicle)
Represents the complex object under construction. ConcreteBuilder builds the
product's internal representation and defines the process by which it's assembled
includes classes that define the constituent parts, including interfaces for assembling
the parts into the final result.

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3.2.3 Sample code in C#

This structural code demonstrates the Builder pattern in which complex objects are
created in a step-by-step fashion. The construction process can create different object
representations and provides a high level of control over the assembly of the objects.

// Builder pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Builder.Structural
{
// MainApp test application

public class MainApp

{
public static void Main()
{
// Create director and builders
Director director = new Director();

Builder b1 = new ConcreteBuilder1();

Builder b2 = new ConcreteBuilder2();

// Construct two products

director.Construct(b1);
Product p1 = b1.GetResult();
p1.Show();

director.Construct(b2);
Product p2 = b2.GetResult();
p2.Show();

// Wait for user

Console.Read();
}
}

// "Director"

class Director
{
// Builder uses a complex series of steps
public void Construct(Builder builder)
{
builder.BuildPartA();
builder.BuildPartB();
}
}

// "Builder"

abstract class Builder

{

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public abstract void BuildPartA();

public abstract void BuildPartB();
public abstract Product GetResult();
}

// "ConcreteBuilder1"

class ConcreteBuilder1 : Builder

{
private Product product = new Product();

public override void BuildPartA()

{
product.Add("PartA");
}

public override void BuildPartB()

{
product.Add("PartB");
}

public override Product GetResult()

{
return product;
}
}

// "ConcreteBuilder2"

class ConcreteBuilder2 : Builder

{
private Product product = new Product();

public override void BuildPartA()

{
product.Add("PartX");
}

public override void BuildPartB()

{
product.Add("PartY");
}

public override Product GetResult()

{
return product;
}
}

// "Product"

class Product
{
ArrayList parts = new ArrayList();

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public void Add(string part)

{
parts.Add(part);
}

public void Show()

{
Console.WriteLine("\nProduct Parts -------");
foreach (string part in parts)
Console.WriteLine(part);
}
}
}

Output
Product Parts -------
PartA
PartB

Product Parts -------

PartX
PartY

This real-world code demonstates the Builder pattern in which different vehicles are
assembled in a step-by-step fashion. The Shop uses VehicleBuilders to construct a
variety of Vehicles in a series of sequential steps.

// Builder pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Builder.RealWorld
{
// MainApp test application

public class MainApp

{
public static void Main()
{
// Create shop with vehicle builders
Shop shop = new Shop();
VehicleBuilder b1 = new ScooterBuilder();
VehicleBuilder b2 = new CarBuilder();
VehicleBuilder b3 = new MotorCycleBuilder();

// Construct and display vehicles

shop.Construct(b1);
b1.Vehicle.Show();

shop.Construct(b2);
b2.Vehicle.Show();

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shop.Construct(b3);
b3.Vehicle.Show();

// Wait for user

Console.Read();
}
}

// "Director"

class Shop
{
// Builder uses a complex series of steps
public void Construct(VehicleBuilder vehicleBuilder)
{
vehicleBuilder.BuildFrame();
vehicleBuilder.BuildEngine();
vehicleBuilder.BuildWheels();
vehicleBuilder.BuildDoors();
}
}

// "Builder"

abstract class VehicleBuilder

{
protected Vehicle vehicle;

// Property
public Vehicle Vehicle
{
get{ return vehicle; }
}

public abstract void BuildFrame();

public abstract void BuildEngine();
public abstract void BuildWheels();
public abstract void BuildDoors();
}

// "ConcreteBuilder1"

class MotorCycleBuilder : VehicleBuilder

{
public override void BuildFrame()
{
vehicle = new Vehicle("MotorCycle");
vehicle["frame"] = "MotorCycle Frame";
}

public override void BuildEngine()

{
vehicle["engine"] = "500 cc";

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public override void BuildWheels()

{
vehicle["wheels"] = "2";
}

public override void BuildDoors()

{
vehicle["doors"] = "0";
}
}

// "ConcreteBuilder2"

class CarBuilder : VehicleBuilder

{
public override void BuildFrame()
{
vehicle = new Vehicle("Car");
vehicle["frame"] = "Car Frame";
}

public override void BuildEngine()

{
vehicle["engine"] = "2500 cc";
}

public override void BuildWheels()

{
vehicle["wheels"] = "4";
}

public override void BuildDoors()

{
vehicle["doors"] = "4";
}
}

// "ConcreteBuilder3"

class ScooterBuilder : VehicleBuilder

{
public override void BuildFrame()
{
vehicle = new Vehicle("Scooter");
vehicle["frame"] = "Scooter Frame";
}

public override void BuildEngine()

{
vehicle["engine"] = "50 cc";
}

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public override void BuildWheels()

{
vehicle["wheels"] = "2";
}

public override void BuildDoors()

{
vehicle["doors"] = "0";
}
}

// "Product"

class Vehicle
{
private string type;
private Hashtable parts = new Hashtable();

// Constructor
public Vehicle(string type)
{
this.type = type;
}

// Indexer (i.e. smart array)

public object this[string key]
{
get{ return parts[key]; }
set{ parts[key] = value; }
}

public void Show()

{
Console.WriteLine("\n---------------------------");
Console.WriteLine("Vehicle Type: {0}", type);
Console.WriteLine(" Frame : {0}", parts["frame"]);
Console.WriteLine(" Engine : {0}", parts["engine"]);
Console.WriteLine(" #Wheels: {0}", parts["wheels"]);
Console.WriteLine(" #Doors : {0}", parts["doors"]);
}
}
}

Output
---------------------------
Vehicle Type: Scooter
Frame : Scooter Frame
Engine : none
#Wheels: 2
#Doors : 0

---------------------------
Vehicle Type: Car
Frame : Car Frame

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Engine : 2500 cc
#Wheels: 4
#Doors : 4

---------------------------
Vehicle Type: MotorCycle
Frame : MotorCycle Frame
Engine : 500 cc
#Wheels: 2
#Doors : 0

3.3 Factory Method Design Pattern

3.3.1 Definition
Define an interface for creating an object, but let subclasses decide
which class to instantiate. Factory Method lets a class defer
instantiation to subclasses.

3.3.2 UML class diagram

The classes and/or objects participating in this pattern are:

Product (Page)
Defines the interface of objects the factory method creates
ConcreteProduct (SkillsPage, EducationPage, ExperiencePage)
Implements the Product interface
Creator (Document)
Declares the factory method, which returns an object of type Product. Creator may also
define a default implementation of the factory method that returns a default
ConcreteProduct object.
may call the factory method to create a Product object.
ConcreteCreator (Report, Resume)
Overrides the factory method to return an instance of a ConcreteProduct.

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3.3.3 Sample code in C#

This structural code demonstrates the Factory method offering great flexibility in
creating different objects. The Abstract class may provide a default object, but each
subclass can instantiate an extended version of the object.
// Factory Method pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Factory.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// An array of creators
Creator[] creators = new Creator[2];
creators[0] = new ConcreteCreatorA();
creators[1] = new ConcreteCreatorB();

// Iterate over creators and create products

foreach(Creator creator in creators)
{
Product product = creator.FactoryMethod();
Console.WriteLine("Created {0}",
product.GetType().Name);
}

// Wait for user

Console.Read();
}
}

// "Product"

abstract class Product

{
}

// "ConcreteProductA"

class ConcreteProductA : Product

{
}

// "ConcreteProductB"

class ConcreteProductB : Product

{
}

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// "Creator"

abstract class Creator

{
public abstract Product FactoryMethod();
}

// "ConcreteCreator"

class ConcreteCreatorA : Creator

{
public override Product FactoryMethod()
{
return new ConcreteProductA();
}
}

// "ConcreteCreator"

class ConcreteCreatorB : Creator

{
public override Product FactoryMethod()
{
return new ConcreteProductB();
}
}
}

Output
Created ConcreteProductA
Created ConcreteProductB

This real-world code demonstrates the Factory method offering flexibility in creating
different documents. The derived Document classes Report and Resume instantiate
extended versions of the Document class. Here, the Factory Method is called in the
constructor of the Document base class.

// Factory Method pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Factory.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Note: constructors call Factory Method
Document[] documents = new Document[2];

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 An Introduction to Design Patterns

documents[0] = new Resume();

documents[1] = new Report();

// Display document pages

foreach (Document document in documents)
{
Console.WriteLine("\n" + document.GetType().Name+ "--");
foreach (Page page in document.Pages)
{
Console.WriteLine(" " + page.GetType().Name);
}
}

// Wait for user

Console.Read();
}
}

// "Product"

abstract class Page

{
}

// "ConcreteProduct"

class SkillsPage : Page

{
}

// "ConcreteProduct"

class EducationPage : Page

{
}

// "ConcreteProduct"

class ExperiencePage : Page

{
}

// "ConcreteProduct"

class IntroductionPage : Page

{
}

// "ConcreteProduct"

class ResultsPage : Page

{
}

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 An Introduction to Design Patterns

// "ConcreteProduct"

class ConclusionPage : Page

{
}

// "ConcreteProduct"

class SummaryPage : Page

{
}

// "ConcreteProduct"

class BibliographyPage : Page

{
}

// "Creator"

abstract class Document

{
private ArrayList pages = new ArrayList();

// Constructor calls abstract Factory method

public Document()
{
this.CreatePages();
}

public ArrayList Pages

{
get{ return pages; }
}

// Factory Method
public abstract void CreatePages();
}

// "ConcreteCreator"

class Resume : Document

{
// Factory Method implementation
public override void CreatePages()
{
Pages.Add(new SkillsPage());
Pages.Add(new EducationPage());
Pages.Add(new ExperiencePage());
}
}

// "ConcreteCreator"

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 An Introduction to Design Patterns

class Report : Document

{
// Factory Method implementation
public override void CreatePages()
{
Pages.Add(new IntroductionPage());
Pages.Add(new ResultsPage());
Pages.Add(new ConclusionPage());
Pages.Add(new SummaryPage());
Pages.Add(new BibliographyPage());
}
}
}

Output
Resume -------
SkillsPage
EducationPage
ExperiencePage

Report -------
IntroductionPage
ResultsPage
ConclusionPage
SummaryPage
BibliographyPage

3.4 Prototype Design Pattern

3.4.1 Definition
Specify the kind of objects to create using a prototypical instance, and
create new objects by copying this prototype.

3.4.2 UML class diagram

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The classes and/or objects participating in this pattern are:

Prototype (ColorPrototype)
Declares an interface for cloning itself
ConcretePrototype (Color)
Implements an operation for cloning itself
Client (ColorManager)
Creates a new object by asking a prototype to clone itself

3.4.3 Sample code in C#

This structural code demonstrates the Prototype pattern in which new objects are
created by copying pre-existing objects (prototypes) of the same class.

// Prototype pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Prototype.Structural
{

// MainApp test application

class MainApp
{

static void Main()

{
// Create two instances and clone each

ConcretePrototype1 p1 = new ConcretePrototype1("I");

ConcretePrototype1 c1 = (ConcretePrototype1)p1.Clone();
Console.WriteLine ("Cloned: {0}", c1.Id);

ConcretePrototype2 p2 = new ConcretePrototype2("II");

ConcretePrototype2 c2 = (ConcretePrototype2)p2.Clone();
Console.WriteLine ("Cloned: {0}", c2.Id);

// Wait for user

Console.Read();
}
}

// "Prototype"

abstract class Prototype

{
private string id;

// Constructor
public Prototype(string id)
{
this.id = id;
}

// Property

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public string Id
{
get{ return id; }
}

public abstract Prototype Clone();

}

// "ConcretePrototype1"

class ConcretePrototype1 : Prototype

{
// Constructor
public ConcretePrototype1(string id) : base(id)
{
}

public override Prototype Clone()

{
// Shallow copy
return (Prototype)this.MemberwiseClone();
}
}

// "ConcretePrototype2"

class ConcretePrototype2 : Prototype

{
// Constructor
public ConcretePrototype2(string id) : base(id)
{
}

public override Prototype Clone()

{
// Shallow copy
return (Prototype)this.MemberwiseClone();
}
}
}

Output
Cloned: I
Cloned: II

This real-world code demonstrates the Prototype pattern in which new Color objects are
created by copying pre-existing, user-defined Colors of the same type.

// Prototype pattern -- Real World example

using System;
using System.Collections;

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namespace DoFactory.GangOfFour.Prototype.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
ColorManager colormanager = new ColorManager();

// Initialize with standard colors

colormanager["red" ] = new Color(255, 0, 0);
colormanager["green"] = new Color( 0, 255, 0);
colormanager["blue" ] = new Color( 0, 0, 255);

// User adds personalized colors

colormanager["angry"] = new Color(255, 54, 0);
colormanager["peace"] = new Color(128, 211, 128);
colormanager["flame"] = new Color(211, 34, 20);

Color color;

// User uses selected colors

string name = "red";
color = colormanager[name].Clone() as Color;

name = "peace";
color = colormanager[name].Clone() as Color;

name = "flame";
color = colormanager[name].Clone() as Color;

// Wait for user

Console.Read();
}
}

// "Prototype"

abstract class ColorPrototype

{
public abstract ColorPrototype Clone();
}

// "ConcretePrototype"

class Color : ColorPrototype

{
private int red;
private int green;
private int blue;

// Constructor

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 An Introduction to Design Patterns

public Color(int red, int green, int blue)

{
this.red = red;
this.green = green;
this.blue = blue;
}

// Create a shallow copy

public override ColorPrototype Clone()
{
Console.WriteLine(
"Cloning color RGB: {0,3},{1,3},{2,3}",
red, green, blue);

return this.MemberwiseClone() as ColorPrototype;

}
}

// Prototype manager

class ColorManager
{
Hashtable colors = new Hashtable();

// Indexer
public ColorPrototype this[string name]
{
get
{
return colors[name] as ColorPrototype;
}
set
{
colors.Add(name, value);
}
}
}
}

Output
Cloning color RGB: 255, 0, 0
Cloning color RGB: 128,211,128
Cloning color RGB: 211, 34, 2

****** 27
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 An Introduction to Design Patterns

3.5 Singleton Design Pattern

3.5.1 Definition
Ensure a class has only one instance and provide a global point of
access to it.

3.5.2 UML class diagram

The classes and/or objects participating in this pattern are:

Singleton (LoadBalancer)
Defines an Instance operation that lets clients access its unique instance. Instance is a
class operation.
Responsible for creating and maintaining its own unique instance.

3.5.3 Sample code in C#

This structural code demonstrates the Singleton pattern which assures only a single
instance (the singleton) of the class can be created.

// Singleton pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Singleton.Structural
{

// MainApp test application

class MainApp
{

static void Main()

{
// Constructor is protected -- cannot use new
Singleton s1 = Singleton.Instance();
Singleton s2 = Singleton.Instance();

if (s1 == s2)
{
Console.WriteLine("Objects are the same instance");
}

// Wait for user

Console.Read();
}
}

// "Singleton"

****** 28
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 An Introduction to Design Patterns

class Singleton
{
private static Singleton instance;

// Note: Constructor is 'protected'

protected Singleton()
{
}

public static Singleton Instance()

{
// Use 'Lazy initialization'
if (instance == null)
{
instance = new Singleton();
}

return instance;
}
}
}

Output
Objects are the same instance

This real-world code demonstrates the Singleton pattern as a LoadBalancing object.

Only a single instance (the singleton) of the class can be created because servers may
dynamically come on- or off-line and every request must go throught the one object that
has knowledge about the state of the (web) farm.

// Singleton pattern -- Real World example

using System;
using System.Collections;
using System.Threading;

namespace DoFactory.GangOfFour.Singleton.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
LoadBalancer b4 = LoadBalancer.GetLoadBalancer();

// Same instance?
if (b1 == b2 && b2 == b3 && b3 == b4)

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 An Introduction to Design Patterns

{
Console.WriteLine("Same instance\n");
}

// All are the same instance -- use b1 arbitrarily

// Load balance 15 server requests
for (int i = 0; i < 15; i++)
{
Console.WriteLine(b1.Server);
}

// Wait for user

Console.Read();
}
}

// "Singleton"

class LoadBalancer
{
private static LoadBalancer instance;
private ArrayList servers = new ArrayList();

private Random random = new Random();

// Lock synchronization object

private static object syncLock = new object();

// Constructor (protected)
protected LoadBalancer()
{
// List of available servers
servers.Add("ServerI");
servers.Add("ServerII");
servers.Add("ServerIII");
servers.Add("ServerIV");
servers.Add("ServerV");
}

public static LoadBalancer GetLoadBalancer()

{
// Support multithreaded applications through
// 'Double checked locking' pattern which (once
// the instance exists) avoids locking each
// time the method is invoked
if (instance == null)
{
lock (syncLock)
{
if (instance == null)
{
instance = new LoadBalancer();
}
}

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 An Introduction to Design Patterns

return instance;
}

// Simple, but effective random load balancer

public string Server

{
get
{
int r = random.Next(servers.Count);
return servers[r].ToString();
}
}
}
}

Output
Same instance

ServerIII
ServerII
ServerI
ServerII
ServerI
ServerIII
ServerI
ServerIII
ServerIV
ServerII
ServerII
ServerIII
ServerIV
ServerII
ServerIV

****** 31
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 An Introduction to Design Patterns

4. Structural Design Patterns

4.1 Adapter Design Pattern
4.1.1 Definition
Convert the interface of a class into another interface clients expect.
Adapter lets classes work together that couldn't otherwise because of
incompatible interfaces.

4.1.2 UML class diagram

The classes and/or objects participating in this pattern are:

Target (ChemicalCompound)
Defines the domain-specific interface that Client uses.
Adapter (Compound)
Adapts the interface Adaptee to the Target interface.
Adaptee (ChemicalDatabank)
Defines an existing interface that needs adapting.
Client (AdapterApp)
Collaborates with objects conforming to the Target interface.

4.1.3 Sample code in C#

This structural code demonstrates the Adapter pattern which maps the interface of one
class onto another so that they can work together. These incompatible classes may
come from different libraries or frameworks.

// Adapter pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Adapter.Structural
{

// Mainapp test application

class MainApp
{
static void Main()

****** 32
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 An Introduction to Design Patterns

{
// Create adapter and place a request
Target target = new Adapter();
target.Request();

// Wait for user

Console.Read();
}
}

// "Target"

class Target
{
public virtual void Request()
{
Console.WriteLine("Called Target Request()");
}
}

// "Adapter"

class Adapter : Target

{
private Adaptee adaptee = new Adaptee();

public override void Request()

{
// Possibly do some other work
// and then call SpecificRequest
adaptee.SpecificRequest();
}
}

// "Adaptee"

class Adaptee
{
public void SpecificRequest()
{
Console.WriteLine("Called SpecificRequest()");
}
}
}

Output
Called SpecificRequest()

This real-world code demonstrates the use of a legacy chemical databank. Chemical
compound objects access the databank through an Adapter interface.
// Adapter pattern -- Real World example

****** 33
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 An Introduction to Design Patterns

using System;

namespace DoFactory.GangOfFour.Adapter.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Non-adapted chemical compound
Compound stuff = new Compound("Unknown");
stuff.Display();

// Adapted chemical compounds

Compound water = new RichCompound("Water");
water.Display();

Compound benzene = new RichCompound("Benzene");

benzene.Display();

Compound alcohol = new RichCompound("Alcohol");

alcohol.Display();

// Wait for user

Console.Read();
}
}

// "Target"

class Compound
{
protected string name;
protected float boilingPoint;
protected float meltingPoint;
protected double molecularWeight;
protected string molecularFormula;

// Constructor
public Compound(string name)
{
this.name = name;
}

public virtual void Display()

{
Console.WriteLine("\nCompound: {0} ------ ", name);
}
}

// "Adapter"

****** 34
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 An Introduction to Design Patterns

class RichCompound : Compound

{
private ChemicalDatabank bank;

// Constructor
public RichCompound(string name) : base(name)
{
}

public override void Display()

{
// Adaptee
bank = new ChemicalDatabank();
boilingPoint = bank.GetCriticalPoint(name, "B");
meltingPoint = bank.GetCriticalPoint(name, "M");
molecularWeight = bank.GetMolecularWeight(name);
molecularFormula = bank.GetMolecularStructure(name);

base.Display();
Console.WriteLine(" Formula: {0}", molecularFormula);
Console.WriteLine(" Weight : {0}", molecularWeight);
Console.WriteLine(" Melting Pt: {0}", meltingPoint);
Console.WriteLine(" Boiling Pt: {0}", boilingPoint);
}
}

// "Adaptee"

class ChemicalDatabank
{
// The Databank 'legacy API'
public float GetCriticalPoint(string compound, string point)
{
float temperature = 0.0F;

// Melting Point
if (point == "M")
{
switch (compound.ToLower())
{
case "water" : temperature = 0.0F; break;
case "benzene" : temperature = 5.5F; break;
case "alcohol" : temperature = -114.1F; break;
}
}
// Boiling Point
else
{
switch (compound.ToLower())
{
case "water" : temperature = 100.0F; break;
case "benzene" : temperature = 80.1F; break;
case "alcohol" : temperature = 78.3F; break;
}

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 An Introduction to Design Patterns

}
return temperature;
}

public string GetMolecularStructure(string compound)

{
string structure = "";

switch (compound.ToLower())
{
case "water" : structure = "H20"; break;
case "benzene" : structure = "C6H6"; break;
case "alcohol" : structure = "C2H6O2"; break;
}
return structure;
}

public double GetMolecularWeight(string compound)

{
double weight = 0.0;
switch (compound.ToLower())
{
case "water" : weight = 18.015; break;
case "benzene" : weight = 78.1134; break;
case "alcohol" : weight = 46.0688; break;
}
return weight;
}
}
}

Output
Compound: Unknown ------

Compound: Water ------

Formula: H20
Weight : 18.015
Melting Pt: 0
Boiling Pt: 100

Compound: Benzene ------

Formula: C6H6
Weight : 78.1134
Melting Pt: 5.5
Boiling Pt: 80.1

Compound: Alcohol ------

Formula: C2H6O2
Weight : 46.0688
Melting Pt: -114.1
Boiling Pt: 78.3

****** 36
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 An Introduction to Design Patterns

4.2 Bridge Design Pattern

4.2.1 Definition
Decouple an abstraction from its implementation so that the two can
vary independently.

4.2.2 UML class diagram

The classes and/or objects participating in this pattern are:

Abstraction (BusinessObject)
Defines the abstraction's interface. Maintains a reference to an object of type
implementor.
RefinedAbstraction (CustomersBusinessObject)
Extends the interface defined by Abstraction.
Implementor (DataObject)
Defines the interface for implementation classes. This interface doesn't have to
correspond exactly to Abstraction's interface; in fact the two interfaces can be quite
different. Typically the Implementation interface provides only primitive operations, and
Abstraction defines higher-level operations based on these primitives.
ConcreteImplementor (CustomersDataObject)
Implements the Implementor interface and defines its concrete implementation.

4.2.3 Sample code in C#

This structural code demonstrates the Bridge pattern which separates (decouples) the
interface from its implementation. The implementation can evolve without changing
clients which use the abstraction of the object.

// Bridge pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Bridge.Structural
{

****** 37
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 An Introduction to Design Patterns

// MainApp test application

class MainApp
{
static void Main()
{
Abstraction ab = new RefinedAbstraction();

// Set implementation and call

ab.Implementor = new ConcreteImplementorA();
ab.Operation();

// Change implemention and call

ab.Implementor = new ConcreteImplementorB();
ab.Operation();

// Wait for user

Console.Read();
}
}

// "Abstraction"

class Abstraction
{
protected Implementor implementor;

// Property
public Implementor Implementor
{
set{ implementor = value; }
}

public virtual void Operation()

{
implementor.Operation();
}
}

// "Implementor"

abstract class Implementor

{
public abstract void Operation();
}

// "RefinedAbstraction"

class RefinedAbstraction : Abstraction

{
public override void Operation()
{
implementor.Operation();

****** 38
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 An Introduction to Design Patterns

}
}

// "ConcreteImplementorA"

class ConcreteImplementorA : Implementor

{
public override void Operation()
{
Console.WriteLine("ConcreteImplementorA Operation");
}
}

// "ConcreteImplementorB"

class ConcreteImplementorB : Implementor

{
public override void Operation()
{
Console.WriteLine("ConcreteImplementorB Operation");
}
}
}

Output
ConcreteImplementorA Operation
ConcreteImplementorB Operation

This real-world code demonstrates the Bridge pattern in which a BusinessObject

abstraction is decoupled from the implementation in DataObject. The DataObject
implementations can evolve dynamically without changing any clients.

// Bridge pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Bridge.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create RefinedAbstraction
Customers customers =
new Customers("Chicago");

// Set ConcreteImplementor
customers.Data = new CustomersData();

****** 39
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 An Introduction to Design Patterns

// Exercise the bridge

customers.Show();
customers.Next();
customers.Show();
customers.Next();
customers.Show();
customers.New("Henry Velasquez");

customers.ShowAll();

// Wait for user

Console.Read();
}
}

// "Abstraction"

class CustomersBase
{
private DataObject dataObject;
protected string group;

public CustomersBase(string group)

{
this.group = group;
}

// Property
public DataObject Data
{
set{ dataObject = value; }
get{ return dataObject; }
}

public virtual void Next()

{
dataObject.NextRecord();
}

public virtual void Prior()

{
dataObject.PriorRecord();
}

public virtual void New(string name)

{
dataObject.NewRecord(name);
}

public virtual void Delete(string name)

{
dataObject.DeleteRecord(name);
}

****** 40
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 An Introduction to Design Patterns

public virtual void Show()

{
dataObject.ShowRecord();
}

public virtual void ShowAll()

{
Console.WriteLine("Customer Group: " + group);
dataObject.ShowAllRecords();
}
}

// "RefinedAbstraction"

class Customers : CustomersBase

{
// Constructor
public Customers(string group) : base(group)
{
}

public override void ShowAll()

{
// Add separator lines
Console.WriteLine();
Console.WriteLine ("------------------------");
base.ShowAll();
Console.WriteLine ("------------------------");
}
}

// "Implementor"

abstract class DataObject

{
public abstract void NextRecord();
public abstract void PriorRecord();
public abstract void NewRecord(string name);
public abstract void DeleteRecord(string name);
public abstract void ShowRecord();
public abstract void ShowAllRecords();
}

// "ConcreteImplementor"

class CustomersData : DataObject

{
private ArrayList customers = new ArrayList();
private int current = 0;

public CustomersData()
{
// Loaded from a database
customers.Add("Jim Jones");

****** 41
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 An Introduction to Design Patterns

customers.Add("Samual Jackson");
customers.Add("Allen Good");
customers.Add("Ann Stills");
customers.Add("Lisa Giolani");
}

public override void NextRecord()

{
if (current <= customers.Count - 1)
{
current++;
}
}

public override void PriorRecord()

{
if (current > 0)
{
current--;
}
}

public override void NewRecord(string name)

{
customers.Add(name);
}

public override void DeleteRecord(string name)

{
customers.Remove(name);
}

public override void ShowRecord()

{
Console.WriteLine(customers[current]);
}

public override void ShowAllRecords()

{
foreach (string name in customers)
{
Console.WriteLine(" " + name);
}
}
}
}

Output
Jim Jones
Samual Jackson
Allen Good
------------------------
Customer Group: Chicago
Jim Jones

****** 42
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 An Introduction to Design Patterns

Samual Jackson
Allen Good
Ann Stills
Lisa Giolani
Henry Velasquez
------------------------

4.3 Composite Design Pattern

4.3.1 Definition
Compose objects into tree structures to represent part-whole
hierarchies. Composite lets clients treat individual objects and
compositions of objects uniformly.

4.3.2 UML class diagram

The classes and/or objects participating in this pattern are:

Component (DrawingElement)
Declares the interface for objects in the composition.
Implements default behavior for the interface common to all classes, as appropriate.
Declares an interface for accessing and managing its child components.
(optional) defines an interface for accessing a component's parent in the recursive
structure, and implements it if that's appropriate.
Leaf (PrimitiveElement)
Represents leaf objects in the composition. A leaf has no children.
defines behavior for primitive objects in the composition.
Composite (CompositeElement)
Defines behavior for components having children. Stores child components. Implements
child-related operations in the Component interface.
Client (CompositeApp)
Manipulates objects in the composition through the Component interface.

****** 43
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 An Introduction to Design Patterns

4.3.3Sample code in C#
This structural code demonstrates the Composite pattern which allows the creation of a
tree structure in which individual nodes are accessed uniformly whether they are leaf
nodes or branch (composite) nodes.

// Composite pattern -- Structural example

using System;
using System.Collections;
namespace DoFactory.GangOfFour.Composite.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create a tree structure
Composite root = new Composite("root");
root.Add(new Leaf("Leaf A"));
root.Add(new Leaf("Leaf B"));

Composite comp = new Composite("Composite X");

comp.Add(new Leaf("Leaf XA"));
comp.Add(new Leaf("Leaf XB"));

root.Add(comp);
root.Add(new Leaf("Leaf C"));

// Add and remove a leaf

Leaf leaf = new Leaf("Leaf D");
root.Add(leaf);
root.Remove(leaf);

// Recursively display tree

root.Display(1);

// Wait for user

Console.Read();
}
}

// "Component"

abstract class Component

{
protected string name;

// Constructor
public Component(string name)
{
this.name = name;
}

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 An Introduction to Design Patterns

public abstract void Add(Component c);

public abstract void Remove(Component c);
public abstract void Display(int depth);
}

// "Composite"

class Composite : Component

{
private ArrayList children = new ArrayList();

// Constructor
public Composite(string name) : base(name)
{
}

public override void Add(Component component)

{
children.Add(component);
}

public override void Remove(Component component)

{
children.Remove(component);
}

public override void Display(int depth)

{
Console.WriteLine(new String('-', depth) + name);

// Recursively display child nodes

foreach (Component component in children)
{
component.Display(depth + 2);
}
}
}

// "Leaf"

class Leaf : Component

{
// Constructor
public Leaf(string name) : base(name)
{
}

public override void Add(Component c)

{
Console.WriteLine("Cannot add to a leaf");
}

public override void Remove(Component c)

****** 45
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 An Introduction to Design Patterns

{
Console.WriteLine("Cannot remove from a leaf");
}

public override void Display(int depth)

{
Console.WriteLine(new String('-', depth) + name);
}
}
}

Output
-root
---Leaf A
---Leaf B
---Composite X
-----Leaf XA
-----Leaf XB
---Leaf C

This real-world code demonstrates the Composite pattern used in building a graphical
tree structure made up of primitive nodes (lines, circles, etc) and composite nodes
(groups of drawing elements that make up more complex elements).

// Composite pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Composite.RealWorld
{

// Mainapp test application

class MainApp
{
static void Main()
{
// Create a tree structure
CompositeElement root =
new CompositeElement("Picture");
root.Add(new PrimitiveElement("Red Line"));
root.Add(new PrimitiveElement("Blue Circle"));
root.Add(new PrimitiveElement("Green Box"));

CompositeElement comp =
new CompositeElement("Two Circles");
comp.Add(new PrimitiveElement("Black Circle"));
comp.Add(new PrimitiveElement("White Circle"));
root.Add(comp);

// Add and remove a PrimitiveElement

****** 46
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 An Introduction to Design Patterns

PrimitiveElement pe =
new PrimitiveElement("Yellow Line");
root.Add(pe);
root.Remove(pe);

// Recursively display nodes

root.Display(1);

// Wait for user

Console.Read();
}
}

// "Component" Treenode

abstract class DrawingElement

{
protected string name;

// Constructor
public DrawingElement(string name)
{
this.name = name;
}

public abstract void Add(DrawingElement d);

public abstract void Remove(DrawingElement d);
public abstract void Display(int indent);
}

// "Leaf"

class PrimitiveElement : DrawingElement

{
// Constructor
public PrimitiveElement(string name) : base(name)
{
}

public override void Add(DrawingElement c)

{
Console.WriteLine(
"Cannot add to a PrimitiveElement");
}

public override void Remove(DrawingElement c)

{
Console.WriteLine(
"Cannot remove from a PrimitiveElement");
}

public override void Display(int indent)

{
Console.WriteLine(

****** 47
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 An Introduction to Design Patterns

new String('-', indent) + " " + name);

}
}

// "Composite"

class CompositeElement : DrawingElement

{
private ArrayList elements = new ArrayList();

// Constructor
public CompositeElement(string name) : base(name)
{
}

public override void Add(DrawingElement d)

{
elements.Add(d);
}

public override void Remove(DrawingElement d)

{
elements.Remove(d);
}

public override void Display(int indent)

{
Console.WriteLine(new String('-', indent) +
"+ " + name);

// Display each child element on this node

foreach (DrawingElement c in elements)
{
c.Display(indent + 2);
}
}
}
}

Output
-+ Picture
--- Red Line
--- Blue Circle
--- Green Box
---+ Two Circles
----- Black Circle
----- White Circle

****** 48
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 An Introduction to Design Patterns

4.4 Decorator Design Pattern

4.4.1 Definition
Attach additional responsibilities to an object dynamically. Decorators
provide a flexible alternative to subclassing for extending functionality.

4.4.2 UML class diagram

The classes and/or objects participating in this pattern are:

Component (LibraryItem)
Defines the interface for objects that can have responsibilities added to them
dynamically.
ConcreteComponent (Book, Video)
Defines an object to which additional responsibilities can be attached.
Decorator (Decorator)
Maintains a reference to a Component object and defines an interface that conforms to
Component's interface.
ConcreteDecorator (Borrowable)
Adds responsibilities to the component.

4.4.3 Sample code in C#

This structural code demonstrates the Decorator pattern which dynamically adds extra
functionality to an existing object.
// Decorator pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Decorator.Structural
{

****** 49
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 An Introduction to Design Patterns

// MainApp test application

class MainApp
{
static void Main()
{
// Create ConcreteComponent and two Decorators
ConcreteComponent c = new ConcreteComponent();
ConcreteDecoratorA d1 = new ConcreteDecoratorA();
ConcreteDecoratorB d2 = new ConcreteDecoratorB();

// Link decorators
d1.SetComponent(c);
d2.SetComponent(d1);

d2.Operation();

// Wait for user

Console.Read();
}
}

// "Component"

abstract class Component

{
public abstract void Operation();
}

// "ConcreteComponent"

class ConcreteComponent : Component

{
public override void Operation()
{
Console.WriteLine("ConcreteComponent.Operation()");
}
}

// "Decorator"

abstract class Decorator : Component

{
protected Component component;

public void SetComponent(Component component)

{
this.component = component;
}

public override void Operation()

{
if (component != null)
{

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 An Introduction to Design Patterns

component.Operation();
}
}
}

// "ConcreteDecoratorA"

class ConcreteDecoratorA : Decorator

{
private string addedState;

public override void Operation()

{
base.Operation();
addedState = "New State";
Console.WriteLine("ConcreteDecoratorA.Operation()");
}
}

// "ConcreteDecoratorB"

class ConcreteDecoratorB : Decorator

{
public override void Operation()
{
base.Operation();
AddedBehavior();
Console.WriteLine("ConcreteDecoratorB.Operation()");
}

void AddedBehavior()
{
}
}
}

Output
ConcreteComponent.Operation()
ConcreteDecoratorA.Operation()
ConcreteDecoratorB.Operation()

This real-world code demonstrates the Decorator pattern in which 'borrowable'

functionality is added to existing library items (books and videos).
// Decorator pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Decorator.RealWorld
{

// MainApp test application

****** 51
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 An Introduction to Design Patterns

class MainApp
{
static void Main()
{
// Create book
Book book = new Book ("Worley", "Inside ASP.NET", 10);
book.Display();

// Create video
Video video = new Video ("Spielberg", "Jaws", 23, 92);
video.Display();

// Make video borrowable, then borrow and display

Console.WriteLine("\nMaking video borrowable:");

Borrowable borrowvideo = new Borrowable(video);

borrowvideo.BorrowItem("Customer #1");
borrowvideo.BorrowItem("Customer #2");

borrowvideo.Display();

// Wait for user

Console.Read();
}
}

// "Component"

abstract class LibraryItem

{
private int numCopies;

// Property
public int NumCopies
{
get{ return numCopies; }
set{ numCopies = value; }
}

public abstract void Display();

}

// "ConcreteComponent"

class Book : LibraryItem

{
private string author;
private string title;

// Constructor
public Book(string author,string title,int numCopies)
{
this.author = author;

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 An Introduction to Design Patterns

this.title = title;
this.NumCopies = numCopies;
}

public override void Display()

{
Console.WriteLine("\nBook ------ ");
Console.WriteLine(" Author: {0}", author);
Console.WriteLine(" Title: {0}", title);
Console.WriteLine(" # Copies: {0}", NumCopies);
}
}

// "ConcreteComponent"

class Video : LibraryItem

{
private string director;
private string title;
private int playTime;

// Constructor
public Video(string director, string title,
int numCopies, int playTime)
{
this.director = director;
this.title = title;
this.NumCopies = numCopies;
this.playTime = playTime;
}

public override void Display()

{
Console.WriteLine("\nVideo ----- ");
Console.WriteLine(" Director: {0}", director);
Console.WriteLine(" Title: {0}", title);
Console.WriteLine(" # Copies: {0}", NumCopies);
Console.WriteLine(" Playtime: {0}\n", playTime);
}
}

// "Decorator"

abstract class Decorator : LibraryItem

{
protected LibraryItem libraryItem;

// Constructor
public Decorator(LibraryItem libraryItem)
{
this.libraryItem = libraryItem;
}

public override void Display()

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 An Introduction to Design Patterns

{
libraryItem.Display();
}
}

// "ConcreteDecorator"

class Borrowable : Decorator

{
protected ArrayList borrowers = new ArrayList();

// Constructor
public Borrowable(LibraryItem libraryItem)
: base(libraryItem)
{
}

public void BorrowItem(string name)

{
borrowers.Add(name);
libraryItem.NumCopies--;
}

public void ReturnItem(string name)

{
borrowers.Remove(name);
libraryItem.NumCopies++;
}

public override void Display()

{
base.Display();

foreach (string borrower in borrowers)

{
Console.WriteLine(" borrower: " + borrower);
}
}
}
}

Output
Book ------
Author: Worley
Title: Inside ASP.NET
# Copies: 10

Video -----
Director: Spielberg
Title: Jaws
# Copies: 23
Playtime: 92

****** 54
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 An Introduction to Design Patterns

Making video borrowable:

Video -----
Director: Spielberg
Title: Jaws
# Copies: 21
Playtime: 92

borrower: Customer #1
borrower: Customer #2

4.5 Facade Design Pattern

4.5.1 Definition
Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface
that makes the subsystem easier to use.

4.5.2 UML class diagram

The classes and/or objects participating in this pattern are:

Facade (MortgageApplication)
Knows which subsystem classes are responsible for a request.
Delegates client requests to appropriate subsystem objects.
Subsystem classes (Bank, Credit, Loan)
Implement subsystem functionality.
Handle work assigned by the Facade object.
Have no knowledge of the facade and keep no reference to it.

4.5.3 Sample code in C#

This structural code demonstrates the Facade pattern which provides a simplified and
uniform interface to a large subsystem of classes.
// Facade pattern -- Structural example

****** 55
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 An Introduction to Design Patterns

using System;

namespace DoFactory.GangOfFour.Facade.Structural
{

// Mainapp test application

class MainApp
{
public static void Main()
{
Facade facade = new Facade();

facade.MethodA();
facade.MethodB();

// Wait for user

Console.Read();
}
}

// "Subsystem ClassA"

class SubSystemOne
{
public void MethodOne()
{
Console.WriteLine(" SubSystemOne Method");
}
}

// Subsystem ClassB"

class SubSystemTwo
{
public void MethodTwo()
{
Console.WriteLine(" SubSystemTwo Method");
}
}

// Subsystem ClassC"

class SubSystemThree
{
public void MethodThree()
{
Console.WriteLine(" SubSystemThree Method");
}
}

// Subsystem ClassD"

class SubSystemFour

****** 56
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 An Introduction to Design Patterns

{
public void MethodFour()
{
Console.WriteLine(" SubSystemFour Method");
}
}

// "Facade"

class Facade
{
SubSystemOne one;
SubSystemTwo two;
SubSystemThree three;
SubSystemFour four;

public Facade()
{
one = new SubSystemOne();
two = new SubSystemTwo();
three = new SubSystemThree();
four = new SubSystemFour();
}

public void MethodA()

{
Console.WriteLine("\nMethodA() ---- ");
one.MethodOne();
two.MethodTwo();
four.MethodFour();
}

public void MethodB()

{
Console.WriteLine("\nMethodB() ---- ");
two.MethodTwo();
three.MethodThree();
}
}
}

Output
MethodA() ----
SubSystemOne Method
SubSystemTwo Method
SubSystemFour Method

MethodB() ----
SubSystemTwo Method
SubSystemThree Method

****** 57
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 An Introduction to Design Patterns

This real-world code demonstrates the Facade pattern as a MortgageApplication object

which provides a simplified interface to a large subsystem of classes measuring the
creditworthyness of an applicant.
// Facade pattern -- Real World example

using System;

namespace DoFactory.GangOfFour.Facade.RealWorld
{
// MainApp test application

class MainApp
{
static void Main()
{
// Facade
Mortgage mortgage = new Mortgage();

// Evaluate mortgage eligibility for customer

Customer customer = new Customer("Ann McKinsey");
bool eligable = mortgage.IsEligible(customer,125000);

Console.WriteLine("\n" + customer.Name +
" has been " + (eligable ? "Approved" : "Rejected"));

// Wait for user

Console.Read();
}
}

// "Subsystem ClassA"

class Bank
{
public bool HasSufficientSavings(Customer c, int amount)
{
Console.WriteLine("Check bank for " + c.Name);
return true;
}
}

// "Subsystem ClassB"

class Credit
{
public bool HasGoodCredit(Customer c)
{
Console.WriteLine("Check credit for " + c.Name);
return true;
}
}

// "Subsystem ClassC"

****** 58
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 An Introduction to Design Patterns

class Loan
{
public bool HasNoBadLoans(Customer c)
{
Console.WriteLine("Check loans for " + c.Name);
return true;
}
}

class Customer
{
private string name;

// Constructor
public Customer(string name)
{
this.name = name;
}

// Property
public string Name
{
get{ return name; }
}
}

// "Facade"

class Mortgage
{
private Bank bank = new Bank();
private Loan loan = new Loan();
private Credit credit = new Credit();

public bool IsEligible(Customer cust, int amount)

{
Console.WriteLine("{0} applies for {1:C} loan\n",
cust.Name, amount);

bool eligible = true;

// Check creditworthyness of applicant

if (!bank.HasSufficientSavings(cust, amount))
{
eligible = false;
}
else if (!loan.HasNoBadLoans(cust))
{
eligible = false;
}
else if (!credit.HasGoodCredit(cust))
{
eligible = false;
}

****** 59
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 An Introduction to Design Patterns

return eligible;
}
}
}

Output
Ann McKinsey applies for $125,000.00 loan

Check bank for Ann McKinsey

Check loans for Ann McKinsey
Check credit for Ann McKinsey

Ann McKinsey has been Approved

4.6 Flyweight Design Pattern

4.6.1 Definition
Use sharing to support large numbers of fine-grained objects
efficiently.

4.6.2 UML class diagram

The classes and/or objects participating in this pattern are:

Flyweight (Character)
Declares an interface through which flyweights can receive and act on extrinsic state.

****** 60
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 An Introduction to Design Patterns

ConcreteFlyweight (CharacterA, CharacterB, ..., CharacterZ)

Implements the Flyweight interface and adds storage for intrinsic state, if any. A
ConcreteFlyweight object must be sharable. Any state it stores must be intrinsic, that
is, it must be independent of the ConcreteFlyweight object's context.
UnsharedConcreteFlyweight ( not used )
Not all Flyweight subclasses need to be shared. The Flyweight interface enables sharing,
but it doesn't enforce it. It is common for UnsharedConcreteFlyweight objects to have
ConcreteFlyweight objects as children at some level in the flyweight object structure (as
the Row and Column classes have).

FlyweightFactory (CharacterFactory)
Creates and manages flyweight objects ensures that flyweight are shared properly.
When a client requests a flyweight, the FlyweightFactory objects supplies an existing
instance or creates one, if none exists.

Client (FlyweightApp)
Maintains a reference to flyweight(s). computes or stores the extrinsic state of
flyweight(s).

4.6.3 Sample code in C#

This structural code demonstrates the Flyweight pattern in which a relatively small
number of objects is shared many times by different clients.

// Flyweight pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Flyweight.Structural
{
// MainApp test application

class MainApp
{
static void Main()
{
// Arbitrary extrinsic state
int extrinsicstate = 22;

FlyweightFactory f = new FlyweightFactory();

// Work with different flyweight instances

Flyweight fx = f.GetFlyweight("X");
fx.Operation(--extrinsicstate);

Flyweight fy = f.GetFlyweight("Y");
fy.Operation(--extrinsicstate);

Flyweight fz = f.GetFlyweight("Z");
fz.Operation(--extrinsicstate);

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 An Introduction to Design Patterns

UnsharedConcreteFlyweight uf = new
UnsharedConcreteFlyweight();

uf.Operation(--extrinsicstate);

// Wait for user

Console.Read();
}
}

// "FlyweightFactory"

class FlyweightFactory
{
private Hashtable flyweights = new Hashtable();

// Constructor
public FlyweightFactory()
{
flyweights.Add("X", new ConcreteFlyweight());
flyweights.Add("Y", new ConcreteFlyweight());
flyweights.Add("Z", new ConcreteFlyweight());
}

public Flyweight GetFlyweight(string key)

{
return((Flyweight)flyweights[key]);
}
}

// "Flyweight"

abstract class Flyweight

{
public abstract void Operation(int extrinsicstate);
}

// "ConcreteFlyweight"

class ConcreteFlyweight : Flyweight

{
public override void Operation(int extrinsicstate)
{
Console.WriteLine("ConcreteFlyweight: " + extrinsicstate);
}
}

// "UnsharedConcreteFlyweight"

class UnsharedConcreteFlyweight : Flyweight

{
public override void Operation(int extrinsicstate)
{

****** 62
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 An Introduction to Design Patterns

Console.WriteLine("UnsharedConcreteFlyweight: " +
extrinsicstate);
}
}
}

Output
ConcreteFlyweight: 21
ConcreteFlyweight: 20
ConcreteFlyweight: 19
UnsharedConcreteFlyweight: 18

This real-world code demonstrates the Flyweight pattern in which a relatively small
number of Character objects is shared many times by a document that has potentially
many characters.
// Flyweight pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Flyweight.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Build a document with text
string document = "AAZZBBZB";
char[] chars = document.ToCharArray();

CharacterFactory f = new CharacterFactory();

// extrinsic state
int pointSize = 10;

// For each character use a flyweight object

foreach (char c in chars)
{
pointSize++;
Character character = f.GetCharacter(c);
character.Display(pointSize);
}

// Wait for user

Console.Read();
}
}

// "FlyweightFactory"

****** 63
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 An Introduction to Design Patterns

class CharacterFactory
{
private Hashtable characters = new Hashtable();

public Character GetCharacter(char key)

{
// Uses "lazy initialization"
Character character = characters[key] as Character;
if (character == null)
{
switch (key)
{
case 'A': character = new CharacterA(); break;
case 'B': character = new CharacterB(); break;
//...
case 'Z': character = new CharacterZ(); break;
}
characters.Add(key, character);
}
return character;
}
}

// "Flyweight"

abstract class Character

{
protected char symbol;
protected int width;
protected int height;
protected int ascent;
protected int descent;
protected int pointSize;

public abstract void Display(int pointSize);

}

// "ConcreteFlyweight"

class CharacterA : Character

{
// Constructor
public CharacterA()
{
this.symbol = 'A';
this.height = 100;
this.width = 120;
this.ascent = 70;
this.descent = 0;
}

public override void Display(int pointSize)

{

****** 64
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 An Introduction to Design Patterns

this.pointSize = pointSize;
Console.WriteLine(this.symbol +
" (pointsize " + this.pointSize + ")");
}
}

// "ConcreteFlyweight"

class CharacterB : Character

{
// Constructor
public CharacterB()
{
this.symbol = 'B';
this.height = 100;
this.width = 140;
this.ascent = 72;
this.descent = 0;
}

public override void Display(int pointSize)

{
this.pointSize = pointSize;
Console.WriteLine(this.symbol +
" (pointsize " + this.pointSize + ")");
}

// ... C, D, E, etc.

// "ConcreteFlyweight"

class CharacterZ : Character

{
// Constructor
public CharacterZ()
{
this.symbol = 'Z';
this.height = 100;
this.width = 100;
this.ascent = 68;
this.descent = 0;
}

public override void Display(int pointSize)

{
this.pointSize = pointSize;
Console.WriteLine(this.symbol +
" (pointsize " + this.pointSize + ")");
}
}
}

****** 65
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 An Introduction to Design Patterns

Output
A (pointsize 11)
A (pointsize 12)
Z (pointsize 13)
Z (pointsize 14)
B (pointsize 15)
B (pointsize 16)
Z (pointsize 17)
B (pointsize 18)

4.7 Proxy Design Pattern

4.7.1 Definition
Provide a surrogate or placeholder for another object to control access
to it.

4.7.2 UML class diagram

The classes and/or objects participating in this pattern are:

Proxy (MathProxy)
Maintains a reference that lets the proxy access the real subject. Proxy may refer to a
Subject if the RealSubject and Subject interfaces are the same.
Provides an interface identical to Subject's so that a proxy can be substituted for for the
real subject.
Controls access to the real subject and may be responsible for creating and deleting it.
other responsibilites depend on the kind of proxy:
remote proxies are responsible for encoding a request and its arguments and for
sending the encoded request to the real subject in a different address space.

****** 66
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 An Introduction to Design Patterns

virtual proxies may cache additional information about the real subject so that they
can postpone accessing it. For example, the ImageProxy from the Motivation caches the
real images's extent.
protection proxies check that the caller has the access permissions required to
perform a request.
Subject (IMath)
Defines the common interface for RealSubject and Proxy so that a Proxy can be used
anywhere a RealSubject is expected.
RealSubject (Math)
Defines the real object that the proxy represents.

4.7.3 Sample code in C#

This structural code demonstrates the Proxy pattern which provides a representative
object (proxy) that controls access to another similar object.

// Proxy pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Proxy.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create proxy and request a service
Proxy proxy = new Proxy();
proxy.Request();

// Wait for user

Console.Read();
}
}

// "Subject"

abstract class Subject

{
public abstract void Request();
}

// "RealSubject"

class RealSubject : Subject

{
public override void Request()

****** 67
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 An Introduction to Design Patterns

{
Console.WriteLine("Called RealSubject.Request()");
}
}

// "Proxy"

class Proxy : Subject

{
RealSubject realSubject;

public override void Request()

{
// Use 'lazy initialization'
if (realSubject == null)
{
realSubject = new RealSubject();
}

realSubject.Request();
}
}
}

Output
Called RealSubject.Request()

This real-world code demonstrates the Proxy pattern for a Math object represented by a
MathProxy object.
// Proxy pattern -- Real World example

using System;

namespace DoFactory.GangOfFour.Proxy.RealWorld
{

// Mainapp test application

class MainApp
{
static void Main()
{
// Create math proxy
MathProxy p = new MathProxy();

// Do the math
Console.WriteLine("4 + 2 = " + p.Add(4, 2));
Console.WriteLine("4 - 2 = " + p.Sub(4, 2));
Console.WriteLine("4 * 2 = " + p.Mul(4, 2));
Console.WriteLine("4 / 2 = " + p.Div(4, 2));

// Wait for user

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 An Introduction to Design Patterns

Console.Read();
}
}

// "Subject"

public interface IMath

{
double Add(double x, double y);
double Sub(double x, double y);
double Mul(double x, double y);
double Div(double x, double y);
}

// "RealSubject"

class Math : IMath

{
public double Add(double x, double y){return x + y;}
public double Sub(double x, double y){return x - y;}
public double Mul(double x, double y){return x * y;}
public double Div(double x, double y){return x / y;}
}

// "Proxy Object"

class MathProxy : IMath

{
Math math;

public MathProxy()
{
math = new Math();
}

public double Add(double x, double y)

{
return math.Add(x,y);
}
public double Sub(double x, double y)
{
return math.Sub(x,y);
}
public double Mul(double x, double y)
{
return math.Mul(x,y);
}
public double Div(double x, double y)
{
return math.Div(x,y);
}
}
}

****** 69
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 An Introduction to Design Patterns

Output
4+2=6
4-2=2
4*2=8
4/2=2

5. Behavioral Design Patterns

5.1 Chain of Responsibility Design pattern
5.1.1 Definition
Avoid coupling the sender of a request to its receiver by giving more
than one object a chance to handle the request. Chain the receiving
objects and pass the request along the chain until an object handles it.

5.1.2 UML class diagram

The classes and/or objects participating in this pattern are:

Handler (Approver)
Defines an interface for handling the requests
(optional) implements the successor link
ConcreteHandler (Director, VicePresident, President)
Handles requests it is responsible for can access its successor
If the ConcreteHandler can handle the request, it does so; otherwise it forwards the
request to its successor
Client (ChainApp)
Initiates the request to a ConcreteHandler object on the chain

5.1.3 Sample code in C#

This structural code demonstrates the Chain of Responsibility pattern in which several
linked objects (the Chain) are offered the opportunity to respond to a request or hand it
off to the object next in line.
// Chain of Responsibility pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Chain.Structural

****** 70
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 An Introduction to Design Patterns

{
// MainApp test application

class MainApp
{
static void Main()
{
// Setup Chain of Responsibility
Handler h1 = new ConcreteHandler1();
Handler h2 = new ConcreteHandler2();
Handler h3 = new ConcreteHandler3();
h1.SetSuccessor(h2);
h2.SetSuccessor(h3);

// Generate and process request

int[] requests = {2, 5, 14, 22, 18, 3, 27, 20};

foreach (int request in requests)

{
h1.HandleRequest(request);
}

// Wait for user

Console.Read();
}
}

// "Handler"

abstract class Handler

{
protected Handler successor;

public void SetSuccessor(Handler successor)

{
this.successor = successor;
}

public abstract void HandleRequest(int request);

}

// "ConcreteHandler1"

class ConcreteHandler1 : Handler

{
public override void HandleRequest(int request)
{
if (request >= 0 && request < 10)
{
Console.WriteLine("{0} handled request {1}",
this.GetType().Name, request);
}
else if (successor != null)
{

****** 71
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 An Introduction to Design Patterns

successor.HandleRequest(request);
}
}
}

// "ConcreteHandler2"

class ConcreteHandler2 : Handler

{
public override void HandleRequest(int request)
{
if (request >= 10 && request < 20)
{
Console.WriteLine("{0} handled request {1}",
this.GetType().Name, request);
}
else if (successor != null)
{
successor.HandleRequest(request);
}
}
}

// "ConcreteHandler3"

class ConcreteHandler3 : Handler

{
public override void HandleRequest(int request)
{
if (request >= 20 && request < 30)
{
Console.WriteLine("{0} handled request {1}",
this.GetType().Name, request);
}
else if (successor != null)
{
successor.HandleRequest(request);
}
}
}
}

Output
ConcreteHandler1 handled request 2
ConcreteHandler1 handled request 5
ConcreteHandler2 handled request 14
ConcreteHandler3 handled request 22
ConcreteHandler2 handled request 18
ConcreteHandler1 handled request 3
ConcreteHandler3 handled request 27
ConcreteHandler3 handled request 20

****** 72
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 An Introduction to Design Patterns

This real-world code demonstrates the Chain of Responsibility pattern in which several
linked managers and executives can respond to a purchase request or hand it off to a
superior. Each position has can have its own set of rules which orders they can
approve.
// Chain of Responsibility pattern -- Real World example

using System;

namespace DoFactory.GangOfFour.Chain.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Setup Chain of Responsibility
Director Larry = new Director();
VicePresident Sam = new VicePresident();
President Tammy = new President();
Larry.SetSuccessor(Sam);
Sam.SetSuccessor(Tammy);

// Generate and process purchase requests

Purchase p = new Purchase(2034, 350.00, "Supplies");
Larry.ProcessRequest(p);

p = new Purchase(2035, 32590.10, "Project X");

Larry.ProcessRequest(p);

p = new Purchase(2036, 122100.00, "Project Y");

Larry.ProcessRequest(p);

// Wait for user

Console.Read();
}
}

// "Handler"

abstract class Approver

{
protected Approver successor;

public void SetSuccessor(Approver successor)

{
this.successor = successor;
}

public abstract void ProcessRequest(Purchase purchase);

}

// "ConcreteHandler"

****** 73
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 An Introduction to Design Patterns

class Director : Approver

{
public override void ProcessRequest(Purchase purchase)
{
if (purchase.Amount < 10000.0)
{
Console.WriteLine("{0} approved request# {1}",
this.GetType().Name, purchase.Number);
}
else if (successor != null)
{
successor.ProcessRequest(purchase);
}
}
}

// "ConcreteHandler"

class VicePresident : Approver

{
public override void ProcessRequest(Purchase purchase)
{
if (purchase.Amount < 25000.0)
{
Console.WriteLine("{0} approved request# {1}",
this.GetType().Name, purchase.Number);
}
else if (successor != null)
{
successor.ProcessRequest(purchase);
}
}
}

// "ConcreteHandler"

class President : Approver

{
public override void ProcessRequest(Purchase purchase)
{
if (purchase.Amount < 100000.0)
{
Console.WriteLine("{0} approved request# {1}",
this.GetType().Name, purchase.Number);
}
else
{
Console.WriteLine(
"Request# {0} requires an executive meeting!",
purchase.Number);
}
}
}

****** 74
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 An Introduction to Design Patterns

// Request details

class Purchase
{
private int number;
private double amount;
private string purpose;

// Constructor
public Purchase(int number, double amount, string purpose)
{
this.number = number;
this.amount = amount;
this.purpose = purpose;
}

// Properties
public double Amount
{
get{ return amount; }
set{ amount = value; }
}

public string Purpose

{
get{ return purpose; }
set{ purpose = value; }
}

public int Number

{
get{ return number; }
set{ number = value; }
}
}
}

Output
Director Larry approved request# 2034
President Tammy approved request# 2035
Request# 2036 requires an executive meeting!

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 An Introduction to Design Patterns

5.2 Command Design Pattern

5.2.1 Definition
Encapsulate a request as an object, thereby letting you parameterize
clients with different requests, queue or log requests, and support
undoable operations.

5.2.2 UML class diagram

The classes and/or objects participating in this pattern are:

Command (Command)
Declares an interface for executing an operation
ConcreteCommand (CalculatorCommand)
Defines a binding between a Receiver object and an action
implements Execute by invoking the corresponding operation(s) on Receiver
Client (CommandApp)
Creates a ConcreteCommand object and sets its receiver
Invoker (User)
Asks the command to carry out the request
Receiver (Calculator)
Knows how to perform the operations associated with carrying out the request.

5.2.3 Sample code in C#

This structural code demonstrates the Command pattern which stores requests as
objects allowing clients to execute or playback the requests.
// Command pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Command.Structural
{

// MainApp test applicatio

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 An Introduction to Design Patterns

class MainApp
{
static void Main()
{
// Create receiver, command, and invoker
Receiver receiver = new Receiver();
Command command = new ConcreteCommand(receiver);
Invoker invoker = new Invoker();

// Set and execute command

invoker.SetCommand(command);
invoker.ExecuteCommand();

// Wait for user

Console.Read();
}
}

// "Command"

abstract class Command

{
protected Receiver receiver;

// Constructor
public Command(Receiver receiver)
{
this.receiver = receiver;
}

public abstract void Execute();

}

// "ConcreteCommand"

class ConcreteCommand : Command

{
// Constructor
public ConcreteCommand(Receiver receiver) :
base(receiver)
{
}

public override void Execute()

{
receiver.Action();
}
}

// "Receiver"

class Receiver
{

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 An Introduction to Design Patterns

public void Action()

{
Console.WriteLine("Called Receiver.Action()");
}
}

// "Invoker"

class Invoker
{
private Command command;

public void SetCommand(Command command)

{
this.command = command;
}

public void ExecuteCommand()

{
command.Execute();
}
}
}

Output
Called Receiver.Action()

This real-world code demonstrates the Command pattern used in a simple calculator
with unlimited number of undo's and redo's. Note that in C# the word 'operator' is a
keyword. Prefixing it with '@' allows using it as an identifier.
// Command pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Command.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create user and let her compute
User user = new User();

user.Compute('+', 100);
user.Compute('-', 50);
user.Compute('*', 10);
user.Compute('/', 2);

// Undo 4 commands
user.Undo(4);

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 An Introduction to Design Patterns

// Redo 3 commands
user.Redo(3);

// Wait for user

Console.Read();
}
}

// "Command"

abstract class Command

{
public abstract void Execute();
public abstract void UnExecute();
}

// "ConcreteCommand"

class CalculatorCommand : Command

{
char @operator;
int operand;
Calculator calculator;

// Constructor
public CalculatorCommand(Calculator calculator,
char @operator, int operand)
{
this.calculator = calculator;
this.@operator = @operator;
this.operand = operand;
}

public char Operator

{
set{ @operator = value; }
}

public int Operand

{
set{ operand = value; }
}

public override void Execute()

{
calculator.Operation(@operator, operand);
}

public override void UnExecute()

{
calculator.Operation(Undo(@operator), operand);
}

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 An Introduction to Design Patterns

// Private helper function

private char Undo(char @operator)
{
char undo;
switch(@operator)
{
case '+': undo = '-'; break;
case '-': undo = '+'; break;
case '*': undo = '/'; break;
case '/': undo = '*'; break;
default : undo = ' '; break;
}
return undo;
}
}

// "Receiver"

class Calculator
{
private int curr = 0;

public void Operation(char @operator, int operand)

{
switch(@operator)
{
case '+': curr += operand; break;
case '-': curr -= operand; break;
case '*': curr *= operand; break;
case '/': curr /= operand; break;
}
Console.WriteLine(
"Current value = {0,3} (following {1} {2})",
curr, @operator, operand);
}
}

// "Invoker"

class User
{
// Initializers
private Calculator calculator = new Calculator();
private ArrayList commands = new ArrayList();

private int current = 0;

public void Redo(int levels)

{
Console.WriteLine("\n---- Redo {0} levels ", levels);
// Perform redo operations
for (int i = 0; i < levels; i++)
{
if (current < commands.Count - 1)

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 An Introduction to Design Patterns

{
Command command = commands[current++] as Command;
command.Execute();
}
}
}

public void Undo(int levels)

{
Console.WriteLine("\n---- Undo {0} levels ", levels);
// Perform undo operations
for (int i = 0; i < levels; i++)
{
if (current > 0)
{
Command command = commands[--current] as Command;
command.UnExecute();
}
}
}

public void Compute(char @operator, int operand)

{
// Create command operation and execute it
Command command = new CalculatorCommand(
calculator, @operator, operand);
command.Execute();

// Add command to undo list

commands.Add(command);
current++;
}
}
}

Output
Current value = 100 (following + 100)
Current value = 50 (following - 50)
Current value = 500 (following * 10)
Current value = 250 (following / 2)

---- Undo 4 levels

Current value = 500 (following * 2)
Current value = 50 (following / 10)
Current value = 100 (following + 50)
Current value = 0 (following - 100)

---- Redo 3 levels

Current value = 100 (following + 100)
Current value = 50 (following - 50)
Current value = 500 (following * 10)

****** 81
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 An Introduction to Design Patterns

5.3 Interpreter Design Pattern

5.3.1 Definition
Given a language, define a representation for its grammar along with
an interpreter that uses the representation to interpret sentences in
the language.

5.3.2 UML class diagram

The classes and/or objects participating in this pattern are:

AbstractExpression (Expression)
Declares an interface for executing an operation
TerminalExpression ( ThousandExpression, HundredExpression, TenExpression,
OneExpression )
Implements an Interpret operation associated with terminal symbols in the grammar.
An instance is required for every terminal symbol in the sentence.
NonterminalExpression ( not used )
One such class is required for every rule R ::= R1R2...Rn in the grammar maintains
instance variables of type AbstractExpression for each of the symbols R1 through Rn.
implements an Interpret operation for nonterminal symbols in the grammar. Interpret
typically calls itself recursively on the variables representing R1 through Rn.
Context (Context)
Contains information that is global to the interpreter
Client (InterpreterApp)
Builds (or is given) an abstract syntax tree representing a particular sentence in the
language that the grammar defines. The abstract syntax tree is assembled from
instances of the NonterminalExpression and TerminalExpression classes
invokes the Interpret operation

5.3.3 Sample code in C#

This structural code demonstrates the Interpreter patterns, which using a defined
grammer, provides the interpreter that processes parsed statements.
// Interpreter pattern -- Structural example
using System;
using System.Collections;

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 An Introduction to Design Patterns

namespace DoFactory.GangOfFour.Interpreter.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
Context context = new Context();

// Usually a tree
ArrayList list = new ArrayList();

// Populate 'abstract syntax tree'

list.Add(new TerminalExpression());
list.Add(new NonterminalExpression());
list.Add(new TerminalExpression());
list.Add(new TerminalExpression());

// Interpret
foreach (AbstractExpression exp in list)
{
exp.Interpret(context);
}

// Wait for user

Console.Read();
}
}

// "Context"

class Context
{
}

// "AbstractExpression"

abstract class AbstractExpression

{
public abstract void Interpret(Context context);
}

// "TerminalExpression"

class TerminalExpression : AbstractExpression

{
public override void Interpret(Context context)
{
Console.WriteLine("Called Terminal.Interpret()");
}
}

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 An Introduction to Design Patterns

// "NonterminalExpression"

class NonterminalExpression : AbstractExpression

{
public override void Interpret(Context context)
{
Console.WriteLine("Called Nonterminal.Interpret()");
}
}
}

Output
Called Terminal.Interpret()
Called Nonterminal.Interpret()
Called Terminal.Interpret()
Called Terminal.Interpret()

This real-world code demonstrates the Interpreter pattern which is used to convert a
Roman numeral to a decimal.
// Interpreter pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Interpreter.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
string roman = "MCMXXVIII";
Context context = new Context(roman);

// Build the 'parse tree'

ArrayList tree = new ArrayList();
tree.Add(new ThousandExpression());
tree.Add(new HundredExpression());
tree.Add(new TenExpression());
tree.Add(new OneExpression());

// Interpret
foreach (Expression exp in tree)
{
exp.Interpret(context);
}

Console.WriteLine("{0} = {1}",
roman, context.Output);

// Wait for user

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 An Introduction to Design Patterns

Console.Read();
}
}

// "Context"

class Context
{
private string input;
private int output;

// Constructor
public Context(string input)
{
this.input = input;
}

// Properties
public string Input
{
get{ return input; }
set{ input = value; }
}

public int Output

{
get{ return output; }
set{ output = value; }
}
}

// "AbstractExpression"

abstract class Expression

{
public void Interpret(Context context)
{
if (context.Input.Length == 0)
return;

if (context.Input.StartsWith(Nine()))
{
context.Output += (9 * Multiplier());
context.Input = context.Input.Substring(2);
}
else if (context.Input.StartsWith(Four()))
{
context.Output += (4 * Multiplier());
context.Input = context.Input.Substring(2);
}
else if (context.Input.StartsWith(Five()))
{
context.Output += (5 * Multiplier());
context.Input = context.Input.Substring(1);

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 An Introduction to Design Patterns

while (context.Input.StartsWith(One()))
{
context.Output += (1 * Multiplier());
context.Input = context.Input.Substring(1);
}
}

public abstract string One();

public abstract string Four();
public abstract string Five();
public abstract string Nine();
public abstract int Multiplier();
}

// Thousand checks for the Roman Numeral M

// "TerminalExpression"

class ThousandExpression : Expression

{
public override string One() { return "M"; }
public override string Four(){ return " "; }
public override string Five(){ return " "; }
public override string Nine(){ return " "; }
public override int Multiplier() { return 1000; }
}

// Hundred checks C, CD, D or CM

// "TerminalExpression"

class HundredExpression : Expression

{
public override string One() { return "C"; }
public override string Four(){ return "CD"; }
public override string Five(){ return "D"; }
public override string Nine(){ return "CM"; }
public override int Multiplier() { return 100; }
}

// Ten checks for X, XL, L and XC

// "TerminalExpression"

class TenExpression : Expression

{
public override string One() { return "X"; }
public override string Four(){ return "XL"; }
public override string Five(){ return "L"; }
public override string Nine(){ return "XC"; }
public override int Multiplier() { return 10; }
}

// One checks for I, II, III, IV, V, VI, VI, VII, VIII, IX
// "TerminalExpression"

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 An Introduction to Design Patterns

class OneExpression : Expression

{
public override string One() { return "I"; }
public override string Four(){ return "IV"; }
public override string Five(){ return "V"; }
public override string Nine(){ return "IX"; }
public override int Multiplier() { return 1; }
}
}

Output
MCMXXVIII = 1928

5.4 Iterator Design pattern

5.4.1 Definition
Provide a way to access the elements of an aggregate object
sequentially without exposing its underlying representation.

5.4.2 UML class diagram

The classes and/or objects participating in this pattern are:

Iterator (AbstractIterator)
Defines an interface for accessing and traversing elements.
ConcreteIterator (Iterator)
Implements the Iterator interface.
Keeps track of the current position in the traversal of the aggregate.
Aggregate (AbstractCollection)
Defines an interface for creating an Iterator object
ConcreteAggregate (Collection)

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 An Introduction to Design Patterns

Implements the Iterator creation interface to return an instance of the proper

ConcreteIterator

5.4.3 Sample code in C#

This structural code demonstrates the Iterator pattern which provides for a way to
traverse (iterate) over a collection of items without detailing the underlying structure of
the collection.
// Iterator pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Iterator.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
ConcreteAggregate a = new ConcreteAggregate();
a[0] = "Item A";
a[1] = "Item B";
a[2] = "Item C";
a[3] = "Item D";

// Create Iterator and provide aggregate

ConcreteIterator i = new ConcreteIterator(a);

Console.WriteLine("Iterating over collection:");

object item = i.First();

while (item != null)
{
Console.WriteLine(item);
item = i.Next();
}

// Wait for user

Console.Read();
}
}

// "Aggregate"

abstract class Aggregate

{
public abstract Iterator CreateIterator();
}

// "ConcreteAggregate"

class ConcreteAggregate : Aggregate

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 An Introduction to Design Patterns

{
private ArrayList items = new ArrayList();

public override Iterator CreateIterator()

{
return new ConcreteIterator(this);
}

// Property
public int Count
{
get{ return items.Count; }
}

// Indexer
public object this[int index]
{
get{ return items[index]; }
set{ items.Insert(index, value); }
}
}

// "Iterator"

abstract class Iterator

{
public abstract object First();
public abstract object Next();
public abstract bool IsDone();
public abstract object CurrentItem();
}

// "ConcreteIterator"

class ConcreteIterator : Iterator

{
private ConcreteAggregate aggregate;
private int current = 0;

// Constructor
public ConcreteIterator(ConcreteAggregate aggregate)
{
this.aggregate = aggregate;
}

public override object First()

{
return aggregate[0];
}

public override object Next()

{
object ret = null;
if (current < aggregate.Count - 1)

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 An Introduction to Design Patterns

{
ret = aggregate[++current];
}

return ret;
}

public override object CurrentItem()

{
return aggregate[current];
}

public override bool IsDone()

{
return current >= aggregate.Count ? true : false ;
}
}
}

Output
Iterating over collection:
Item A
Item B
Item C
Item D

This real-world code demonstrates the Iterator pattern which is used to iterate over a
collection of items and skip a specific number of items each iteration.
// Iterator pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Iterator.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Build a collection
Collection collection = new Collection();
collection[0] = new Item("Item 0");
collection[1] = new Item("Item 1");
collection[2] = new Item("Item 2");
collection[3] = new Item("Item 3");
collection[4] = new Item("Item 4");
collection[5] = new Item("Item 5");
collection[6] = new Item("Item 6");
collection[7] = new Item("Item 7");
collection[8] = new Item("Item 8");

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 An Introduction to Design Patterns

// Create iterator
Iterator iterator = new Iterator(collection);

// Skip every other item

iterator.Step = 2;

Console.WriteLine("Iterating over collection:");

for(Item item = iterator.First();

!iterator.IsDone; item = iterator.Next())
{
Console.WriteLine(item.Name);
}

// Wait for user

Console.Read();
}
}

class Item
{
string name;

// Constructor
public Item(string name)
{
this.name = name;
}

// Property
public string Name
{
get{ return name; }
}
}

// "Aggregate"

interface IAbstractCollection
{
Iterator CreateIterator();
}

// "ConcreteAggregate"

class Collection : IAbstractCollection

{
private ArrayList items = new ArrayList();

public Iterator CreateIterator()

{
return new Iterator(this);
}

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 An Introduction to Design Patterns

// Property
public int Count
{
get{ return items.Count; }
}

// Indexer
public object this[int index]
{
get{ return items[index]; }
set{ items.Add(value); }
}
}

// "Iterator"

interface IAbstractIterator
{
Item First();
Item Next();
bool IsDone{ get; }
Item CurrentItem{ get; }
}

// "ConcreteIterator"

class Iterator : IAbstractIterator

{
private Collection collection;
private int current = 0;
private int step = 1;

// Constructor
public Iterator(Collection collection)
{
this.collection = collection;
}

public Item First()

{
current = 0;
return collection[current] as Item;
}

public Item Next()

{
current += step;
if (!IsDone)
return collection[current] as Item;
else
return null;
}

// Properties

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public int Step

{
get{ return step; }
set{ step = value; }
}

public Item CurrentItem

{
get
{
return collection[current] as Item;
}
}

public bool IsDone

{
get
{
return current >= collection.Count ? true : false;
}
}
}
}

Output
Iterating over collection:
Item 0
Item 2
Item 4
Item 6
Item 8

****** 93
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 An Introduction to Design Patterns

5.5 Mediator Design Pattern

5.5.1 Definition
Define an object that encapsulates how a set of objects interact.
Mediator promotes loose coupling by keeping objects from referring to
each other explicitly, and it lets you vary their interaction
independently.

5.5.2 UML class diagram

The classes and/or objects participating in this pattern are:

Mediator (IChatroom)
Defines an interface for communicating with Colleague objects
ConcreteMediator (Chatroom)
Implements cooperative behavior by coordinating Colleague objects
Knows and maintains its colleagues
Colleague classes (Participant)
Each Colleague class knows its Mediator object
Each colleague communicates with its mediator whenever it would have otherwise
communicated with another colleague

5.5.3 Sample code in C#

This structural code demonstrates the Mediator pattern facilitating loosely coupled
communication between different objects and object types. The mediator is a central
hub through which all interaction must take place.
// Mediator pattern -- Structural example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Mediator.Structural
{

// Mainapp test application

class MainApp
{
static void Main()
{

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 An Introduction to Design Patterns

ConcreteMediator m = new ConcreteMediator();

ConcreteColleague1 c1 = new ConcreteColleague1(m);

ConcreteColleague2 c2 = new ConcreteColleague2(m);

m.Colleague1 = c1;
m.Colleague2 = c2;

c1.Send("How are you?");

c2.Send("Fine, thanks");

// Wait for user

Console.Read();
}
}

// "Mediator"

abstract class Mediator

{
public abstract void Send(string message,
Colleague colleague);
}

// "ConcreteMediator"

class ConcreteMediator : Mediator

{
private ConcreteColleague1 colleague1;
private ConcreteColleague2 colleague2;

public ConcreteColleague1 Colleague1

{
set{ colleague1 = value; }
}

public ConcreteColleague2 Colleague2

{
set{ colleague2 = value; }
}

public override void Send(string message,

Colleague colleague)
{
if (colleague == colleague1)
{
colleague2.Notify(message);
}
else
{
colleague1.Notify(message);
}
}
}

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 An Introduction to Design Patterns

// "Colleague"

abstract class Colleague

{
protected Mediator mediator;

// Constructor
public Colleague(Mediator mediator)
{
this.mediator = mediator;
}
}

// "ConcreteColleague1"

class ConcreteColleague1 : Colleague

{
// Constructor
public ConcreteColleague1(Mediator mediator)
: base(mediator)
{
}

public void Send(string message)

{
mediator.Send(message, this);
}

public void Notify(string message)

{
Console.WriteLine("Colleague1 gets message: "
+ message);
}
}

// "ConcreteColleague2"

class ConcreteColleague2 : Colleague

{
// Constructor
public ConcreteColleague2(Mediator mediator)
: base(mediator)
{
}

public void Send(string message)

{
mediator.Send(message, this);
}

public void Notify(string message)

{
Console.WriteLine("Colleague2 gets message: "

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 An Introduction to Design Patterns

+ message);
}
}
}

Output
Colleague2 gets message: How are you?
Colleague1 gets message: Fine, thanks

This real-world code demonstrates the Mediator pattern facilitating loosely coupled
communication between different Participants registering with a Chatroom. The
Chatroom is the central hub through which all communication takes place. At this
point only one-to-one communication is implemented in the Chatroom, but would be
trivial to change to one-to-many.
// Mediator pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Mediator.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create chatroom
Chatroom chatroom = new Chatroom();

// Create participants and register them

Participant George = new Beatle("George");
Participant Paul = new Beatle("Paul");
Participant Ringo = new Beatle("Ringo");
Participant John = new Beatle("John") ;
Participant Yoko = new NonBeatle("Yoko");

chatroom.Register(George);
chatroom.Register(Paul);
chatroom.Register(Ringo);
chatroom.Register(John);
chatroom.Register(Yoko);

// Chatting participants
Yoko.Send ("John", "Hi John!");
Paul.Send ("Ringo", "All you need is love");
Ringo.Send("George", "My sweet Lord");
Paul.Send ("John", "Can't buy me love");
John.Send ("Yoko", "My sweet love") ;

// Wait for user

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Console.Read();
}
}

// "Mediator"

abstract class AbstractChatroom

{
public abstract void Register(Participant participant);
public abstract void Send(
string from, string to, string message);
}

// "ConcreteMediator"

class Chatroom : AbstractChatroom

{
private Hashtable participants = new Hashtable();

public override void Register(Participant participant)

{
if (participants[participant.Name] == null)
{
participants[participant.Name] = participant;
}

participant.Chatroom = this;
}

public override void Send(

string from, string to, string message)
{
Participant pto = (Participant)participants[to];
if (pto != null)
{
pto.Receive(from, message);
}
}
}

// "AbstractColleague"

class Participant
{
private Chatroom chatroom;
private string name;

// Constructor
public Participant(string name)
{
this.name = name;
}

// Properties

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public string Name

{
get{ return name; }
}

public Chatroom Chatroom

{
set{ chatroom = value; }
get{ return chatroom; }
}

public void Send(string to, string message)

{
chatroom.Send(name, to, message);
}

public virtual void Receive(

string from, string message)
{
Console.WriteLine("{0} to {1}: '{2}'",
from, Name, message);
}
}

//" ConcreteColleague1"

class Beatle : Participant

{
// Constructor
public Beatle(string name) : base(name)
{
}

public override void Receive(string from, string message)

{
Console.Write("To a Beatle: ");
base.Receive(from, message);
}
}

//" ConcreteColleague2"

class NonBeatle : Participant

{
// Constructor
public NonBeatle(string name) : base(name)
{
}

public override void Receive(string from, string message)

{
Console.Write("To a non-Beatle: ");
base.Receive(from, message);
}

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}
}

Output
To a Beatle: Yoko to John: 'Hi John!'
To a Beatle: Paul to Ringo: 'All you need is love'
To a Beatle: Ringo to George: 'My sweet Lord'
To a Beatle: Paul to John: 'Can't buy me love'
To a non-Beatle: John to Yoko: 'My sweet love'

5.6 Memento Design Pattern

5.6.1 Definition
Without violating encapsulation, capture and externalize an object's
internal state so that the object can be restored to this state later.

5.6.2 UML class diagram

The classes and/or objects participating in this pattern are:

Memento (Memento)
Stores internal state of the Originator object. The memento may store as much or as
little of the originator's internal state as necessary at its originator's discretion.
Protect against access by objects of other than the originator. Mementos have effectively
two interfaces. Caretaker sees a narrow interface to the Memento -- it can only pass the
memento to the other objects. Originator, in contrast, sees a wide interface, one that
lets it access all the data necessary to restore itself to its previous state. Ideally, only
the originator that produces the memento would be permitted to access the memento's
internal state.
Originator (SalesProspect)
Creates a memento containing a snapshot of its current internal state.
Uses the memento to restore its internal state.
Caretaker (Caretaker)
Is responsible for the memento's safekeeping
Never operates on or examines the contents of a memento.

****** 100
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 An Introduction to Design Patterns

5.6.3 Sample code in C#

This structural code demonstrates the Memento pattern which temporary saves and
restores another object's internal state.
// Memento pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Memento.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
Originator o = new Originator();
o.State = "On";

// Store internal state

Caretaker c = new Caretaker();
c.Memento = o.CreateMemento();

// Continue changing originator

o.State = "Off";

// Restore saved state

o.SetMemento(c.Memento);

// Wait for user

Console.Read();
}
}

// "Originator"

class Originator
{
private string state;

// Property
public string State
{
get{ return state; }
set
{
state = value;
Console.WriteLine("State = " + state);
}
}

public Memento CreateMemento()

{

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 An Introduction to Design Patterns

return (new Memento(state));

}

public void SetMemento(Memento memento)

{
Console.WriteLine("Restoring state:");
State = memento.State;
}
}

// "Memento"

class Memento
{
private string state;

// Constructor
public Memento(string state)
{
this.state = state;
}

// Property
public string State
{
get{ return state; }
}
}

// "Caretaker"

class Caretaker
{
private Memento memento;

// Property
public Memento Memento
{
set{ memento = value; }
get{ return memento; }
}
}
}

Output
State = On
State = Off
Restoring state:
State = On

This real-world code demonstrates the Memento pattern which temporarily saves and
then restores the SalesProspect's internal state.
// Memento pattern -- Real World example

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 An Introduction to Design Patterns

using System;

namespace DoFactory.GangOfFour.Memento.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
SalesProspect s = new SalesProspect();
s.Name = "Noel van Halen";
s.Phone = "(412) 256-0990";
s.Budget = 25000.0;

// Store internal state

ProspectMemory m = new ProspectMemory();
m.Memento = s.SaveMemento();

// Continue changing originator

s.Name = "Leo Welch";
s.Phone = "(310) 209-7111";
s.Budget = 1000000.0;

// Restore saved state

s.RestoreMemento(m.Memento);

// Wait for user

Console.Read();
}
}

// "Originator"

class SalesProspect
{
private string name;
private string phone;
private double budget;

// Properties
public string Name
{
get{ return name; }
set
{
name = value;
Console.WriteLine("Name: " + name);
}
}

public string Phone

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 An Introduction to Design Patterns

{
get{ return phone; }
set
{
phone = value;
Console.WriteLine("Phone: " + phone);
}
}

public double Budget

{
get{ return budget; }
set
{
budget = value;
Console.WriteLine("Budget: " + budget);
}
}

public Memento SaveMemento()

{
Console.WriteLine("\nSaving state --\n");
return new Memento(name, phone, budget);
}

public void RestoreMemento(Memento memento)

{
Console.WriteLine("\nRestoring state --\n");
this.Name = memento.Name;
this.Phone = memento.Phone;
this.Budget = memento.Budget;
}
}

// "Memento"

class Memento
{
private string name;
private string phone;
private double budget;

// Constructor
public Memento(string name, string phone, double budget)
{
this.name = name;
this.phone = phone;
this.budget = budget;
}

// Properties
public string Name
{
get{ return name; }

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 An Introduction to Design Patterns

set{ name = value; }

}

public string Phone

{
get{ return phone; }
set{ phone = value; }
}

public double Budget

{
get{ return budget; }
set{ budget = value; }
}
}

// "Caretaker"

class ProspectMemory
{
private Memento memento;

// Property
public Memento Memento
{
set{ memento = value; }
get{ return memento; }
}
}
}

Output
Name: Noel van Halen
Phone: (412) 256-0990
Budget: 25000

Saving state --

Name: Leo Welch

Phone: (310) 209-7111
Budget: 1000000

Restoring state --

Name: Noel van Halen

Phone: (412) 256-0990
Budget: 25000

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 An Introduction to Design Patterns

5.7 Observer Design Pattern

5.7.1 Definition
Define a one-to-many dependency between objects so that when one
object changes state, all its dependents are notified and updated
automatically.

5.7.2 UML class diagram

The classes and/or objects participating in this pattern are:

Subject (Stock)
Knows its observers. Any number of Observer objects may observe a subject provides
an interface for attaching and detaching Observer objects.
ConcreteSubject (IBM)
Stores state of interest to ConcreteObserver sends a notification to its observers when
its state changes
Observer (IInvestor)
Defines an updating interface for objects that should be notified of changes in a subject.
ConcreteObserver (Investor)
Maintains a reference to a ConcreteSubject object stores state that should stay
consistent with the subject's implements the Observer updating interface to keep its
state consistent with the subject's.

5.7.3 Sample code in C#

This structural code demonstrates the Observer pattern in which registered objects are
notified of and updated with a state change.
// Observer pattern -- Structural example

using System;
using System.Collections;

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 An Introduction to Design Patterns

namespace DoFactory.GangOfFour.Observer.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Configure Observer pattern
ConcreteSubject s = new ConcreteSubject();

s.Attach(new ConcreteObserver(s,"X"));
s.Attach(new ConcreteObserver(s,"Y"));
s.Attach(new ConcreteObserver(s,"Z"));

// Change subject and notify observers

s.SubjectState = "ABC";
s.Notify();

// Wait for user

Console.Read();
}
}

// "Subject"

abstract class Subject

{
private ArrayList observers = new ArrayList();

public void Attach(Observer observer)

{
observers.Add(observer);
}

public void Detach(Observer observer)

{
observers.Remove(observer);
}

public void Notify()

{
foreach (Observer o in observers)
{
o.Update();
}
}
}

// "ConcreteSubject"

class ConcreteSubject : Subject

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{
private string subjectState;

// Property
public string SubjectState
{
get{ return subjectState; }
set{ subjectState = value; }
}
}

// "Observer"

abstract class Observer

{
public abstract void Update();
}

// "ConcreteObserver"

class ConcreteObserver : Observer

{
private string name;
private string observerState;
private ConcreteSubject subject;

// Constructor
public ConcreteObserver(
ConcreteSubject subject, string name)
{
this.subject = subject;
this.name = name;
}

public override void Update()

{
observerState = subject.SubjectState;
Console.WriteLine("Observer {0}'s new state is {1}",
name, observerState);
}

// Property
public ConcreteSubject Subject
{
get { return subject; }
set { subject = value; }
}
}
}

Output
Observer X's new state is ABC
Observer Y's new state is ABC
Observer Z's new state is ABC

****** 108
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 An Introduction to Design Patterns

This real-world code demonstrates the Observer pattern in which registered investors
are notified every time a stock changes value.
// Observer pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Observer.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Create investors
Investor s = new Investor("Sorros");
Investor b = new Investor("Berkshire");

// Create IBM stock and attach investors

IBM ibm = new IBM("IBM", 120.00);
ibm.Attach(s);
ibm.Attach(b);

// Change price, which notifies investors

ibm.Price = 120.10;
ibm.Price = 121.00;
ibm.Price = 120.50;
ibm.Price = 120.75;

// Wait for user

Console.Read();
}
}

// "Subject"

abstract class Stock

{
protected string symbol;
protected double price;
private ArrayList investors = new ArrayList();

// Constructor
public Stock(string symbol, double price)
{
this.symbol = symbol;
this.price = price;
}

public void Attach(Investor investor)

{
investors.Add(investor);

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public void Detach(Investor investor)

{
investors.Remove(investor);
}

public void Notify()

{
foreach (Investor investor in investors)
{
investor.Update(this);
}
Console.WriteLine("");
}

// Properties
public double Price
{
get{ return price; }
set
{
price = value;
Notify();
}
}

public string Symbol

{
get{ return symbol; }
set{ symbol = value; }
}
}

// "ConcreteSubject"

class IBM : Stock

{
// Constructor
public IBM(string symbol, double price)
: base(symbol, price)
{
}
}

// "Observer"

interface IInvestor
{
void Update(Stock stock);
}

// "ConcreteObserver"

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 An Introduction to Design Patterns

class Investor : IInvestor

{
private string name;
private Stock stock;

// Constructor
public Investor(string name)
{
this.name = name;
}

public void Update(Stock stock)

{
Console.WriteLine("Notified {0} of {1}'s " +
"change to {2:C}", name, stock.Symbol, stock.Price);
}

// Property
public Stock Stock
{
get{ return stock; }
set{ stock = value; }
}
}
}

Output
Notified Sorros of IBM's change to $120.10
Notified Berkshire of IBM's change to $120.10

Notified Sorros of IBM's change to $121.00

Notified Berkshire of IBM's change to $121.00

Notified Sorros of IBM's change to $120.50

Notified Berkshire of IBM's change to $120.50

Notified Sorros of IBM's change to $120.75

Notified Berkshire of IBM's change to $120.75

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 An Introduction to Design Patterns

5.8 State Design Pattern

5.8.1 Definition
Allow an object to alter its behavior when its internal state changes.
The object will appear to change its class.

5.8.2 UML class diagram

The classes and/or objects participating in this pattern are:

Context (Account)
Defines the interface of interest to clients maintains an instance of a ConcreteState
subclass that defines the current state.
State (State)
Defines an interface for encapsulating the behavior associated with a particular state of
the Context.
Concrete State (RedState, SilverState, GoldState)
Each subclass implements a behavior associated with a state of Context.

5.8.3 Sample code in C#

This structural code demonstrates the State pattern which allows an object to behave
differently depending on its internal state. The difference in behavior is delegated to
objects that represent this state.
// State pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.State.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Setup context in a state
Context c = new Context(new ConcreteStateA());

// Issue requests, which toggles state

c.Request();

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 An Introduction to Design Patterns

c.Request();
c.Request();
c.Request();

// Wait for user

Console.Read();
}
}

// "State"

abstract class State

{
public abstract void Handle(Context context);
}

// "ConcreteStateA"

class ConcreteStateA : State

{
public override void Handle(Context context)
{
context.State = new ConcreteStateB();
}
}

// "ConcreteStateB"

class ConcreteStateB : State

{
public override void Handle(Context context)
{
context.State = new ConcreteStateA();
}
}

// "Context"

class Context
{
private State state;

// Constructor
public Context(State state)
{
this.State = state;
}

// Property
public State State
{
get{ return state; }
set
{

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 An Introduction to Design Patterns

state = value;
Console.WriteLine("State: " +
state.GetType().Name);
}
}

public void Request()

{
state.Handle(this);
}
}
}

Output
State: ConcreteStateA
State: ConcreteStateB
State: ConcreteStateA
State: ConcreteStateB
State: ConcreteStateA

This real-world code demonstrates the State pattern which allows an Account to behave
differently depending on its balance. The difference in behavior is delegated to State
objects called RedState, SilverState and GoldState. These states represent overdrawn
accounts, starter accounts, and accounts in good standing.
// State pattern -- Real World example

using System;

namespace DoFactory.GangOfFour.State.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
// Open a new account
Account account = new Account("Jim Johnson");

// Apply financial transactions

account.Deposit(500.0);
account.Deposit(300.0);
account.Deposit(550.0);
account.PayInterest();
account.Withdraw(2000.00);
account.Withdraw(1100.00);

// Wait for user

Console.Read();
}
}

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// "State"

abstract class State

{
protected Account account;
protected double balance;

protected double interest;

protected double lowerLimit;
protected double upperLimit;

// Properties
public Account Account
{
get{ return account; }
set{ account = value; }
}

public double Balance

{
get{ return balance; }
set{ balance = value; }
}

public abstract void Deposit(double amount);

public abstract void Withdraw(double amount);
public abstract void PayInterest();
}

// "ConcreteState"

// Account is overdrawn

class RedState : State

{
double serviceFee;

// Constructor
public RedState(State state)
{
this.balance = state.Balance;
this.account = state.Account;
Initialize();
}

private void Initialize()

{
// Should come from a datasource
interest = 0.0;
lowerLimit = -100.0;
upperLimit = 0.0;
serviceFee = 15.00;
}

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public override void Deposit(double amount)

{
balance += amount;
StateChangeCheck();
}

public override void Withdraw(double amount)

{
amount = amount - serviceFee;
Console.WriteLine("No funds available for withdrawal!");
}

public override void PayInterest()

{
// No interest is paid
}

private void StateChangeCheck()

{
if (balance > upperLimit)
{
account.State = new SilverState(this);
}
}
}

// "ConcreteState"

// Silver is non-interest bearing state

class SilverState : State

{
// Overloaded constructors

public SilverState(State state) :

this( state.Balance, state.Account)
{
}

public SilverState(double balance, Account account)

{
this.balance = balance;
this.account = account;
Initialize();
}

private void Initialize()

{
// Should come from a datasource
interest = 0.0;
lowerLimit = 0.0;
upperLimit = 1000.0;
}

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public override void Deposit(double amount)

{
balance += amount;
StateChangeCheck();
}

public override void Withdraw(double amount)

{
balance -= amount;
StateChangeCheck();
}

public override void PayInterest()

{
balance += interest * balance;
StateChangeCheck();
}

private void StateChangeCheck()

{
if (balance < lowerLimit)
{
account.State = new RedState(this);
}
else if (balance > upperLimit)
{
account.State = new GoldState(this);
}
}
}

// "ConcreteState"

// Interest bearing state

class GoldState : State

{
// Overloaded constructors
public GoldState(State state)
: this(state.Balance,state.Account)
{
}

public GoldState(double balance, Account account)

{
this.balance = balance;
this.account = account;
Initialize();
}

private void Initialize()

{
// Should come from a database
interest = 0.05;

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lowerLimit = 1000.0;
upperLimit = 10000000.0;
}

public override void Deposit(double amount)

{
balance += amount;
StateChangeCheck();
}

public override void Withdraw(double amount)

{
balance -= amount;
StateChangeCheck();
}

public override void PayInterest()

{
balance += interest * balance;
StateChangeCheck();
}

private void StateChangeCheck()

{
if (balance < 0.0)
{
account.State = new RedState(this);
}
else if (balance < lowerLimit)
{
account.State = new SilverState(this);
}
}
}

// "Context"

class Account
{
private State state;
private string owner;

// Constructor
public Account(string owner)
{
// New accounts are 'Silver' by default
this.owner = owner;
state = new SilverState(0.0, this);
}

// Properties
public double Balance
{
get{ return state.Balance; }

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public State State

{
get{ return state; }
set{ state = value; }
}

public void Deposit(double amount)

{
state.Deposit(amount);
Console.WriteLine("Deposited {0:C} --- ", amount);
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}\n" ,
this.State.GetType().Name);
Console.WriteLine("");
}

public void Withdraw(double amount)

{
state.Withdraw(amount);
Console.WriteLine("Withdrew {0:C} --- ", amount);
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}\n" ,
this.State.GetType().Name);
}

public void PayInterest()

{
state.PayInterest();
Console.WriteLine("Interest Paid --- ");
Console.WriteLine(" Balance = {0:C}", this.Balance);
Console.WriteLine(" Status = {0}\n" ,
this.State.GetType().Name);
}
}
}

Output
Deposited $500.00 ---
Balance = $500.00
Status = SilverState

Deposited $300.00 ---

Balance = $800.00
Status = SilverState

Deposited $550.00 ---

Balance = $1,350.00
Status = GoldState

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 An Introduction to Design Patterns

Interest Paid ---

Balance = $1,417.50
Status = GoldState

Withdrew $2,000.00 ---

Balance = ($582.50)
Status = RedState

No funds available for withdrawal!

Withdrew $1,100.00 ---
Balance = ($582.50)
Status = RedState

5.9 Strategy Design Pattern

5.9.1 Definition
Define a family of algorithms, encapsulate each one, and make them
interchangeable. Strategy lets the algorithm vary independently from
clients that use it.

5.9.2 UML class diagram

The classes and/or objects participating in this pattern are:

Strategy (SortStrategy)
Declares an interface common to all supported algorithms. Context uses this interface
to call the algorithm defined by a ConcreteStrategy.
ConcreteStrategy (QuickSort, ShellSort, MergeSort)
Implements the algorithm using the Strategy interface
Context (SortedList)
Is configured with a ConcreteStrategy object maintains a reference to a Strategy object
may define an interface that lets Strategy access its data.

5.9.3 Sample code in C#

This structural code demonstrates the Strategy pattern which encapsulates
functionality in the form of an object. This allows clients to dynamically change
algorithmic strategies.
// Strategy pattern -- Structural example

using System;

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namespace DoFactory.GangOfFour.Strategy.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
Context context;

// Three contexts following different strategies

context = new Context(new ConcreteStrategyA());
context.ContextInterface();

context = new Context(new ConcreteStrategyB());

context.ContextInterface();

context = new Context(new ConcreteStrategyC());

context.ContextInterface();

// Wait for user

Console.Read();
}
}

// "Strategy"

abstract class Strategy

{
public abstract void AlgorithmInterface();
}

// "ConcreteStrategyA"

class ConcreteStrategyA : Strategy

{
public override void AlgorithmInterface()
{
Console.WriteLine(
"Called ConcreteStrategyA.AlgorithmInterface()");
}
}

// "ConcreteStrategyB"

class ConcreteStrategyB : Strategy

{
public override void AlgorithmInterface()
{
Console.WriteLine(
"Called ConcreteStrategyB.AlgorithmInterface()");
}
}

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 An Introduction to Design Patterns

// "ConcreteStrategyC"

class ConcreteStrategyC : Strategy

{
public override void AlgorithmInterface()
{
Console.WriteLine(
"Called ConcreteStrategyC.AlgorithmInterface()");
}
}

// "Context"

class Context
{
Strategy strategy;

// Constructor
public Context(Strategy strategy)
{
this.strategy = strategy;
}

public void ContextInterface()

{
strategy.AlgorithmInterface();
}
}
}

Output
Called ConcreteStrategyA.AlgorithmInterface()
Called ConcreteStrategyB.AlgorithmInterface()
Called ConcreteStrategyC.AlgorithmInterface()

This real-world code demonstrates the Strategy pattern which encapsulates sorting
algorithms in the form of sorting objects. This allows clients to dynamically change
sorting strategies including Quicksort, Shellsort, and Mergesort.
// Strategy pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Strategy.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{

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 An Introduction to Design Patterns

// Two contexts following different strategies

SortedList studentRecords = new SortedList();

studentRecords.Add("Samual");
studentRecords.Add("Jimmy");
studentRecords.Add("Sandra");
studentRecords.Add("Vivek");
studentRecords.Add("Anna");

studentRecords.SetSortStrategy(new QuickSort());
studentRecords.Sort();

studentRecords.SetSortStrategy(new ShellSort());
studentRecords.Sort();

studentRecords.SetSortStrategy(new MergeSort());
studentRecords.Sort();

// Wait for user

Console.Read();
}
}

// "Strategy"

abstract class SortStrategy

{
public abstract void Sort(ArrayList list);
}

// "ConcreteStrategy"

class QuickSort : SortStrategy

{
public override void Sort(ArrayList list)
{
list.Sort(); // Default is Quicksort
Console.WriteLine("QuickSorted list ");
}
}

// "ConcreteStrategy"

class ShellSort : SortStrategy

{
public override void Sort(ArrayList list)
{
//list.ShellSort(); not-implemented
Console.WriteLine("ShellSorted list ");
}
}

// "ConcreteStrategy"

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 An Introduction to Design Patterns

class MergeSort : SortStrategy

{
public override void Sort(ArrayList list)
{
//list.MergeSort(); not-implemented
Console.WriteLine("MergeSorted list ");
}
}

// "Context"

class SortedList
{
private ArrayList list = new ArrayList();
private SortStrategy sortstrategy;

public void SetSortStrategy(SortStrategy sortstrategy)

{
this.sortstrategy = sortstrategy;
}

public void Add(string name)

{
list.Add(name);
}

public void Sort()

{
sortstrategy.Sort(list);

// Display results
foreach (string name in list)
{
Console.WriteLine(" " + name);
}
Console.WriteLine();
}
}
}

Output
QuickSorted list
Anna
Jimmy
Samual
Sandra
Vivek

ShellSorted list
Anna
Jimmy
Samual
Sandra
Vivek

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 An Introduction to Design Patterns

MergeSorted list
Anna
Jimmy
Samual
Sandra
Vivek

5.10 Template Method Design Pattern

5.10.1 Definition
Define the skeleton of an algorithm in an operation, deferring some
steps to subclasses. Template Method lets subclasses redefine certain
steps of an algorithm without changing the algorithm's structure.

5.10.2 UML class diagram

The classes and/or objects participating in this pattern are:

AbstractClass (DataObject)
Defines abstract primitive operations that concrete subclasses define to implement
steps of an algorithm.
Implements a template method defining the skeleton of an algorithm. The template
method calls primitive operations as well as operations defined in AbstractClass or
those of other objects.
ConcreteClass (CustomerDataObject)
Implements the primitive operations ot carry out subclass-specific steps of the
algorithm

5.10.2 Sample code in C#

This structural code demonstrates the Template method which provides a skeleton
calling sequence of methods. One or more steps can be deferred to subclasses which
implement these steps without changing the overall calling sequence.
// Template pattern -- Structural example

using System;

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namespace DoFactory.GangOfFour.Template.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
AbstractClass c;

c = new ConcreteClassA();
c.TemplateMethod();

c = new ConcreteClassB();
c.TemplateMethod();

// Wait for user

Console.Read();
}
}

// "AbstractClass"

abstract class AbstractClass

{
public abstract void PrimitiveOperation1();
public abstract void PrimitiveOperation2();

// The "Template method"

public void TemplateMethod()
{
PrimitiveOperation1();
PrimitiveOperation2();
Console.WriteLine("");
}
}

// "ConcreteClass"

class ConcreteClassA : AbstractClass

{
public override void PrimitiveOperation1()
{
Console.WriteLine("ConcreteClassA.PrimitiveOperation1()");
}
public override void PrimitiveOperation2()
{
Console.WriteLine("ConcreteClassA.PrimitiveOperation2()");
}
}

class ConcreteClassB : AbstractClass

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{
public override void PrimitiveOperation1()
{
Console.WriteLine("ConcreteClassB.PrimitiveOperation1()");
}
public override void PrimitiveOperation2()
{
Console.WriteLine("ConcreteClassB.PrimitiveOperation2()");
}
}
}

Output
ConcreteClassA.PrimitiveOperation1()
ConcreteClassA.PrimitiveOperation2()

This real-world code demonstrates a Template method named Run() which provides a
skeleton calling sequence of methods. Implementation of these steps are deferred to the
CustomerDataObject subclass which implements the Connect, Select, Process, and
Disconnect methods.
// Template pattern -- Real World example
using System;
using System.Data;
using System.Data.OleDb;

namespace DoFactory.GangOfFour.Template.RealWorld
{

// MainApp test application

class MainApp
{
static void Main()
{
DataAccessObject dao;

dao = new Categories();

dao.Run();

dao = new Products();

dao.Run();

// Wait for user

Console.Read();
}
}

// "AbstractClass"

abstract class DataAccessObject

{
protected string connectionString;

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 An Introduction to Design Patterns

protected DataSet dataSet;

public virtual void Connect()

{
// Make sure mdb is on c:\
connectionString =
"provider=Microsoft.JET.OLEDB.4.0; " +
"data source=c:\\nwind.mdb";
}

public abstract void Select();

public abstract void Process();

public virtual void Disconnect()

{
connectionString = "";
}

// The "Template Method"

public void Run()

{
Connect();
Select();
Process();
Disconnect();
}
}

// "ConcreteClass"

class Categories : DataAccessObject

{
public override void Select()
{
string sql = "select CategoryName from Categories";
OleDbDataAdapter dataAdapter = new OleDbDataAdapter(
sql, connectionString);

dataSet = new DataSet();

dataAdapter.Fill(dataSet, "Categories");
}

public override void Process()

{
Console.WriteLine("Categories ---- ");

DataTable dataTable = dataSet.Tables["Categories"];

foreach (DataRow row in dataTable.Rows)
{
Console.WriteLine(row["CategoryName"]);
}
Console.WriteLine();
}

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 An Introduction to Design Patterns

class Products : DataAccessObject

{
public override void Select()
{
string sql = "select ProductName from Products";
OleDbDataAdapter dataAdapter = new OleDbDataAdapter(
sql, connectionString);

dataSet = new DataSet();

dataAdapter.Fill(dataSet, "Products");
}

public override void Process()

{
Console.WriteLine("Products ---- ");
DataTable dataTable = dataSet.Tables["Products"];
foreach (DataRow row in dataTable.Rows)
{
Console.WriteLine(row["ProductName"]);
}
Console.WriteLine();
}
}
}

Output
Categories ----
Beverages
Condiments
Confections
Dairy Products
Grains/Cereals
Meat/Poultry
Produce
Seafood

Products ----
Chai
Chang
Aniseed Syrup
Chef Anton's Cajun Seasoning
Chef Anton's Gumbo Mix
Grandma's Boysenberry Spread
Uncle Bob's Organic Dried Pears
Northwoods Cranberry Sauce
Mishi Kobe Niku

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 An Introduction to Design Patterns

5.11 Visitor Design Pattern

5.11.1 Definition
Represent an operation to be performed on the elements of an object
structure. Visitor lets you define a new operation without changing the
classes of the elements on which it operates.

5.11.2 UML class diagram

The classes and/or objects participating in this pattern are:

Visitor (Visitor)
Declares a Visit operation for each class of ConcreteElement in the object structure. The
operation's name and signature identifies the class that sends the Visit request to the
visitor. That lets the visitor determine the concrete class of the element being visited.
Then the visitor can access the elements directly through its particular interface
ConcreteVisitor (IncomeVisitor, VacationVisitor)
Implements each operation declared by Visitor. Each operation implements a fragment
of the algorithm defined for the corresponding class or object in the structure.

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ConcreteVisitor provides the context for the algorithm and stores its local state. This
state often accumulates results during the traversal of the structure.
Element (Element)
Defines an Accept operation that takes a visitor as an argument.
ConcreteElement (Employee)
Implements an Accept operation that takes a visitor as an argument
ObjectStructure (Employees)
Can enumerate its elements
May provide a high-level interface to allow the visitor to visit its elements
May either be a Composite (pattern) or a collection such as a list or a set

5.11.3 Sample code in C#

This structural code demonstrates the Visitor pattern in which an object traverses an
object structure and performs the same operation on each node in this structure.
Different visitor objects define different operations.
// Visitor pattern -- Structural example
using System;
using System.Collections;

namespace DoFactory.GangOfFour.Visitor.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
// Setup structure
ObjectStructure o = new ObjectStructure();
o.Attach(new ConcreteElementA());
o.Attach(new ConcreteElementB());

// Create visitor objects

ConcreteVisitor1 v1 = new ConcreteVisitor1();
ConcreteVisitor2 v2 = new ConcreteVisitor2();

// Structure accepting visitors

o.Accept(v1);
o.Accept(v2);

// Wait for user

Console.Read();
}
}

// "Visitor"

abstract class Visitor

{
public abstract void VisitConcreteElementA(
ConcreteElementA concreteElementA);
public abstract void VisitConcreteElementB(

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ConcreteElementB concreteElementB);
}

// "ConcreteVisitor1"

class ConcreteVisitor1 : Visitor

{
public override void VisitConcreteElementA(
ConcreteElementA concreteElementA)
{
Console.WriteLine("{0} visited by {1}",
concreteElementA.GetType().Name, this.GetType().Name);
}

public override void VisitConcreteElementB(

ConcreteElementB concreteElementB)
{
Console.WriteLine("{0} visited by {1}",
concreteElementB.GetType().Name, this.GetType().Name);
}
}

// "ConcreteVisitor2"

class ConcreteVisitor2 : Visitor

{
public override void VisitConcreteElementA(
ConcreteElementA concreteElementA)
{
Console.WriteLine("{0} visited by {1}",
concreteElementA.GetType().Name, this.GetType().Name);
}

public override void VisitConcreteElementB(

ConcreteElementB concreteElementB)
{
Console.WriteLine("{0} visited by {1}",
concreteElementB.GetType().Name, this.GetType().Name);
}
}

// "Element"

abstract class Element

{
public abstract void Accept(Visitor visitor);
}

// "ConcreteElementA"

class ConcreteElementA : Element

{
public override void Accept(Visitor visitor)
{

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 An Introduction to Design Patterns

visitor.VisitConcreteElementA(this);
}

public void OperationA()

{
}
}

// "ConcreteElementB"

class ConcreteElementB : Element

{
public override void Accept(Visitor visitor)
{
visitor.VisitConcreteElementB(this);
}

public void OperationB()

{
}
}

// "ObjectStructure"

class ObjectStructure
{
private ArrayList elements = new ArrayList();

public void Attach(Element element)

{
elements.Add(element);
}

public void Detach(Element element)

{
elements.Remove(element);
}

public void Accept(Visitor visitor)

{
foreach (Element e in elements)
{
e.Accept(visitor);
}
}
}
}

Output
ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1
ConcreteElementA visited by ConcreteVisitor2
ConcreteElementB visited by ConcreteVisitor2

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 An Introduction to Design Patterns

This real-world code demonstrates the Visitor pattern in which two objects traverse a
list of Employees and performs the same operation on each Employee. The two visitor
objects define different operations -- one adjusts vacation days and the other income.
// Visitor pattern -- Real World example

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Visitor.RealWorld
{

// MainApp startup application

class MainApp
{
static void Main()
{
// Setup employee collection
Employees e = new Employees();
e.Attach(new Clerk());
e.Attach(new Director());
e.Attach(new President());

// Employees are 'visited'

e.Accept(new IncomeVisitor());
e.Accept(new VacationVisitor());

// Wait for user

Console.Read();
}
}

// "Visitor"

interface IVisitor
{
void Visit(Element element);
}

// "ConcreteVisitor1"

class IncomeVisitor : IVisitor

{
public void Visit(Element element)
{
Employee employee = element as Employee;

// Provide 10% pay raise

employee.Income *= 1.10;
Console.WriteLine("{0} {1}'s new income: {2:C}",
employee.GetType().Name, employee.Name,
employee.Income);
}
}

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 An Introduction to Design Patterns

// "ConcreteVisitor2"

class VacationVisitor : IVisitor

{
public void Visit(Element element)
{
Employee employee = element as Employee;

// Provide 3 extra vacation days

Console.WriteLine("{0} {1}'s new vacation days: {2}",
employee.GetType().Name, employee.Name,
employee.VacationDays);
}
}

class Clerk : Employee

{
// Constructor
public Clerk() : base("Hank", 25000.0, 14)
{
}
}

class Director : Employee

{
// Constructor
public Director() : base("Elly", 35000.0, 16)
{
}
}

class President : Employee

{
// Constructor
public President() : base("Dick", 45000.0, 21)
{
}
}

// "Element"

abstract class Element

{
public abstract void Accept(IVisitor visitor);
}

// "ConcreteElement"

class Employee : Element

{
string name;
double income;
int vacationDays;

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// Constructor
public Employee(string name, double income,
int vacationDays)
{
this.name = name;
this.income = income;
this.vacationDays = vacationDays;
}

// Properties
public string Name
{
get{ return name; }
set{ name = value; }
}

public double Income

{
get{ return income; }
set{ income = value; }
}

public int VacationDays

{
get{ return vacationDays; }
set{ vacationDays = value; }
}

public override void Accept(IVisitor visitor)

{
visitor.Visit(this);
}
}

// "ObjectStructure"

class Employees
{
private ArrayList employees = new ArrayList();

public void Attach(Employee employee)

{
employees.Add(employee);
}

public void Detach(Employee employee)

{
employees.Remove(employee);
}

public void Accept(IVisitor visitor)

{
foreach (Employee e in employees)

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{
e.Accept(visitor);
}
Console.WriteLine();
}
}
}

Output
Clerk Hank's new income: $27,500.00
Director Elly's new income: $38,500.00
President Dick's new income: $49,500.00

Clerk Hank's new vacation days: 14

Director Elly's new vacation days: 16
President Dick's new vacation days: 21

: END :

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