Factory Method Pattern

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. The key objective of the Factory Method is extensibility. Factory Methods are frequently used in applications that manage, maintain, or manipulate collections of objects that are different but at the same time have many characteristics in common.

using System;
using System.Collections.Generic;

namespace Patterns
{
/// <summary>
/// MainApp startup class for .NET optimized
/// Factory Method Design Pattern.
/// </summary>
class MainApp
{
static void Main()
{
// Note: document constructors call Factory Method
List<Document> documents = new List<Document> { new Resume(), new Report() };

// Display document pages
foreach (Document document in documents)
{
Console.WriteLine(document + "--");
foreach (Page page in document.Pages)
{
Console.WriteLine(" " + page);
}
Console.WriteLine();
}

// Wait for user
Console.ReadKey();
}
}

/// <summary>
/// The 'Product' abstract class
/// </summary>
abstract class Page
{
// Override. Display class name
public override string ToString()
{
return this.GetType().Name;
}
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class SkillsPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class EducationPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class ExperiencePage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class IntroductionPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class ResultsPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class ConclusionPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class SummaryPage : Page
{
}

/// <summary>
/// A 'ConcreteProduct' class
/// </summary>
class BibliographyPage : Page
{
}

/// <summary>
/// The 'Creator' abstract class
/// </summary>
abstract class Document
{
// Constructor invokes Factory Method
public Document()
{
this.CreatePages();
}

// Gets list of document pages
public List<Page> Pages { get; protected set; }

// Factory Method
public abstract void CreatePages();

// Override
public override string ToString()
{
return this.GetType().Name;
}
}

/// <summary>
/// A 'ConcreteCreator' class
/// </summary>
class Resume : Document
{
// Factory Method implementation
public override void CreatePages()
{
Pages = new List<Page>
{ new SkillsPage(),
new EducationPage(),
new ExperiencePage() };
}
}

/// <summary>
/// A 'ConcreteCreator' class
/// </summary>
class Report : Document
{
// Factory Method implementation
public override void CreatePages()
{
Pages = new List<Page>
{ new IntroductionPage(),
new ResultsPage(),
new ConclusionPage(),
new SummaryPage(),
new BibliographyPage() };
}
}
}