Factory Method¶

Factory Method
UML class diagram
Participants
Structural code in C# .{10}
- Output
Real-world code in C# .{10} - Output
.NET Optimized code in C# .{10}

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

Frequency of use: High

UML class diagram¶

alt

Participants¶

The classes and 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.

Structural code in C# .{10}¶

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.

using System;

namespace DoFactory.GangOfFour.Factory.Structural {
  /// <summary>
  /// MainApp startup class for Structural
  /// Factory Method Design Pattern.
  /// </summary>

  class MainApp

  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>

    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.ReadKey();
    }
  }

  /// <summary>
  /// The 'Product' abstract class
  /// </summary>

  abstract class Product

  {}

  /// <summary>
  /// A 'ConcreteProduct' class
  /// </summary>

  class ConcreteProductA: Product

  {}

  /// <summary>
  /// A 'ConcreteProduct' class
  /// </summary>

  class ConcreteProductB: Product

  {}

  /// <summary>
  /// The 'Creator' abstract class
  /// </summary>

  abstract class Creator

  {
    public abstract Product FactoryMethod();
  }

  /// <summary>
  /// A 'ConcreteCreator' class
  /// </summary>

  class ConcreteCreatorA: Creator

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

  /// <summary>
  /// A 'ConcreteCreator' class
  /// </summary>

  class ConcreteCreatorB: Creator

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

Output¶

Created ConcreteProductA
Created ConcreteProductB

Real-world code in C# .{10}¶

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.

using System;
using System.Collections.Generic;

namespace DoFactory.GangOfFour.Factory.RealWorld {
  /// <summary>
  /// MainApp startup class for Real-World
  /// Factory Method Design Pattern.
  /// </summary>

  class MainApp

  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>

    static void Main() {
      // Note: constructors call Factory Method

      Document[] documents = new Document[2];

      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.ReadKey();
    }
  }

  /// <summary>
  /// The 'Product' abstract class
  /// </summary>

  abstract class Page

  {}

  /// <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

  {
    private List < Page > _pages = new List < Page > ();

    // Constructor calls abstract Factory method

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

    public List < Page > Pages {
      get {
        return _pages;
      }
    }

    // Factory Method

    public abstract void CreatePages();
  }

  /// <summary>
  /// A 'ConcreteCreator' class
  /// </summary>

  class Resume: Document

  {
    // Factory Method implementation

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

  /// <summary>
  /// A 'ConcreteCreator' class
  /// </summary>

  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

.NET Optimized code in C# .{10}¶

alt

```cs{107,119} using System; using System.Collections.Generic;

namespace DoFactory.GangOfFour.Factory.NETOptimized { ///

/// MainApp startup class for .NET optimized /// Factory Method Design Pattern. ///

class MainApp { ///

/// Entry point into console application. ///

static void Main() { // Document constructors call Factory Method var documents = new List { new Resume(), new Report() };

        // Display document pages
        foreach (var document in documents)
        {
            Console.WriteLine(document + "--");
            foreach (var 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() };
    }
}

} ```