Factory Method

Frequency of use:

High

UML class diagram

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#

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

Real-world code in C#

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

.NET Optimized code in C#

The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, LINQ, lambda functions, and more. You can find an example on our Singleton pattern page.

All other patterns, and so much more, are available in our Dofactory .NET product.

Dofactory .NET includes the Gang of Four and Enterprise patterns, but also many other innovations including our Ultra-Clean™ Architecture, powerful low-code tactics, Rapid Application Development (RAD) techniques, and much more.

Accelerate your application development to where you can write entire solutions in just 33 days!. This unique package will change your outlook on development and your career.  Here's what is included:

Dofactory .NET
.NET Developer Pack




Learn More

• 69 gang-of-four pattern projects
• 46 head-first pattern projects
• Fowler's enterprise patterns
• Multi-tier patterns
• Convention over configuration
• Active Record and CQRS patterns
• Repository and Unit-of-Work patterns
• MVC, MVP, & MVVM patterns
• Spark^TM Rapid App Dev (RAD) data access
• Complete Analytics, Dashboard App
• Complete Art Shop, Ecommerce App
• Complete SaaS, Multi-Tenant App
• Complete CRM, Customer Relationship App
• 33-Day App Factory™
• Everything 100% source code