C# Factory Method

The Factory Method design pattern defines an interface for creating an object, but let subclasses decide which class to instantiate. This pattern lets a class defer instantiation to subclasses.

Frequency of use:
high
C# Design Patterns

UML class diagram

A visualization of the classes and objects participating in this pattern.


Participants

The classes and objects participating in this pattern include:

  • 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
Created ConcreteProductA
Created ConcreteProductB

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
Resume -------
 SkillsPage
 EducationPage
 ExperiencePage

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

.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, etc. You can find an example on our Singleton pattern page.

All other patterns (and much more) are available in our Dofactory .NET product.


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