Earn income with your .NET skills
Sign up and we'll send you the best freelance opportunities straight to your inbox.
We're building the largest self-service freelancing marketplace for people like you.

C# Decorator

The Decorator design pattern attaches additional responsibilities to an object dynamically. This pattern provide a flexible alternative to subclassing for extending functionality.

Frequency of use:
medium
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:

  • 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.

Structural code in C#

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

using System;

namespace Decorator.Structural
{
    /// <summary>
    /// Decorator Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // 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.ReadKey();
        }
    }

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

    public abstract class Component
    {
        public abstract void Operation();
    }

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>

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

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

    public abstract class Decorator : Component
    {
        protected Component component;

        public void SetComponent(Component component)
        {
            this.component = component;
        }

        public override void Operation()
        {
            if (component != null)
            {
                component.Operation();
            }
        }
    }

    /// <summary>
    /// The 'ConcreteDecoratorA' class
    /// </summary>

    public class ConcreteDecoratorA : Decorator
    {
        public override void Operation()
        {
            base.Operation();
            Console.WriteLine("ConcreteDecoratorA.Operation()");
        }
    }

    /// <summary>
    /// The 'ConcreteDecoratorB' class
    /// </summary>

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

        void AddedBehavior()
        {
        }
    }
}
Output
ConcreteComponent.Operation()
ConcreteDecoratorA.Operation()
ConcreteDecoratorB.Operation()

Real-world code in C#

This real-world code demonstrates the Decorator pattern in which 'borrowable' functionality is added to existing library items (books and videos).

using System;
using System.Collections.Generic;

namespace Decorator.RealWorld
{
    /// <summary>
    /// Decorator Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // 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.ReadKey();
        }
    }
    /// <summary>
    /// The 'Component' abstract class
    /// </summary>

    public abstract class LibraryItem
    {
        private int numCopies;

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

        public abstract void Display();
    }

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>

    public class Book : LibraryItem
    {
        private string author;
        private string title;

        // Constructor

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

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>

    public 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);
        }
    }

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

    public abstract class Decorator : LibraryItem
    {
        protected LibraryItem libraryItem;

        // Constructor

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

        public override void Display()
        {
            libraryItem.Display();
        }
    }

    /// <summary>
    /// The 'ConcreteDecorator' class
    /// </summary>

    public class Borrowable : Decorator
    {
        protected readonly List<string> borrowers = new List<string>();

        // 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


Making video borrowable:

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

borrower: Customer #1
borrower: Customer #2

.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.


Not only does Dofactory .NET cover the Gang of Four and Enterprise patterns, it also includes pattern architectures, low-code, and RAD (Rapid Application Development) techniques. Accelerate development to where you can write entire solutions in just 33 days!.

This unique package will change your developer lifestyle.  Here's what is included:



#1 .NET Success Platform
  • 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
  • REST patterns with Web API
  • SparkTM Rapid App Dev (RAD) data access
  • Complete Art Shop, Ecommerce App
  • Complete Analytics, Dashboard App
  • Complete Art Shop, Ecommerce App
  • Complete SaaS, Multi-Tenant App
  • Everything 100% source code


Earn income with your .NET skills
Sign up and we'll send you the best freelance opportunities straight to your inbox.
We're building the largest self-service freelancing marketplace for people like you.
Guides


vsn 3.1