Skip to content

Decorator

Summary: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Frequency of use: Medium

UML class diagram

alt

Participants

The classes and objects participating in this pattern are:

  • 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# .{10}

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

using System;

namespace DoFactory.GangOfFour.Decorator.Structural
{
    /// <summary>
    /// MainApp startup class for Structural
    /// Decorator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            // 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>
    abstract class Component
    {
        public abstract void Operation();
    }

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>
    class ConcreteComponent : Component
    {
        public override void Operation()
        {
            Console.WriteLine("ConcreteComponent.Operation()");
        }
    }

    /// <summary>
    /// The 'Decorator' abstract class
    /// </summary>
    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>
    class ConcreteDecoratorA : Decorator
    {
        public override void Operation()
        {
            base.Operation();
            Console.WriteLine("ConcreteDecoratorA.Operation()");
        }
    }

    /// <summary>
    /// The 'ConcreteDecoratorB' class
    /// </summary>
    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# .{10}

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 DoFactory.GangOfFour.Decorator.RealWorld
{
    /// <summary>
    /// MainApp startup class for Real-World
    /// Decorator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            // 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>
    abstract class LibraryItem
    {
        private int _numCopies;

        // Property
        public int NumCopies
        {
            get { return _numCopies; }
            set { _numCopies = value; }
        }

        public abstract void Display();
    }

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>
    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>
    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>
    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>
    class Borrowable : Decorator
    {
        protected 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# .{10}

alt

using System;
using System.Collections.Generic;

namespace DoFactory.GangOfFour.Decorator.NETOptimized
{
    /// <summary>
    /// MainApp startup class for .NET optimized
    /// Decorator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            // Create book
            var book = new Book("Worley", "Inside ASP.NET", 10);
            book.Display();

            // Create video
            var video = new Video("Spielberg", "Jaws", 23, 92);
            video.Display();

            // Make video borrowable, then borrow and display
            Console.WriteLine("\nMaking video borrowable:");

            var borrow = new Borrowable<Video>(video);
            borrow.BorrowItem("Customer #1");
            borrow.BorrowItem("Customer #2");

            borrow.Display();

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

    /// <summary>
    /// The 'Component' abstract class
    /// </summary>
    abstract class LibraryItem<T>
    {
        // Each T has its own NumCopies
        public static int NumCopies { get; set; }

        public abstract void Display();
    }

    /// <summary>
    /// The 'ConcreteComponent' class
    /// </summary>
    class Book : LibraryItem<Book>
    {
        private string _author;
        private string _title;

        // Constructor
        public Book(string author, string title, int numCopies)
        {
            this._author = author;
            this._title = title;
            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>
    class Video : LibraryItem<Video>
    {
        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;
            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>
    abstract class Decorator<T> : LibraryItem<T>
    {
        private LibraryItem<T> _libraryItem;

        // Constructor
        public Decorator(LibraryItem<T> libraryItem)
        {
            _libraryItem = libraryItem;
        }

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

    /// <summary>
    /// The 'ConcreteDecorator' class
    /// </summary>
    class Borrowable<T> : Decorator<T>
    {
        private List<string> _borrowers = new List<string>();

        // Constructor
        public Borrowable(LibraryItem<T> libraryItem)
            : base(libraryItem)
        {
        }

        public void BorrowItem(string name)
        {
            _borrowers.Add(name);
            NumCopies--;
        }

        public void ReturnItem(string name)
        {
            _borrowers.Remove(name);
            NumCopies++;
        }

        public override void Display()
        {
            base.Display();
            _borrowers.ForEach(b =>  Console.WriteLine(" borrower: " + b));
        }
    }
}