Decorator Design Pattern

// Decorator pattern -- Structural example

using System;

namespace DoFactory.GangOfFour.Decorator.Structural
{

  // MainApp test application

  class MainApp
  {
    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.Read();
    }
  }

  // "Component"

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

  // "ConcreteComponent"

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

  // "Decorator"

  abstract class Decorator : Component
  {
    protected Component component;

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

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

  // "ConcreteDecoratorA"

  class ConcreteDecoratorA : Decorator
  {
    private string addedState;

    public override void Operation()
    {
      base.Operation();
      addedState = "New State";
      Console.WriteLine("ConcreteDecoratorA.Operation()");
    }
  }

  // "ConcreteDecoratorB"

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

    void AddedBehavior()
    {
    }
  }
}

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Decorator.RealWorld
{

  // MainApp test application

  class MainApp
  {
    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.Read();
    }
  }

  // "Component"

  abstract class LibraryItem
  {
    private int numCopies;

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

    public abstract void Display();
  }

  // "ConcreteComponent"

  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);
    }
  }

  // "ConcreteComponent"

  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);
    }
  }

  // "Decorator"

  abstract class Decorator : LibraryItem
  {
    protected LibraryItem libraryItem;

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

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

  // "ConcreteDecorator"

  class Borrowable : Decorator
  {
    protected ArrayList borrowers = new ArrayList();

    // 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);
      }
    }
  }
}

// Decorator pattern -- .NET optimized

Two editions: C# and VB.NET

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