Decorator Design Pattern

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

using System;
using System.Collections.Generic;

namespace Patterns
{
  
    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.ReadKey();
        }
     }

    /// <summary>
    /// The 'Component' abstract class
    /// </summary>
    abstract class LibraryItem
    {
         public int NumCopies { get; set; }

         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);
             }
        }
     }
}