Decorator Design Pattern -- Design Pattern

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

Participants:

    The classes and/or objects participating in this pattern are:

• Component(Library Item)

       Defines the interface for objects that can have responsibilities added to them dynamically.

• Concrete Component   (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.

         Concrete Decorator (browse able). Adds responsibilities to the component.

Example of Decorator Design pattern

using System;  namespace 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()