Decorator Design Pattern in C#

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

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

using System;

  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()
    {
    }
  }