Definition

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

UML class diagram

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Participants

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

• Component - defines the interface for objects that can have responsibilities added to them dynamically.
• ConcreteComponent - defines an object to which additional responsibilities can be attached.
• Decorator - maintains a reference to a Component object and defines an interface that conforms to Component's interface.
• ConcreteDecorator - adds responsibilities to the component.

Sample code in C#

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  /// <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()
    {
    }
  }

 

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