Structural Pattern – Decorator

“Attach additional responsibilities to an object dynamically.”

• Related to the Composite pattern

• Most commonly used when drawing objects.

Example:

Form.Draw(rect) {

// … draw form …

foreach(Element e in elements)

e.Draw(rect)

}

Example-

namespace Patterns {
  interface IComponent {
    string Operation();
  }

  class Original : IComponent {
    public string Operation() {
      return "I'm original but ";
    }
  }

  class Decorate : IComponent {
    IComponent _compo;
    public Decorate(IComponent c) {
      _compo = c;
    }

    public string Operation() {
      string s = _compo.Operation();
      s += "I'm decorated too.";
      return s;
    }
  }

  public class DecoExample {
    public void test() {
      IComponent c = new Original();
      Decorate d = new Decorate(c);
      Console.WriteLine(d.Operation());
    }
  }
}

Links:

• http://www.dofactory.com/Patterns/PatternDecorator.aspx
• Decorator pattern discussion