访问者模式（Visitor）

// Visitor pattern 
// Structural example 
using System;
using System.Collections.Generic;
  
namespace DoFactory.GangOfFour.Visitor.Structural
{
    // MainApp test application
    class MainApp
    {
        static void Main()
        {
            // Setup structure
            ObjectStructure o = new ObjectStructure();
            o.Attach(new ConcreteElementA());
            o.Attach(new ConcreteElementB());
  
            // Create visitor objects
            ConcreteVisitor1 v1 = new ConcreteVisitor1();
            ConcreteVisitor2 v2 = new ConcreteVisitor2();
  
            // Structure accepting visitors
            o.Accept(v1);
            o.Accept(v2);
  
            // Wait for user
            Console.ReadKey();
        }
    }
  
    // "Visitor"
    abstract class Visitor
    {
        public abstract void VisitConcreteElementA(ConcreteElementA concreteElementA);
        public abstract void VisitConcreteElementB(ConcreteElementB concreteElementB);
    }
  
    // "ConcreteVisitor"
    class ConcreteVisitor1 : Visitor
    {
        public override void VisitConcreteElementA(ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0} visited by {1}",concreteElementA.GetType().Name, this.GetType().Name);
        }
  
        public override void VisitConcreteElementB(ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0} visited by {1}",concreteElementB.GetType().Name, this.GetType().Name);
        }
    }
  
    // "ConcreteVisitor"
    class ConcreteVisitor2 : Visitor
    {
        public override void VisitConcreteElementA(ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0} visited by {1}", concreteElementA.GetType().Name, this.GetType().Name);
        }
  
        public override void VisitConcreteElementB(ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0} visited by {1}", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }
  
    // "Element"
    abstract class Element
    {
        public abstract void Accept(Visitor visitor);
    }
  
    // "ConcreteElement"
    class ConcreteElementA : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementA(this);
        }
  
        public void OperationA()
        {
        }
    }
  
    // "ConcreteElement"
    class ConcreteElementB : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementB(this);
        }
  
        public void OperationB()
        {
        }
    }
  
    // "ObjectStructure"
    class ObjectStructure
    {
        private List<Element> _elements = new List<Element>();
  
        public void Attach(Element element)
        {
            _elements.Add(element);
        }
  
        public void Detach(Element element)
        {
            _elements.Remove(element);
        }
  
        public void Accept(Visitor visitor)
        {
            foreach (Element element in _elements)
            {
                element.Accept(visitor);
            }
        }
    }
}