访问者模式(Visitor Pattern)

意图：表示一个作用于某对象结构中的各元素的操作。它使你可以在不改变各元素的类的前提下定义作用于这些元素的新操作。

  using System;
   using ObjectStructure;
 
     // Visitor Pattern - Example    Judith Bishop October 2007
     // Sets up an object structure and visits it 
     // in two ways - for printing and for collecting elements in groups
     // using dynamic-dispatch
    
   namespace ObjectStructure {
    
     abstract class IElement {
       // Added to make Elements Visitor-ready
       public abstract void Accept(IVisitor visitor);
     }
 
     class Element : IElement {
       public Element Next {get; set;}
       public Element Link {get; set;}
       public Element () {}
       public Element (Element next) {
         Next = next;
       }
       public override void Accept(IVisitor visitor) {
         visitor.Visit(this);
       }
     }
   
     class ElementWithLink : Element {
       public ElementWithLink (Element link, Element next) {
         Next = next; 
         Link = link;
       }
       public override void Accept(IVisitor visitor) {
         visitor.Visit(this);
       }
     }
   } // end ObjectStructure
   
     // Visitor interface 
     interface IVisitor {
       void Visit (Element element);
       void Visit (ElementWithLink element);
     }
 
     // Visitor
     class CountVisitor : IVisitor {
     
     public int Count {get; set;}
     
     public void CountElements(Element element) {
       element.Accept(this);
       if (element.Link!=null) CountElements(element.Link);
       if (element.Next!=null) CountElements(element.Next);
     } 
     
     // Elements with links are not counted
     public void Visit(ElementWithLink element) {
        Console.WriteLine("Not counting");
     }
     
     // Only Elements are counted
     public void Visit (Element element) {
       Count++;
     }
   }
 
   // Client
   class Client {
     
     static void Main() {
       // Set up the object structure
       Element objectStructure = 
         new Element(
             new Element(
             new ElementWithLink(
              new Element(           
                    new Element(  
                      new ElementWithLink(
                  new Element(null),        
                    new Element(
                    null)))),
         new Element(
             new Element(
             new Element(null))))));
 
       Console.WriteLine ("Count the Elements");
       CountVisitor visitor = new CountVisitor();
       visitor.CountElements(objectStructure);
       Console.WriteLine("Number of Elements is: "+visitor.Count);
       Console.ReadKey();
     }
   }
 /*
 Count the As
 Found Element
 Found Element
 Number of elements is: 9
 */