访问者模式(Visitor Pattern)

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
*/