设计模式：访问者模式

访问者模式（Visitor）：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。

namespace VisitorDesign
{
    public abstract class Visitor
    {
        public abstract void VisitConcreteElementA(ConcreteElementA concreteElementA);
        public abstract void VisitConcreteElementB(ConcreteElementB concreteElementB);
    }
    public class ConcreteVisitor1 : Visitor
    {
        public override void VisitConcreteElementA(ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0}被{1}访问", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void VisitConcreteElementB(ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0}被{1}访问", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }
    public class ConcreteVisitor2 : Visitor
    {
        public override void VisitConcreteElementA(ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0}被{1}访问", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void VisitConcreteElementB(ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0}被{1}访问", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }
    public abstract class Element
    {
        public abstract void Accept(Visitor visitor);
    }
    public class ConcreteElementA : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementA(this);
        }
        public void OperationA()
        {

        }
    }
    public class ConcreteElementB : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementB(this);
        }
        public void OperationB()
        {

        }
    }
    public class ObjectStructure
    {
        private IList<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 e in elements)
            {
                e.Accept(visitor);
            }
        }
    }
}

测试代码：

            ObjectStructure o = new ObjectStructure();
            o.Attach(new ConcreteElementA());
            o.Attach(new ConcreteElementB());
            ConcreteVisitor1 v1 = new ConcreteVisitor1();
            ConcreteVisitor2 v2 = new ConcreteVisitor2();
            o.Accept(v1);
            o.Accept(v2);