访问者模式 c#

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {

        public interface IVisitor
        {
            void Visit(PrinterA a);
            void Visit(PrinterB b);
        }

        public abstract class Printer
        {
            public abstract void Accept(IVisitor visitor);
            public abstract void Print();
        }

        public class PrinterA:Printer
        {
            public override void Accept(IVisitor visitor)
            {
                visitor.Visit(this);
            }

            public override void Print()
            {
                Console.WriteLine("i am is a");
            }
        }

        public class  PrinterB:Printer
        {
            public override void Accept(IVisitor visitor)
            {
                visitor.Visit(this);
            }

            public override void Print()
            {
                Console.WriteLine("i am is b");
            }
        }

        public class ConcreteVisitor:IVisitor
        {
            public void Visit(PrinterA a)
            {
                a.Print();
            }

            public void Visit(PrinterB b)
            {
                b.Print();
            }
        }

        static void Main(string[] args)
        {
            Printer a = new PrinterA();
            Printer b = new PrinterB();

            a.Accept(new ConcreteVisitor());
            b.Accept(new ConcreteVisitor());
        }
    }
}