扩展方法(类)或访问者模式

在进行良好的设计时，您会选择扩展方法还是访问者模式?

哪个更容易设计，什么时候应该在访问者模式上使用扩展方法，反之亦然?

在访问者类上使用扩展方法，除了语法上的好处来提高程序的可读性之外，还有什么好的正当理由吗?

您将如何设计一个包含扩展方法的系统，您会在UML图中对它们进行分类吗?

namespace ExtensionMethods
{
    public static class MyExtensions
    {
        public static int WordCount(this String str)
        {
            return str.Split(new char[] { ' ', '.', '?' }, 
                             StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }   
}

我可能有错误的模式，它看起来像上面代码中的访问者模式。所以我认为我的比较站得住脚。

一些代码，我想说扩展方法看起来像一个访问者模式。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
    #region Interfaces
    public interface IFred
    {
        string Data
        {
            get;
            set;
        }        
        string doSomething();
    }

    public interface IBob
    {
        string Data
        {
            get;
            set;
        }
    }
    #endregion
    #region fred stuff
    public partial class Fred : IFred
    {
        public string doSomething()
        {
            return this.Data + " is really cool";
        }
        public string Value()
        {
            throw new NotImplementedException();
        }
    }
    public partial class Fred
    {
        public string Data
        {
            get;
            set;
        }
    }
    #endregion

    #region bob stuff
    public class BobData : IBob
    {
        public string Data
        {
            get;
            set;
        }
    }
    public class BobData2 : IBob
    {
        private string pData;
        public string Data
        {
            get
            {
                return pData + " and then some!";
            }
            set
            {
                pData = value;
            }
        }
    }
    public class BobVisitor
    {
        public string dosomething(IBob bobData)
        {
            Console.WriteLine(bobData.Data);
            return "ok";
        }
        public string dosomethingOnlyToBob(BobData bobData)
        {
            Console.WriteLine("hello bob version 1");
            return "ok";
        }

        public string dosomethingOnlyToBob2(BobData2 bobData)
        {
            Console.WriteLine("hello bob version 2");
            return "ok";
        }
    }
    #endregion

    public static class Visitor
    {
        public static string visit(this IBob bobObj)
        {
            Console.WriteLine(bobObj.Data);
            return "ok";
        }
        public static string visit(this IFred fredObj)
        {
            Console.WriteLine(fredObj.Data);
            return "ok";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            //Another way of abstracting methods from data, using Partial Classes.
            var fredObj = new Fred();
            fredObj.Data = "fred data";
            fredObj.doSomething();

            //Create the bob classes version 1 and 2
            var bobObj = new BobData();
            bobObj.Data = "bob data";
            var bob2Obj = new BobData2();
            bob2Obj.Data = "bob 2 data";

            //using the bobVisitor Class
            var bobVisitor = new BobVisitor();
            bobVisitor.dosomething(bobObj);
            bobVisitor.dosomething(bob2Obj);
            bobVisitor.dosomethingOnlyToBob(bobObj);
            bobVisitor.dosomethingOnlyToBob2(bob2Obj);

            //using the extension methods in the extension class
            bobObj.visit();
            fredObj.visit();
            Console.Read();
        }
    }
}