扩展方法 -- 可以基于接口

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApp
{
    public partial class ExFunction : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            AA aa = new AA() { Code="zzy" };
            this.Label1.Text=aa.GetName();
        }
    }
    public interface IFun
    {
        string Code {get;set; }
    }

    public class AA : IFun
    {
        public string Code { get; set; }
    }
    public class BB : IFun
    {
        public string Code { get; set; }
    }

    public static class CC
    {
        public static  string GetName(this IFun fun)
        {
            return fun.Code + "XXX";
        }
    }
}