在C#中创建扩展方法

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc; //Correctly referencing the necessary namespaces,right?

namespace MvcApplication1.HelperMethods
{
    public static class NavigationalMenu
    {
        public static string MyMenu(this HtmlHelper helper)
        {
            CategoryRepository categoryRepo = new CategoryRepository();
            var categories = categoryRepo.FindAllCategories();

            foreach (Category c in categories)
            {
                helper.RouteLink(blablabla); //Construct links and return them.
            }

            //helper.RouteLink doesn't show up! C# wipeouuuuuttttt.
            //It's as if 'helper' doesn't have the RouteLink method there.
        }
    }
}

第一次在C#编程时发生这种情况.其他人遇到这个问题？