What's wrong with my HtmlHelper?

I've created an Html extension method in Helper class, but I can not get it to work. I've implemented it as seen on different tutorials.

My MenuItemHelper static class:

public static string MenuItem(this HtmlHelper helper, string linkText, string actionName, string controllerName)
    {
        var currentControllerName = (string)helper.ViewContext.RouteData.Va开发者_如何学编程lues["controller"];
        var currentActionName = (string)helper.ViewContext.RouteData.Values["action"];

        var sb = new StringBuilder();

        if (currentControllerName.Equals(controllerName, StringComparison.CurrentCultureIgnoreCase) && currentActionName.Equals(actionName, StringComparison.CurrentCultureIgnoreCase))
            sb.Append("<li class=\"selected\">");
        else
            sb.Append("<li>");

        sb.Append(helper.ActionLink(linkText, actionName, controllerName));
        sb.Append("</li>");
        return sb.ToString();
    }

import namespace

<%@ Import Namespace="MYAPP.Web.App.Helpers" %>

Implementation on my master.page

<%= Html.MenuItem("TEST LINK", "About", "Site") %> 

The error message I get:

Method not found: 'System.String System.Web.Mvc.Html.LinkExtensions.ActionLink(System.Web.Mvc.HtmlHelper, System.String, System.String, System.String)

EDIT: Seem like the problem is the application name. the folder is called MYAPP-MVC.Web, but in the classes it translates to MYAPP_MVC.Web

I just tried it on a fresh application and it works

Try rewriting your helper in a more ASP.NET MVCish 2.0 style. Also don't forget to add using System.Web.Mvc.Html inside your helper namespace so that you have access to the ActionLink method:

namespace MYAPP.Web.App.Helpers
{
    using System.Web.Mvc;
    using System.Web.Mvc.Html;

    public static class HtmlExtensions
    {
        public static MvcHtmlString MenuItem(this HtmlHelper helper, string linkText, string actionName, string controllerName)
        {
            var currentControllerName = (string)helper.ViewContext.RouteData.Values["controller"];
            var currentActionName = (string)helper.ViewContext.RouteData.Values["action"];

            var li = new TagBuilder("li");
            if (string.Equals(currentControllerName, controllerName, StringComparison.CurrentCultureIgnoreCase) &&
                string.Equals(currentActionName, actionName, StringComparison.CurrentCultureIgnoreCase))
            {
                li.AddCssClass("selected");
            }

            li.InnerHtml = helper.ActionLink(linkText, actionName, controllerName).ToHtmlString();
            return MvcHtmlString.Create(li.ToString());
        }
    }
}

If this doesn't work you are definitely having some version problems with the System.Web.Mvc assembly being used.

You need to make the extension method available to the page/viewpage. You can do this for the individual page by adding a directive to the top of it

<%@ Import namespace="Namespace.Where.Your.HtmlHelper.Extension.Is.Defined" %>

or you can make it available to all pages by adding it to the

<pages> 
    <namespaces>
        <add namespace="Namespace.Where.Your.HtmlHelper.Extension.Is.Defined" />
    </namespaces>
</pages>

section in web.config.

EDIT:

The problem might be to do with the fact that HtmlHelper.ActionLink() returns an MvcHtmlString and not a string. I think you should call ToString() on it when calling append with the StringBuilder. As has been pointed out, ideally you ought to be returning MvcHtmlString so that others can use your extension method with the <%: ... %> syntax and not have the output encoded again.