Question

I’m trying to add an extension method to my MVC 2 project without success and after several hours of googling and looking here I’m at a loss. I’ve created a brand new MVC 2 project to make sure there was not anything weird about my existing project and I’m still facing the same problem. I’m sure this is a situation of I “can’t see the forest for the trees” so any help would be greatly appreciated. Here is the code for the extension method.

using System.Web.Mvc;

namespace ExtensionTest.Helper
{
    public static class UrlExtensions
    {
        public static string Image(this UrlHelper helper, string fileName)
        {
            return helper.Content("~/Content/Images/" + fileName);
        }

    }
}

and here is the code in the view (standard home index view created by default for a new MVC 2 project)

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage" %>
<%@ Import Namespace="ExtensionTest.Helper" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Home Page
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <%= UrlHelper.Image("test") %>
    <h2><%: ViewData["Message"] %></h2>
    <p>
        To learn more about ASP.NET MVC visit <a href="http://asp.net/mvc" title="ASP.NET MVC Website">http://asp.net/mvc</a>.
    </p>
</asp:Content>

In design mode, when I type UrlHelper, intellisense does not show my extension method Image and if I run the project I get the following error:

CS0117: 'System.Web.Mvc.UrlHelper' does not contain a definition for 'Image'

At first I thought it was as simple as not adding a reference (Import statement), but that does not appear to be the case. The really weird thing to me is that I can add extension methods to the HtmlHelper object without issue in this same project.

Thanks in advance for any help provided.

Answer 1

Extension methods in .NET should be invoked on an object instance and not on the class itself (even though they are static).

So instead of:

<%= UrlHelper.Image("test") %>

try:

<%= Url.Image("test") %>