MVC has let you add Areas to your web project for a couple versions now.  Areas help you group large chunks of code in your web app.  You can group along a technical axes (like "Api") or a functional axes (like "Admin").

The first error

I created a new MVC4 project, added an area called "Test", and browsed to the home page (http://localhost:59669).  Works!  Then I browse to the new area (http://localhost:59669/Test) and get this:

data/admin/2012/12/image_thumb_5.png

What's going on with those underscores?  The error message shows the problem is in my Web.config, but not the application root one, the one under /Areas/Test/Views/Web.config.  I open it in Visual Studio, and sure enough, underscores.

<configSections>
  <sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=__WebPagesVersion__.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
    <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=__WebPagesVersion__.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
    <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=__WebPagesVersion__.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
  </sectionGroup>
</configSections>

I compare that to the other Web.config under the root /Views/Web.config, and copy and paste those values in.

<configSections>
  <sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
    <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
    <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
  </sectionGroup>
</configSections>

At this point, I'm assuming there is a bug in the "Add Area" template.  So let's build and refresh the browser and it should work now, right?

The second error

Of course not.

data/admin/2012/12/image_thumb_6.png

Hmm.  Lots of details in that stack trace, but not much to go on here.  I checked my route registration code and checked my area registration code.  Both looked fine:

using System.Web.Mvc;
using System.Web.Routing;

namespace MvcApplication2.App_Start
{
    public class RouteConfig
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                name: "Default",
                url: "{controller}/{action}/{id}",
                defaults: new {controller = "Home", action = "Index", id = UrlParameter.Optional},
                namespaces: new[] { "MvcApplication2.Controllers" }
                );
        }
    }
}

using System.Web.Mvc;

namespace MvcApplication2.Areas.Test
{
    public class TestAreaRegistration : AreaRegistration
    {
        public override string AreaName
        {
            get { return "Test"; }
        }

        public override void RegisterArea(AreaRegistrationContext context)
        {
            context.MapRoute(
                name: "Test_default",
                url: "Test/{controller}/{action}/{id}",
                defaults: new {action = "Index", id = UrlParameter.Optional},
                namespaces: new[] {"MvcApplication2.Areas.Test.Controllers"}
                );
        }
    }
}

I also added the namespaces to the area (the last parameter above) to help the routing engine find the right controller.  That looked fine, too.

The error message was about a bad assembly ("The given assembly name or codebase was invalid.").  Maybe something didn't get built or loaded right ?  I tried a Clean and Rebuild.  No luck.

Maybe ASP.NET temp files were cached?  Let's clear browser cache, close and re-open the browser, close and re-open Visual Studio, restart IIS Express, and see if that helps….and no, no luck.

The solution

I was running out of ideas, but since the first problem was the Web.config, maybe this was, too.  I opened up the generated Web.config under /Areas/Test/Views/Web.config again and compared it to the one under the root /Views/Web.config.

Sure enough, more freaking underscores.  I just copied ALL the values from the root view's Web.config into the areas one, built, refreshed, and finally got the areas showing up.

Maybe I have a wacky install.  Maybe I got hold of some bad pre-release NuGet package.  But it seems there is a template bug here when adding Areas to an MVC project.  I have the Fall Update on Visual Studio 2012, so my About screen looks like this:

data/admin/2012/12/image_thumb_8.png

I'm writing this up in the hopes that I save someone else some time.  If you have suggestions on where I got an underscore happy template, please let me know in the comments below.