[asp.net-mvc-3] Razor HtmlHelper Extensions (or other namespaces for views) Not Found

I don't know if this was happening in the PR or Beta, but if I create an extension method on HtmlHelper, it is not recognized in a Razor powered page:

namespace SomeNamespace.Extensions {
    public static class HtmlExtensions {
        public static string Foo(this HtmlHelper html) {
            return "Foo";
        }
    }
}

I added it to the <Namespaces> section in Web.config:

<pages>
  <namespaces>
    <add namespace="System.Web.Mvc" />
    <!-- snip -->
    <add namespace="SomeNamespace.Extensions"/>
  </namespaces>
</pages>

But it throws a compile error when trying to view the page:

@Html.Foo()

If I recreate the page with WebForms it works fine. What's the deal?

Workaround

If I include @using SomeNamespace.Extensions in my Razor view, then it works, but I'd much rather just have it in Web.config

In my case use VS 2013, and It's not support MVC 3 natively (even of you change ./Views/web.config): https://stackoverflow.com/a/28155567/1536197

As the accepted answer suggests you can add "using" to all views by adding to section of config file.

But for a single view you could just use

@using SomeNamespace.Extensions

I had this issue in VS 2015. The following solved it for me:

Find "webpages:Version" in the appsettings and update it to version 3.0.0.0. My web.config had

<add key="webpages:Version" value="2.0.0.0" />

and I updated it to

<add key="webpages:Version" value="3.0.0.0" />

I had this same error in an MVC 4 application using Razor. In an attempt to clean up the web.config files, I removed the two webpages: configuration values:

<appSettings>
  <add key="webpages:Version" value="2.0.0.0" />
  <add key="webpages:Enabled" value="false" />

Once I restored these configuration values, the pages would compile correctly and the errors regarding the .Partial() extension method disappeared.

Since ASP.NET MVC 3 RTM is out there is no need for config section for Razor. And these sections can be safely removed.

I found that putting this section in my web.config for each view folder solved it.

<runtime>
        <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
            <dependentAssembly>
                <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
                <bindingRedirect oldVersion="1.0.0.0-2.0.0.0" newVersion="4.0.0.0" />
            </dependentAssembly>
        </assemblyBinding>
    </runtime>

This error tells you that you do not have the razor engine properly associated with your project.

Solution: In the Solution Explorer window right click on your web project and select "Manage Nuget Packages..." then install "Microsoft ASP.NET Razor". This will make sure that the properly package is installed and it will add the necessary entries into your web.config file.