I wrote a simple extension method for UrlHelper:
public static class ExtensionMethods
{
    private const string ImagesFolder = "~/Images";
    public static string Images(this UrlHelper url)
    {
        return url.Content(ImagesFolder);
    }
}
The above code resides in /Helper/ExtensionMethods.cs. It works just fine but I need to add using MyNamespace.Helper; in every cshtml where I want to use the Url.Images(). I the old days we would add another line to web.config:
<system.web>
    <pages>
        <namespaces>
            <add namespace="MyNamespace.Helper"/>
        </namespaces>
    </pages>
</system.web>
But the above does not seem to be picked up by Razor. I tried adding my using statement to _ViewStart.cshtml, with the same result.
So, what's Razor's way of specifying a using across the entire site?
 
    