Yesterday I was trying to upgrade an ASP.NET MVC 2 web application to ASP.NET MVC 3 RC2. I also wanted to use the new Razor view engine that allows a more compact and less obtrusive way of writing ASP.NET MVC view templates.

There already is an excellent guide of how to upgrade your MVC 2 application to MVC 3 RC2. If you are running your MVC 2 application in Windows Azure, you must take some additional steps, which are described here. Visual Studio 2010 doesn’t (yet) allow you to specify an ASP.NET MVC 3 Web Role, so when running in Azure, you always have to upgrade if you want to use MVC 3.

You should reference some extra DLL’s from your MVC project and make sure that for all these DLL’s you set Copy Local to True. This is necessary because on the virtual machine your application is deployed to these DLL’s are not in the GAC. The DLL’s you have to reference are:

• Microsoft.Web.Infrastructure
• System.Web.Helpers
• System.Web.Mvc
• System.Web.Razor
• System.Web.WebPages
• System.Web.WebPages.Razor
• WebMatrix.Data (I’m not sure why this one is necessary)

and these are located either somewhere below C:\Program Files (x86)\Microsoft ASP.NET for 64bits or below C:\Program Files\Microsoft ASP.NET for 32bits. So far so good.

Having taken all the required steps (or so I thought) I tried to run my first Razor template in the Windows Azure emulator:

No what I expected: The name ‘ViewBag’ does not exist in the current context. I must have forgotten something. After double-checking I had all the required dependencies that all had there Copy Local property set to True, I took a better look at the Compilation Source. My page appeared to extend from System.Web.WebPages.WebPage. This class doesn’t have a ViewBag property so that explains the compilation error.

Back to the upgrade instructions. It appeared I had forgotten one small step. In the Views sub directory there is an additional Web.config file. In an MVC 3 project, it contains a new configuration section: system.web.webPages.razor. In this section, a new page base type is specified: System.Web.Mvc.WebViewPage. After copying this Web.config file from another MVC 3 project, everything worked as expected.

A classic case of RTFM. But if you ever happen to see this compiler error when using the Razor view engine, check your Web.config file.