If you’re working with the ASP.NET MVC 3 MvcMusicStore demo and you run into the error message:

The name ‘ViewBag’ does not exist in the current context

It’s probably a sign that you are running on an older version of ASP.NET MVC (or a pre-release of MVC 3). You can download the latest version of ASP.NET MVC 3 here. This will install the Web Platform Installer (version 3) and will install ASP.NET MVC 3:

Once MVC 3 (released version) is installed, recompile the MVC Music Store demo and your “ViewBag does not exist in current context” errors should disappear.

Update: You may still see this error if you have an application that you built using the pre-release versions of MVC 3. In that case, the fix that worked for me is to update the web.config in the /Views folder to use the one from the RTM version of MVC 3 (you can find one such as part of themvc music store source code, here).

Update 2: There’s also an upgrade tool that, if used, should prevent all of the above problems:

http://blogs.msdn.com/b/marcinon/archive/2011/01/13/mvc-3-project-upgrade-tool.aspx