Error Loading MVC 3 Project after MVC 4 is Installed or in VS 2011

Since installing the ASP.net MVC 4 Beta, it has happened to me a few times that MVC 3 websites will not load when the solution is loaded into VS 2010 (I have also read about this happening with VS 2011).

You can fix this by doing the following:

  1. In the Solution Explorer, right click on the project that wont load and click on “Edit [ProjectName].csproj”. This will open up the project definition file in VS. You can also edit this manually using your favorite text editor, opening the file through Windows Exporer.
  2. Find the line in the file that starts with ProjectTypeGuids and remove the entry “{E53F8FEA-EAE0-44A6-8774-FFD645390401}” from the list (this is code that tells Visual Studio that it is an MVC 3 project – for some reason, including this in the project file after installing MVC 4 messes things up).
  3. Save the .csproj. file and [Reload] the project through the Right Click menu in Solution Explorer.

The project should now load properly (if the MVC project has been the startup project for the solution, you may have to reset this as well).