Could not load type 'System.Web.WebPages.DisplayModes' from assembly 'System.Web.WebPages, Version=2.0.0.0

I got this annoying error recently on my ASP.NET MVC 4 Beta project and didn't find any real solutions/explanations on Google. So after investigating it here are my findings.

The basic problem is that there is a mixup between MVC 4 Developer Preview and MVC 4 Beta .dlls in the project. To see which version you have right click the .dll file in Windows Explorer and choose Properties->Details. Look for  'File Version' :

• MVC 4 Developer Preview is version 4.0.10906.0
• MVC 4 Beta is version 4.0.20126.16343

Make sure all says the MVC 4 beta version number (replace the .dlls from a fresh MVC 4 Beta project if necessary) for these files:

• System.Web.Mvc
• System.Web.WebPages
• System.Web.Razor
• System.Web.WebPages.Deployment
• System.Web.WebPages.Razor

It should then work.

(Technical: It is actually the older version of System.Web.Mvc.dll that tries to use a removed/changed item in the newer System.Dll.WebPages library. DisplayMode have been replaced with DisplayModeProvider)