c# - [A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to... web.config issue

I am getting the following error:

[A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast
to [B]System.Web.WebPages.Razor.Configuration.HostSection. Type A
originates from 'System.Web.WebPages.Razor, Version=2.0.0.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context

'Default' at location
'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'.
Type B originates from 'System.Web.WebPages.Razor, Version=3.0.0.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context
'Default' at location 'C:\Users\MyName\AppData\Local\Temp\Temporary
ASP.NET
Files\root\63e7ff36\a1cb775d\assembly\dl3\8f568c18\9b7ddacf_d04dcf01\System.Web.WebPages.Razor.dll'.

I have searched across stackoverflow for similar issues and I used their solutions but it doesn't seem to fix the issue I am experiencing.

It suddenly occurred, I just pressed clean solution and this error popped up. I also started using nuget package restore on the solution. I updated my packages as well (using Update-package) so one of those might have something to do with this issue.

Answer

The cause of this error is the web.config in the Views folder referencing System.Web.WebPages.Razor, Version=2.0.0.0 instead of 3.0.0.0.

Typically this can happen after a nuget-package upgrade, which does not account for this folder.

Update the Views\Web.config file:

for more information on upgrading to MVC 5 http://www.asp.net/mvc/tutorials/mvc-5/how-to-upgrade-an-aspnet-mvc-4-and-web-api-project-to-aspnet-mvc-5-and-web-api-2