Razor 2 to Razor 3 MVC 5

I've been working on an MVC 4 solution, and I've been trying to upgrade it to MVC 5. I've followed the steps outlined here.

I've followed it, and now whenever I run the MVC Application, it gives me this error message:

[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\User\AppData\Local\Temp\Temporary ASP.NET 
Files\root\665ac028\de53a189\assembly\dl3\c2c0a4b5\56e8099e_40e0ce01\System.Web.WebPages.Razor.dll'.

Does anyone know how this could have originated? or how it can be solved? I've looked around thus far? I've tried changing the web.config files, with no avail...

In your Web.config(-s) make sure assemblyBinding contains the proper version for the assembly System.Web.WebPages.Razor and System.Web.Mvc.

<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
  <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages.Razor" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0"/>
  </dependentAssembly>

  <dependentAssembly>
    <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
    <bindingRedirect oldVersion="1.0.0.0-5.0.0.0" newVersion="5.0.0.0" />
  </dependentAssembly>
</assemblyBinding>

And make sure that razor sectionGroup in ConfigSections reference latest versions as well:

<sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
    <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" />
    <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" />
</sectionGroup>

Check for version in web.config. If it again gives an error, try to clean the solution and rebuild it. Also check for project's Bin folder, removes old references from bin folder and rebuild the project solution.

Looks like you still have a reference to Razor 2.0 dll.

Check your project references' properties - especially 'Use specific version' one.