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

Question

I am using VS2013, MVC 5.2.2.0, Web Api 2. I have just changed the all versions from 2.0.0.0 to 3.0.0.0 of the following section of Web.config resides inside the View folder of my project.

<configSections>
<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>

And the problem gets solved. BINGO !!

Just adding my two cents to this issue.

I found that the cause of this error for me was that the Views folders web.config was referencing System.Web.WebPages.Razor, Version=2.0.0.0 when everything else was using 3.0.0.0

Seems the nuget-package upgrade didn't account for this folder somehow.

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:

<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>

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

Tried all the methods above, and the issue was still not solved until I inserted the following lines in web.config directly under project folder.

  <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>

If you get this with the Route Debugger then you need to update the web.config in the Views folder under the Area for the Route Debugger.