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

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.

In my case, I've solved this by also updating the Web.config on the live server which the VS has updated upon upgrading to MVC 5