How to Resolve the 'System.Web.WebPages.Razor.Configuration.HostSection Cannot be Cast' Error During ASP.NET MVC 4 to 5 Upgrade?

Error Encountered While Upgrading ASP.NET MVC from 4 to 5

When upgrading ASP.NET MVC projects from version 4 to 5, a perplexing error may surface. This error occurs when the updated project references an incompatible version of the System.Web.WebPages.Razor.dll assembly.

Error Description

The error message typically includes the following information:

System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to System.Web.WebPages.Razor.Configuration.HostSection

The error originates from a type mismatch between two versions of the assembly:

• Version 1.0.0.0 located in the GAC
• Version 3.0.0.0, which is the expected version for MVC 5

Troubleshooting Steps

To resolve this error, it is essential to manually review the project files (.csproj) and update the assembly references accordingly.

1. Unload and Edit Project File: Unload the project in Solution Explorer and manually edit its .csproj file.
2. Search for Assembly References: Search the file for references to "System.Web.WebPages.Razor" and verify that the version number and HintPath are correct.
3. Update References: Update the version number to 3.0.0.0 and ensure the HintPath points to an existing file.
4. Repeat for Dependent Projects: Repeat these steps for all dependent projects in the solution and any that reference external DLLs.

Example:

Old Reference:

<Reference Include="System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
    <Private>True</Private>        
    <HintPath>..\packages\Microsoft.AspNet.WebPages.1.0.20105.408\lib\net40\System.Web.WebPages.Razor.dll</HintPath>
</Reference>

Updated Reference:

<Reference Include="System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <Private>True</Private>
  <HintPath>..\packages\Microsoft.AspNet.WebPages.3.0.0\lib\net45\System.Web.WebPages.Razor.dll</HintPath>
</Reference>

Additional Considerations:

• Review web.config and web.config files to ensure they do not reference old versions of the assembly.
• Exclude third-party libraries that may be referencing the old version of the DLL.
• Refer to the provided [question](https://stackoverflow.com/questions/18976083/system-web-webpages-razor-configuration-hostsection-cannot-be-cast-to-system-web-) for further insights.

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