How to Resolve Casting Errors During ASP.NET MVC 4 to 5 Upgrade?

Unexpected Error During MVC 4 to 5 Upgrade

When upgrading an ASP.NET MVC project from version 4 to 5, developers may encounter a perplexing error related to a casting issue between two versions of the 'System.Web.WebPages.Razor.Configuration.HostSection' class. This issue can be attributed to lingering references to an older version of the assembly.

Identifying the Root Cause

Visual Studio, while generally efficient in handling dependency upgrades, sometimes overlooks certain project configurations. To resolve this error, it's crucial to manually review and modify the .csproj file. Typically, one or more project files continue to reference the obsolete version of 'System.Web.WebPages.Razor' (version 1.0.0.0).

Resolving the Issue

To rectify the problem, follow these steps for all projects involved in the solution:

1. Unload the project in Solution Explorer.
2. Edit the project's .csproj file.
3. Search for any references to 'System.Web.WebPages.Razor' and update the version to 3.0.0.0. Ensure the 'HintPath' property points to the correct location of the newer assembly.
4. Repeat steps 1-3 for any dependent projects or DLLs outside the solution.

Example of 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 Checks

Inspect the 'web.config' and '/Views/web.config' files to ensure they don't contain references to the old assembly. In rare cases, third-party libraries may be responsible for maintaining outdated dependencies. Consider obtaining updated versions of these libraries if necessary.

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