Why Am I Getting a Casting Error When Upgrading My ASP.NET MVC Application from Version 4 to 5?

Weird Error Upgrading ASP.NET MVC from 4 to 5

When upgrading ASP.NET MVC from version 4 to version 5, it's possible to encounter an error that appears to result from a casting operation between two different versions of the System.Web.WebPages.Razor.Configuration.HostSection class. Understanding the cause and identifying a solution to this problem is crucial.

Cause of the Error

After thoroughly checking the project files and ensuring that all of them are updated to MVC 5, the issue persists. This indicates that the problem may not be within the Visual Studio solution.

As per the error message, the casting operation fails because:

• Type A: Originates from the older version of the assembly (System.Web.WebPages.Razor, Version=1.0.0.0), which is present in the Global Assembly Cache (GAC).
• Type B: Originates from the newer version of the assembly (System.Web.WebPages.Razor, Version=3.0.0.0) that has been referenced successfully in the project files.

How to Find the Origin of the Old DLL

Visual Studio may not always make accurate choices during dependency upgrades. Therefore, it's crucial to manually inspect the project files (.csproj) to verify the references. When upgrading from MVC 4 to MVC 5, it's possible that one or more project files still contains a reference to the older version of System.Web.WebPages.Razor.

Steps to Update the References

1. Unload the project by right-clicking on its node in Solution Explorer and selecting "Unload Project."
2. Right-click the project node again and choose "Edit .csproj."
3. Search for all references to System.Web.WebPages.Razor in the file.
4. Update the version and HintPath accordingly as shown below:

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>

These steps should be repeated for all dependent projects in the solution.

Preventing Future Occurrences

To prevent similar issues in the future, it's recommended to thoroughly review the project files and ensure that all references are updated properly when upgrading dependencies. Additionally, it's beneficial to check the web.config and /Views/web.config files to make sure they don't reference outdated versions of the assembly.

In the rare case that none of the above solutions resolve the issue, it's possible that the problem lies outside of the solution, potentially in a third-party library that might be referencing the old version of the file. In such cases, obtaining an updated version of the DLL is necessary.

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