Compiler error CS0433 - The type exists in both GAC and Temporary ASP.NET files

Question

I have a situation where I get a runtime error when running ASP.NET MVC4. I recently migrated to MVC4 from MVC3.

Upon starting the debugger, I instantly get faced with this error message:

The type 'System.Web.Mvc.WebViewPage' exists in both 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\promotionweb\20a5681f\9ce59023\assembly\dl3\29f671cf\37f0bcf2_4619d001\System.Web.Mvc.DLL' and 'c:\Windows\Microsoft.NET\assembly\GAC_MSIL\System.Web.Mvc\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Mvc.dll'

I'm running in IIS 7.5, with a solution setup of the MVC project itself, another C# library plus a console application project.

I've tried all steps mentioned in this answer, descended from a similar question. However it doesnt help at all in my case.

I had some doubts it had to do to different compilations models, so I've tried compiling for x86 and AnyCPU, cleaning all catalogs as mentioned above in-between. Nothing seems to help.

Any ideas what might cause this interference between the GAC and my Temporary ASP.NET files?

Answer 1

Below is quite a story, but I think it might benefit someone else.

First of all, I opened my project inside Visual Studio 2013, and got a complaint. I followed a link from inside VS2013 (cannot find it) to a Microsoft site where it explained how to manually upgrade an MVC3 to MVC4 project. The steps I followed required manual changes to web.config.

One of the changes I made was to modify the version number of the System.Web.Mvc reference. Previously it was 3.0.0.0, and according to the guide 4.0.0.0 was the number to change to. Another step was to go into the NuGet package manager, search for ASP.NET MVC4 and install it. This downloaded version 4.0.0.1 of System.Web.Mvc, as I think that was the correct version of the time the guide was written.

Once done, I compiled the project and got into the never-ending loop of the runtime error in the question that I posted. My project was compiled against version 4.0.0.1 (which seems to be the correct version number according to a windows update). The compilation was correct, but I had the following line in my web.config:

<add assembly="System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

The application was compiled against 4.0.0.1, but was running and searching for 4.0.0.0 and found it in these two places:

c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET 
c:\Windows\Microsoft.NET\assembly\GAC_MSIL

and thew the error. I still do think this part is strange.

My solution was to modify the web.config, changing the assembly reference version as mentioned, and the binding redirects for System.Web.Mvc.

Some other sources:

• System.Web.Mvc not functioning as expected after Windows Update
• System.Web.Mvc broken after security update

Answer 2

I once had this error. I noticed that it is important that the reference in the project and the add assembly-tag in web.config is the same. What I did was remove the reference to the assembly that was not the same as in web.config and then adding reference with the correct version. I assume you could also do it the other way around.