I have an app in VS 2005 that contains both VB and C# code in the same website project. At compile time I kept getting this error:
"The files '/WebSite/App_Code/VB/test.vb' and '/WebSite/App_Code/CS/test.cs' use a different language, which is not allowed since they need to be compiled together."
To go around this issue, simply add the following to your web.config file under the compilation tag:
In this case, all the VB files and C# files were already separated into different folders inside the App_Code Folder.
Hope this helps.