The App_Code folder is not explicitly marked as containing files written in any one programming language. Instead, the ASP.NET infers which compiler to invoke for the App_Code folder based on the files it contains. If the App_Code folder contains .vb files, ASP.NET uses the Visual Basic compiler; if it contains .cs files, ASP.NET uses the C# compiler, and so on.
If the App_Code folder contains only files where the programming language is ambiguous, such as a .wsdl file, ASP.NET uses the default compiler for Web applications, as established in the compilation element of the application Web.config file or the machine-level Web.config file. Compilers are named build providers and a build provider is specified for each file extension in an extension element.
See the documentation here.