The source code in the App_Code folder is compiled into a single assembly, all the files in the App_Code folder must be in the same programming language. For example, the App_Code folder cannot include source code in both Visual Basic and C#.
However, you can configure your Web application to treat subfolders of the App_Code folder as separate compilable units. Each folder can then contain source code in a different programming language. The configuration is specified by creating a codeSubDirectories element in the compilation element of the Web.config file and adding a reference to the subfolder. The following example illustrates how you would configure subfolders named VBCode and CSCode to compile into separate assemblies:
compilation debug="false"> <codeSubDirectories> <add directoryName="VBCode" /> <add directoryName="CSCode" /> </codeSubDirectories> </compilation> The references to the VBCode and CSCode subfolders do not need to include any information about what programming language is contained in the subfolder. with the App_Code folder itself, ASP.NET infers the compiler to use based on the files in the subfolder.