If you are taking advantage of the App_Code folder to develop an ASP.NET WebSite some times you need to use code files that are written in different .NET languages. For example, you may want to use C# files and VB.NET files in the same web site:

image

To be able to do this you can use a feature of the compilation section in web.config to configure sub folders inside App_Code to be compiled separately.

<compilation debug="true">
    <codeSubDirectories>
        <add directoryName="VBFiles" />
    </codeSubDirectories>
</compilation>

Federico Silva Armas 
SDET, ASP.NET QA Team