What is App_Code ?
The \App_Code folder is meant to store your classes, .wsdl files, and typed datasets. Any of these items stored in this folder are then automatically available to all the pages within your solution.
Everything placed in the \App_Code folder is compiled into a single assembly. The class files placed within the \App_Code folder are not required to use a specific language. This means that even if all the pages of the solution are written in Visual Basic 2008, the class in the \App_Code folder of the solution can be built in C#.
Here is a small catch...
Because all the classes contained in this folder are built into a single assembly, you cannot have classes of different languages sitting in the root \App_Code folder, as in the following example:
Having two classes made up of different languages in the \App_Code folder causes an
error to be thrown. It is impossible for the assigned compiler to work with two different languages.
Therefore, in order to be able to work with multiple languages in your \App_Code folder, you must make some changes to the folder structure and to the web.config file.
The first step is to add two new subfolders to the \App_Code folder
— a \VB folder and a \CS folder.
This gives you the following folder structure:
This still will not correctly compile these class files into separate assemblies, at least not until you make some additions to the web.config file.
In the web.config file , change the <compilation>
node so that it is structured as following:
Now that this is in place in your web.config file, you can work with each of the classes in your ASP.NET pages. In addition, any C# class placed in the CS folder is now automatically compiled just like any of the classes placed in the VB folder.
NOTE: The name of sub directories are'VB' and 'CS' for this example. You can put any name you wish to.