How to Access VB class from c# class which both Classes in App_code folder

If you are trying to link the two in the manner that the unmanaged linker does, then you cannot do it using managed code. It just is downright impossible. In other words, unmanged programs such as C++ generate an object file for each input source code file and then the linker does a static link to merge all the object files into an executable. Managed programs do not work that way. That is the main way that software is built starting about half a century ago. It is a static link.

Dynamic linking is newer technology. There are many benefits to it. Unmanaged Windows programs can be created using DLLs. Most of the Windows API (I think all of it) is actually a DLL. The only way I know of to use Managed code developed using different languages is to create a "library" from one or both of them. A library is essentially a DLL or similar to a DLL. There is no equivalent to static linking of multiple languages for managed code.