I tried to use the original Gaussian.cpp in Visual Studio 2019 to create a dll project. I created a default dll project and copied the Gaussian.cpp and tried to build the file. The header file is left empty. The build was successful but when I tried to load into Zemax, it gives error saying cannot load DLL as in the attachment.
I don't have a file Gaussian.cpp in my default installation of OpticStudio. Can you elaborate what DLL you are trying to build? Is that a source, or a surface? Can you also share the file you are trying to compile for troubleshooting?