Using a common class from another EXE/DLL

I want to create a "black box" of sorts with a class I've written. There is no visible UI to it; it's just connection/validation stuff. I want to use it across multiple projects. Do you suggest it be a DLL or EXE? I do not want this code reverse-engineered so I don't want to distribute the VCX nor make an .APP (as you can view code in the .APP). I want to be able to instantiate an object of this class and use it to validate some things like login and license validation.

tia, --Mike