>Use Singleton.
>
> Best regards

How does a singleton help with this? I use singletons when I have a class that is needed multiple places in my application but I can only have one instance of that class instantiated. I do work with RFID readers, Cameras, scales, and GPS devices that are connected to a Com port and the class can only be connected once but I need to access the instantiated class in several different places in the application.

Project common functionality like Viv suggested could go in a seperate project such as MyProject.Common where all the common stuff goes and can be referenced by any of the projects without concern of cyclic references.
Tim