DLL - Using main project classes in library class

I've never tried separating down my projects into DLLs but something I'm working on at the moment calls for a more modular approach. What I want to be able to do is switch out versions of a number of my classes easily 开发者_如何学Goto push updates to users without them having to re install my entire application.

These classes create instances of other classes which are defined within the application and shouldn't be part of the modules which are switched in and out - they're central. Obviously in the class Library project these classes are undefined as they are only defined in the main application. How do I go about enabling the class library to use these main application classes?

All help is much appreciated.