Access permitions to my dlls

Hello, I’m not sure this is a security question, but it doesn’t fit in any other topic, so I would like to ask the following: How do I guarantee that a dll I implemented only is accessed by one I authorize? Imagine that I separate my application in 3 levels (GUI – Business – Data) and only want that my GUI classes can access to Business classes. Another question, is about versions. Imagine I have the GUI code in version 3 and Business in version 2. I build another version of each and want to force that version 4 only works with version 3. How do I do this? Thank you, in advance, for your time.