Composing Parts Exported in the Application Class

Question

When I make class library projects (dlls) in a solution contain export classes they compose just fine.

When I try to access export classes that are simply classes within the application (compile into the .exe that is doing the composing)  I get rejection.

Do export classes have to be deployed externally to the main application to be composed?

Forgive my ignorance, I'm new at this.