I have a N tier application with a UI, BI, and Data layer.
The BI layer performs many operations, say returning a List(of DBLayer.SomeObject)
However the UI layer does not reference the BI layer, so how can i return it to the UI layer in a format that the UI can handle?
Do i have to re-define the objects into the BI layer?
Often entity objects are put into a component shared by presentation layer, business layer, and data access layer.
The way you structure your layers and the entities depends on the size and complexity of your application. A valid approach tacking domain-driven design (DDD) into account is the separation of application and domain logic into separate layers. The entities actually build up a domain model not only containing data structures but also domain logic and therefore being part of the domain layer.
Obviously the domain model is used by the application layer, the entities can also be used by the UI. It’s easiest if also data access layer knows the domain model and handles the entity directly. Alternatively the DAL only provides DTOs which are mapped to and from domain objects.