>If I create a customer object with the ui, a customerbiz object and a customerdat layer that is self contained basically, does that violate this principle?

Idealy the UI instantiates the Businsses Object which uses the Data Layer.

There are different ideas as to how to do this. Some people have a Data Layer which is just a helper class that wraps ADO and the BizObj uses these methods. Some people have the ADO wrapper, AND they create a data object for each table. The data object uses the data layer helper class (these class combined are called the data layer and could consit of two dlls. Then the business object talks only to the data object.

The bottom line, your UI never talks to an object in the data layer, just in the business layer.

BOb