I'm new to WCF. I'm using it for the first time to package a Business Logic Layer, making it usable over the network. In my business dll (class library) there are several interfaces and derived classes. I read about KnownTypeAttribute, and I understand how it works when you declare the data contract in the same project of the WCF service, but I haven't quite grasped how to specify a data contract for an already existing type which is packaged in a referenced library. Any help would be appreciated.