I have a WCF data service that works well with an Internet application I developed (using Forms authentication). Now I'm trying to create a WPF desktop application that uses this same data service and does its own custom authentication. The problem is my WCF data service always sees Membership.GetUser() as null. I've tried calling Membership.ValidateUser() within both the WPF application and the WCF data service with no luck. What am I missing, what am I doing wrong?