In this article, Mahesh Sabnis designs a
WCF service which has the custom validation built and then creates a
Silverlight client application that will call this service.
Source: Dot Net Curry
Most of the times, when you develop an application that communicates to a service-oriented solution (like WCF services), it becomes necessary that the service must have some mechanism for authenticating the user, who is making a call to the service. If it is a Silverlight application that is making a call to such a service, then as a developer you should be aware of the technique to pass user name/password to the service and the service must have the capability to verify these credentials passed to it and authenticate the user.