Instead of creating a separate application that hosted a WCF service used by a Silverlight 3.0 application within a asp.net website I just added the service to the parent web site. Over time the service would throw this error. It would seem that having a WCF service in a silverlight asp.net website that doesn’t use RIA has all sorts of compile problems. To work around this problem I removed the WCF service from the parent application and moved it to it’s WCF project and update the Silverlight application to make a call from a separate URL.