.net – Can a .net project discover and use a WCF service

You can configure a WCF endpoint to use 2-way SSL authentication. That means that you can require clients to present an X.509 certificate that confirms their identity whenever they make a request to the service.

On the server side of things, you can use one of the built-in validation schemes in WCF or provide your own validation logic to check the X.509 certificate.
If you were hosting your service in IIS, it would be trivial to configure SSL to require client certificates at the transport-level. However, you can find a good guide on how to implement this behaviour in a self-hosted WCF service here:

http://leastprivilege.com/2007/08/25/certificate-based-authentication-and-wcf-message-security/

I haven't tried this myself but, since this creates a security requirement at the message-level, I think you will have to use wsHttpBinding to enforce it in your WSDL contract, since imposing security requirements to access a web service is part of the WS-* standards.

If you have to use basicHttpBinding, you can try this solution instead that moves things up at the transport-level:

http://leastprivilege.com/2007/08/26/certificate-based-authentication-and-wcf-mode-independent/

Hope this helps

"This error can arise if you are calling the service in a class library and calling the class library from another project."

In this case you will need to include the WS configuration settings into the main projects app.config if its a winapp or web.config if its a web app. This is the way to go even with PRISM and WPF/Silverlight.