You likely need to include a reference to System.IdentityModel when denoting a CertificateValidationMode when using a WCF web service.

Here I don't seem to mention it. But this...

x.ClientCredentials.ServiceCertificate.Authentication.CertificateValidationMode =
      System.ServiceModel.Security.X509CertificateValidationMode.None;

 
 

...strangely requires the System.IdentityModel namespace.