a System.Security.Cryptography.X509Certificates.X509CertificateCollection
This property allows a client to pass one or more client certificates, also known as Authenticode X.509 v.3 certificates, when calling an XML Web service method. If the XML Web service method has been configured to use client certificates, a client certificate can be used as one mechanism for authenticating a client. For details on setting up client certificates, see the Internet Information Services (IIS) documentation.