The MetadataExample project demonstrates how to retrieve X.509 certificates from SAML metadata.

The relevant code from the MetadataExample project is shown below.

        // Reads the X.509 certificates contained within an IdP or SP SSO descriptor
        private static void ReadX509Certificates(RoleDescriptorType roleDescriptor) {
            foreach (KeyDescriptor keyDescriptor in roleDescriptor.KeyDescriptors) {
                KeyInfo keyInfo = new KeyInfo();
                keyInfo.LoadXml(keyDescriptor.KeyInfo);

                IEnumerator enumerator = keyInfo.GetEnumerator(typeof(KeyInfoX509Data));

                while (enumerator.MoveNext()) {
                    KeyInfoX509Data keyInfoX509Data = (KeyInfoX509Data)enumerator.Current;

                    foreach (X509Certificate2 x509Certificate in keyInfoX509Data.Certificates) {
                        Console.WriteLine("X509 certificate: " + x509Certificate.ToString());
                    }
                }

                foreach (XmlElement xmlElement in keyDescriptor.EncryptionMethods) {
                    Console.WriteLine("Encryption method: " + KeyDescriptor.GetEncryptionMethodAlgorithm(xmlElement));
                }
            }
        }