GetAttributeValue method in SAMLAssertion object gets only the first child element and the document states. It also mentions to use other attribute related method, what are those?
In most uses cases a SAML attribute will have a single attribute value. The SAMLAssertion.GetAttributeValue is a convenience method that retrieves the single attribute value for the SAML attribute. If the SAML attribute may contain one or more attribute values then you should use the SAMLAssertion.GetAttributes method which returns a list of SAMLAttribute objects with the specified attribute name. The SAMLAttribute class includes a SAMLAttribute.Values property that returns the list of AttributeValue objects. The AttributeValue.Data property returns the actual SAML attribute value.
The SAMLAssertion.GetAttributes and associated methods give you maximum control over accessing the SAML attribute information but requires more code. The SAMLAssertion.GetAttributeValue method covers the typical use case of a single attribute value and is simpler to use.
If you use the high-level API rather than these low-level APIs, it’s even simpler.
http://www.componentspace.com/Forums/36/SAML-Highlevel-API-vs-Lowlevel-API