You must use the classes available in System.Xmlor System.Xml.Linqfor XML parsing.
XDocument LINQ XML , . , XmlDocument ( .NET 3.5).
XDocument StringBuilder :
var doc = XDocument.Parse( stringBuilder.ToString() );
FirstNode, Descendents() , XML. XDocument LINQ, , :
var someData = from node in doc.Descendants ("yourNodeType")
select node.Value;