You can achieve reading without holding the whole object graph in memory by using
System.Xml.XmlReader
(using just this base class will be quite enough), and the usage can be easily figured out from the documentation:
https://msdn.microsoft.com/en-us/library/system.xml.xmlreader%28v=vs.110%29.aspx[
^].
If you cannot read and understand documentation to the degree which allows you to solve this simple problem, I'll regret to say that I cannot lend you my eyes.
If you have some particular problem, you probably need to ask some more special question, starting with informing us:
what have you tried so far?
—SA