You'd have to do it yourself, it isn't implemented by default.
See this page
Quote:Q: Why can't I serialize hashtables?
A: The XmlSerializer cannot process classes implementing the IDictionary interface. This was partly due to schedule constraints and partly due to the fact that a hashtable does not have a counterpart in the XSD type system. The only solution is to implement a custom hashtable that does not implement the IDictionary interface.