In order to be serialized to XML, Uri class should have a parameterless constructor, which it doesn't: Uri is designed to be immutable. Honestly, I can't see why it cannot be serialized without having a parameterless constructor.
To circumvent this, either change URI property type to string, or add one more property called _URI, mark URI with XmlIgnoreAttribute and rewrite it's get method as get { return new Uri(_URI); }.
