Why Does the UnknownNode Event keep Firing?

[Charles Law]

I have an xml file that I created by serializing a collection that I have
created. It contains attributes and elements that correspond to my classes.

I want to use the UnknownNode, UnknownAttribute and UnknownElement events
when I deserialize the file to tell if the file has been changed to contain
unrecognised data.

When I deserialize the file, the UnknownNode and UnknownAttribute events
fire for just about every node and attribute where e.Name is "xsi:type" and
e.Text is the name of one of my classes.

Firstly, I don't know why the file contains xsi:type all over the place, and
secondly, how can I stop these events from firing for nodes and attributes
that really are recognised?

I should add that the file actually deserializes correctly, but I always get
these events raised.

TIA

Charles