Open-source Xml Validators

Hello all

I'm looking for an open-source xml validator that fully supports xml schemas (including key and keyref constraints) and is easy to extend. I'm especially interested in validators written in C#. So far the only open-source validator I've encountered is Xerces. I'm also interested in any resource that explains how validation is implemented in Xerces-J (the java version of Xerces) - not just javadoc style documentation, but explanations regarding which classes are involved and how it is done. Also - does Xerces support in-memory validation? That is, can I load a document from a file, change it using dom and validate the new document (without saving to a file)? Or can I only validate a document by parsing it?

Sharon