How to verify a xml file with another Schema

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I have a xml file, it have a DTD declaration like below

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE HealthData SYSTEM "Health.dtd">
....

Now I want to verify the xml file with another Schema file when use
XmlValidatingReader.

But when I verify schema file, it will always report an error about that
there is a DTD declaration.

How can I catch these exception and ignore it?

or how can I delete the dtd delcaration before vailidate it?