.NET : How to validate XML file with DTD without DOCTYPE declaration

I have an XML file with no DOCTYPE declaration that I would like to validate with an external DTD upon reading.

Dim x_set As Xml.XmlReaderSettings = New Xml.XmlReaderSettings()
x_set.XmlResolver = Nothing
x_set.CheckCharacters = False
x_set.ProhibitDtd = False
x = XmlTextReader.Create(sChemin, x_set)

How do you set the path for that external DTD? How do you validate?