VB.Net - Validating XML using DTD - asp.net

Question

I've got myself a bit of a problem that I can't seem to figure out. I'm trying to programatically validate a generated XML file using a DTD using the following instructions from Microsoft (Validation Using a DTD with XmlReader
Here's my code thus far:
Private Sub BTN_ValidateXMLdoc_Click(sender As Object, e As EventArgs) Handles BTN_ValidateXMLdoc.Click
Dim JobNumber As String = Me.HNF_JobName.Value
Dim XMLPath As String = "N:\" & JobNumber & "\" & JobNumber & ".xml"
' Set the validation settings.
Dim settings As XmlReaderSettings = New XmlReaderSettings()
settings.DtdProcessing = DtdProcessing.Parse
settings.ValidationType = ValidationType.DTD
settings.ValidationFlags = XmlSchemaValidationFlags.ReportValidationWarnings
settings.ValidationFlags = XmlSchemaValidationFlags.ProcessInlineSchema
AddHandler settings.ValidationEventHandler, AddressOf ValidationCallBack
' Create the XmlReader object.
Dim reader As XmlReader = XmlReader.Create(XMLPath, settings)
' Parse the file.
While reader.Read()
End While
End Sub
' Display any validation errors.
Private Shared Sub ValidationCallBack(sender As Object, e As ValidationEventArgs)
MsgBox(String.Format("Validation Error: {0}", e.Message), MsgBoxStyle.OkOnly)
End Sub
Now.... Here in lies my problem. For some reason, the ValidationCallBack EventHandler never gets fired. If an error occurs during the reader.Read() executing it throws an exception error.
Correct me if I'm wrong, but shouldn't the ValidationCallBack sub be called?
There are errors in the XML file but instead of it being handled by the event handler it's just throwing an exception....
I'm open to any ideas as to what may be going on here?

Answer 1

Your code implies the DTD is embedded in the XML document itself. While that's possible, it's unlikely to be what you want. I suspect your test is failing because there is no embedded DTD in the actual XML document.
If the DTD is indeed external to the XML document, you need to tell your XmlReaderSettings where to find it.
Remove the line
settings.ValidationFlags = XmlSchemaValidationFlags.ProcessInlineSchema from your code.
Make sure there's a DOCTYPE in your XML document to specify the DTD. If there isn't, you may need to insert it before validation.
If you want to escape the overhead of downloading the DTD each time, you should create an XmlResolver. See Problem validation a XML file with a local DTD file in C# for an example.
Consider converting your DTD to an XML Schema. Schemas are more natural in XmlReader. DTDs don't understand XML namespaces.