[XML]DTDファイルを用いての検証ができない。

    Private isValid As Boolean = True

    Private Sub Form1_Load _
        (ByVal sender As System.Object, ByVal e As System.EventArgs) _
        Handles MyBase.Load

        Dim xmlReader As New XmlTextReader("Product.xml")
        Dim xmlParser As New XmlValidatingReader(xmlReader)

        xmlParser.ValidationType = ValidationType.DTD
        AddHandler xmlParser.ValidationEventHandler, _
            AddressOf XmlParserEventHandler

        While xmlReader.Read()
            If xmlReader.NodeType = XmlNodeType.Text Then _
               Call MessageBox.Show(xmlReader.Value)
        End While

        Call xmlReader.Close()

        If isValid Then
            Call MessageBox.Show("XML is Valid !!")
        Else
            Call MessageBox.Show("XML is not Valid")
        End If

    End Sub

    Private Sub XmlParserEventHandler _
        (ByVal sender As Object, ByVal e As ValidationEventArgs)

        isValid = False
        Debug.WriteLine("Validation event" & ControlChars.NewLine & e.Message)

    End Sub

While xmlReader.Read()

While xmlParser.Read()