Need to check that XML file has the right structure

I am writing a small application that generates unique codes for me. As I do not whish to have duplicates I want to make use of an xml file to store the generated codes for checking against any newly created codes.

I want to be able to assign the xml file that is used for the checking and as such have to validate that the file contains the correct xml.

I have created an XSD file and tried to validate it that way, but it does not seem to work. I intentionally use an incorrect xml file yet it validates.

This is the xsd file:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="XSDSchema1" targetNamespace="http://tempuri.org/XSDSchema1.xsd" elementFormDefault="qualified" xmlns="http://tempuri.org/XSDSchema1.xsd" xmlns:mstns="http://tempuri.org/XSDSchema1.xsd" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="machineKeys">
    <xs:sequence>
      <xs:element name="machinekey" minOccurs="0">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="validationKey" minOccurs="1" maxOccurs="1">
              <xs:simpleType>
                <xs:restriction base="xs:string">
                  <xs:pattern value="^[A-Za-z0-9]{128}$" />
                </xs:restriction>
              </xs:simpleType>
            </xs:element>
            <xs:element name="decryptionKey">
              <xs:simpleType>
                <xs:restriction base="xs:string">
                  <xs:pattern value="^[A-Za-z0-9]{48}$" />
                </xs:restriction>
              </xs:simpleType>
            </xs:element>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

Here is the code that is supposed to validate the xml file:

If ofd.ShowDialog = Windows.Forms.DialogResult.OK Then
      Try
        Dim rdr = New XmlTextReader(ofd.FileName)
        Dim sr As StreamReader = New StreamReader(AppDomain.CurrentDomain.BaseDirectory & "MachineKeys.xsd")
        Dim sch As XmlSchema = New XmlSchema

        sch = XmlSchema.Read(sr, New ValidationEventHandler(AddressOf ValidationEventHandler))
        Dim rdrSettings As XmlReaderSettings = New XmlReaderSettings
        rdrSettings.ValidationType = ValidationType.Schema
        rdrSettings.Schemas.Add(sch)

        AddHandler rdrSettings.ValidationEventHandler, New ValidationEventHandler(AddressOf ValidationEventHandler)

        Dim objXmlReader As XmlReader = XmlReader.Create(rdr, rdrSettings)

        While objXmlReader.Read
        End While
      Catch ex As Exception
        MsgBox("Error: " & ex.Message, MsgBoxStyle.Critical, "Machine Key Generator")
      End Try
End If

Private Sub ValidationEventHandler(ByVal sender As Object, ByVal e As Xml.Schema.ValidationEventArgs)
    Select Case e.Severity
      Case Xml.Schema.XmlSeverityType.Error
        _xmlValidationOutcome &= "Error: " & e.Message & vbNewLine
      Case Xml.Schema.XmlSeverityType.Warning
        _xmlValidationOutcome &= "Warning {0}" & e.Message & vbNewLine
    End Select
End Sub

PS: The XSD file is created if not found.