Private Sub SignatureDefinitionCommandHandler(sender As Object, e As RoutedEventArgs)
    Using sigDefDialog As New SignatureDefinition()
        If sigDefDialog.ShowDialog() = System.Windows.Forms.DialogResult.OK Then
            Dim signatureDefinition As New XpsSignatureDefinition With {
                .RequestedSigner = sigDefDialog.RequestedSigner.Text,
                .Intent = sigDefDialog.Intent.Text,
                .SigningLocale = sigDefDialog.SigningLocale.Text
            }
            Try
                signatureDefinition.SignBy = Date.Parse(sigDefDialog.SignBy.Text)
            Catch e1 As FormatException
            End Try
            signatureDefinition.SpotId = Guid.NewGuid()
            Dim docSeq As IXpsFixedDocumentSequenceReader = _xpsDocument.FixedDocumentSequenceReader '_xpsDocument is type System.Windows.Xps.Packaging.XpsDocument
            Dim doc As IXpsFixedDocumentReader = docSeq.FixedDocuments(0)
            doc.AddSignatureDefinition(signatureDefinition)
            doc.CommitSignatureDefinition()
            InitializeSignatureDisplay()
        End If
    End Using
End Sub

ShowStatus("   Getting FixedDocumentSequenceReader.")
Dim fixedDocSeqReader As IXpsFixedDocumentSequenceReader = xpsDocument.FixedDocumentSequenceReader

ShowStatus("   Getting FixedDocumentReaders.")
Dim fixedDocuments As ICollection(Of IXpsFixedDocumentReader) = fixedDocSeqReader.FixedDocuments

ShowStatus("   Getting FixedPageReaders.")
Dim enumerator As IEnumerator(Of IXpsFixedDocumentReader) = fixedDocuments.GetEnumerator()
enumerator.MoveNext()
Dim fixedPages As ICollection(Of IXpsFixedPageReader) = enumerator.Current.FixedPages


' Add a document structure to each fixed page.
Dim i As Integer = 0
For Each fixedPageReader As IXpsFixedPageReader In fixedPages
    Dim pageStructure As XpsResource
    ShowStatus("   Adding page structure resource:" & vbLf & "       '" & Filename(_fixedPageStructures(i)) & "'")
    Try
        pageStructure = fixedPageReader.AddStoryFragment()
    Catch e2 As InvalidOperationException
        System.Windows.MessageBox.Show(xpsUnstructuredFile & vbLf & vbLf & "Document structure cannot be added." & vbLf & vbLf & Filename(xpsUnstructuredFile) & " might already " & "contain an existing document structure.",
                        "Cannot Add Document Structure",
                        MessageBoxButton.OK,
                        MessageBoxImage.Error)
        Exit For
    End Try

    ' Copy the page structure to the new StoryFragment.
    WriteResource(pageStructure, _fixedPageStructures(i))
    i += 1
Next fixedPageReader

ShowStatus("   Saving and closing the new document." & vbLf)
xpsDocument.Close()