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' For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
Using doc As SlidesDocument = Document.Load(Of SlidesDocument)("D:\test.pptx")
    Dim watermark As New TextWatermark("Test watermark", New Font("Arial", 8))

    ' Add watermark to all master slides
    For Each slide As SlidesMasterSlide In doc.MasterSlides
        slide.AddWatermark(watermark)
    Next

    ' Add watermark to all layout slides
    If doc.LayoutSlides IsNot Nothing Then
        For Each slide As SlidesLayoutSlide In doc.LayoutSlides
            slide.AddWatermark(watermark)
        Next
    End If

    ' Add watermark to all notes slides
    For Each slide As SlidesSlide In doc.Slides
        If slide.NotesSlide IsNot Nothing Then
            slide.NotesSlide.AddWatermark(watermark)
        End If
    Next

    ' Add watermark to handout master
    If doc.MasterHandoutSlide IsNot Nothing Then
        doc.MasterHandoutSlide.AddWatermark(watermark)
    End If

    ' Add watermark to notes master
    If doc.MasterNotesSlide IsNot Nothing Then
        doc.MasterNotesSlide.AddWatermark(watermark)
    End If
    doc.Save()
End Using