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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (SlidesDocument doc = Document.Load<SlidesDocument>(@"D:\test.pptx"))
{
    TextWatermark watermark = new TextWatermark("Test watermark", new Font("Arial", 8));

    // Add watermark to all master slides
    foreach (SlidesMasterSlide slide in doc.MasterSlides)
    {
        slide.AddWatermark(watermark);
    }

    // Add watermark to all layout slides
    if (doc.LayoutSlides != null)
    {
        foreach (SlidesLayoutSlide slide in doc.LayoutSlides)
        {
            slide.AddWatermark(watermark);
        }
    }

    // Add watermark to all notes slides
    foreach (SlidesSlide slide in doc.Slides)
    {
        if (slide.NotesSlide != null)
        {
            slide.NotesSlide.AddWatermark(watermark);
        }
    }

    // Add watermark to handout master
    if (doc.MasterHandoutSlide != null)
    {
        doc.MasterHandoutSlide.AddWatermark(watermark);
    }

    // Add watermark to notes master
    if (doc.MasterNotesSlide != null)
    {
        doc.MasterNotesSlide.AddWatermark(watermark);
    }
    doc.Save();
}