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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (DiagramDocument doc = Document.Load<DiagramDocument>(Utilities.MapSourceFilePath(FilePath)))
{
    TextWatermark watermark = new TextWatermark("Test watermark", new Font("Arial", 19));

    // Editing of the shape in Visio is forbidden
    doc.AddWatermark(watermark, new DiagramShapeSettings { IsLocked = true });

    doc.Save();
}