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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (CellsDocument doc = Document.Load<CellsDocument>(@"D:\test.xlsx"))
{
    // Initialize image or text watermark
    TextWatermark watermark = new TextWatermark("Protected image", new Font("Arial", 8));
    watermark.HorizontalAlignment = HorizontalAlignment.Center;
    watermark.VerticalAlignment = VerticalAlignment.Center;
    watermark.RotateAngle = 45;
    watermark.SizingType = SizingType.ScaleToParentDimensions;
    watermark.ScaleFactor = 1;

    foreach (CellsWorksheet worksheet in doc.Worksheets)
    {
        foreach (CellsShape shape in worksheet.Shapes)
        {
            if (shape.Image != null)
            {
                // Add watermark to the image
                shape.Image.AddWatermark(watermark);
            }
        }
    }

    doc.Save();
}