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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>(Utilities.MapSourceFilePath (FilePath)))
{
    // 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 (CellsHeaderFooter headerFooter in worksheet.HeadersFooters)
        {
            foreach (CellsHeaderFooterSection section in headerFooter.Sections)
            {
                if (section.Image != null)
                {
                    // Add watermark to the image
                    section.Image.AddWatermark(watermark);
                }
            }
        }
    }
}