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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (Stream stream = File.Open(@"C:\test.xlsx", FileMode.Open, FileAccess.ReadWrite))
{
   using (Document doc = Document.Load(stream))
   {
     // Here we can use document instance to add or remove watermarks
   }
}