title: What is the best way to store XML data in an Excel file
body: <p>I am looking for a way to store XML data in an Excel file.
  The data should be completely hidden to the user, it should not be in a cell or comment, even hidden.
  Also, the data should be preserved when the user opens and then saves the file with Excel.
  I am not looking for a way to map cells to external XML data. The XML data should be inside the xlsx file.</p>
  
  <p>The data will be entered using a C# tool, not Excel itself.</p>