C# Opening an Excel work sheet file from FileStream
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I need to open an existing worksheet and using OPENXML tools, modify it, but I don't seem to have the opening down


     // Append a new worksheet and associate it with the workbook.
       XmlDocument doc = new XmlDocument();
       doc.Load(new FileStream(sheetfilename + ".xls", FileMode.Open));
       Sheet sheet;
       //?

Problem with the above is how to take the XML and open it as a Sheet. 

Below is another attempt I made earlier to do something similar using a FileStream but I got the error 

    Cannot convert type System.IO.FileStream to Document.OpenXML.SpreadSheet.Sheet

    Sheet sheet = (Sheet)new FileStream(sheetfilename + ".xls",FileMode.Open);


Can I please get some help with this please? 