Check out example codes for "convert from xls to xlsx C#". It will help you in understanding the concepts better.

Code Example 1

/// <summary>/// Using Microsoft.Office.Interop to convert XLS to XLSX format, to work with EPPlus library/// </summary>/// <param name="filesFolder"></param>
public static string ConvertXLS_XLSX(FileInfo file) 
{
  var app = new Microsoft.Office.Interop.Excel.Application();
  var xlsFile = file.FullName;
  var wb = app.Workbooks.Open(xlsFile);
  var xlsxFile = xlsFile + "x";
  wb.SaveAs(Filename: xlsxFile, FileFormat: Microsoft.Office.Interop.Excel.XlFileFormat.xlOpenXMLWorkbook);
  wb.Close();
  app.Quit();
  return xlsxFile;
}

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