Sample code:

Microsoft.Office.Interop.Excel.

Application oExcel = new Application();

object oMissing = Type.Missing;

string fileName = @"D:\Test\WorksheetNamesExpected.xls";

Workbook _book = oExcel.Workbooks.Open(fileName, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing, oMissing);

object fileName2 = @"D:\Test\WorksheetNamesExpected.xlsx";

object fileFormat = XlFileFormat.xlOpenXMLWorkbook;

_book.SaveAs(fileName2, fileFormat, oMissing, oMissing, oMissing, oMissing,

XlSaveAsAccessMode.xlNoChange, oMissing, oMissing, oMissing, oMissing, oMissing);

_book.Close();