求助：怎么导不出xlsx的Excel？？？

        private void SaveExcel(string excelName)
        {
            _excelApp.Visible = false;            FileInfo file = new FileInfo(excelName);
            if (!file.Exists) { throw new Exception("文件不存在"); }
            string extension = file.Extension;            switch (extension)
            {
                case ".xls":
                    _book.SaveAs(excelName, Microsoft.Office.Interop.Excel.XlFileFormat.xlExcel8, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
                    break;
                case ".xlsx":
                    _book.SaveAs(excelName, Excel.XlFileFormat.xlOpenXMLWorkbook, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
                    break;
                default:
                    _book.SaveAs(excelName, Microsoft.Office.Interop.Excel.XlFileFormat.xlExcel8, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
                    break;
            }            //保存为Office2003和Office2007都兼容的格式  .xlExcel8
            
            //_book.SaveAs(excelName, Excel.XlFileFormat.xlOpenXMLWorkbook, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            _excelApp.Quit();        }