Untitled

How to verify if a file is a valid excel spreadsheet?
_app = new Application()
        {
            Visible = false,
            DisplayAlerts = false,     // <-- I don't want to remove this.
            AskToUpdateLinks = false,
        };

_workbooks.Open(filename);
   _workbook = _workbooks.get_Item(1); // 1-based.
   _worksheets = _workbook.Sheets;

XlFileFormat.xlTextMac
    XlFileFormat.xlTextMSDOS
    XlFileFormat.xlTextPrinter
    XlFileFormat.xlTextWindows

XlFileFormat.xlExcel12
    XlFileFormat.xlExcel7
    XlFileFormat.xlExcel8
    XlFileFormat.xlExcel9795

bool validFile = ( f == XlFileFormat.xlExcel12   )
                  || ( f == XlFileFormat.xlExcel7    )
                  || ( f == XlFileFormat.xlExcel8    )
                  || ( f == XlFileFormat.xlExcel9795 );

int countBefore = _workbooks.get_Count();
_workbooks.Open(filename);
int countAfter = _workbooks.get_Count();
if (countBefore == countAfter) {
    // The file failed to load.
}