public static void CloseExcel()
{
    GC.Collect();
    GC.WaitForPendingFinalizers();
    if (_range != null)
    {
        Marshal.FinalReleaseComObject(_range);
        _range = null;
    }
    if (_workSheet != null)
    {
        Marshal.FinalReleaseComObject(_workSheet);
        _workSheet = null;
    }
    if (_workBook != null)
    {
        _workBook.Close(false, String.Empty, false);
        Marshal.FinalReleaseComObject(_workBook);
        _workBook = null;
    }
    if (_application != null)
    {
        try
        {
            _application.Quit();
        }
        Marshal.FinalReleaseComObject(_application);
        _application = null;
    }
}