private void setDocumentMetadata()
{
if (_documentPreferences.DocumentMetadata == null) return;
_package.Workbook.Properties.Title = _documentPreferences.DocumentMetadata.Title;
_package.Workbook.Properties.Author = _documentPreferences.DocumentMetadata.Author;
_package.Workbook.Properties.Subject = _documentPreferences.DocumentMetadata.Subject;
_package.Workbook.Properties.Keywords = _documentPreferences.DocumentMetadata.Keywords;
}