private static WorksheetPart GetWorksheetPartByName( SpreadsheetDocument document, string sheetName )
{
    IEnumerable<Sheet> sheets =
       document.WorkbookPart.Workbook.GetFirstChild<Sheets>().
       Elements<Sheet>().Where( s => s.Name == sheetName );

    if ( sheets?.Count() == 0 )
    {
        // The specified worksheet does not exist.

        return null;
    }

    string relationshipId = sheets?.First().Id.Value;

    WorksheetPart worksheetPart = ( WorksheetPart ) document.WorkbookPart.GetPartById( relationshipId );

    return worksheetPart;
}