Novacode.DocX.GetNextFreeRelationshipID C# (CSharp) Method

        private string GetNextFreeRelationshipID()
        {
            int id = (
                 from r in mainPart.GetRelationships()
                 where r.Id.Substring(0, 3).Equals("rId")
                 select int.Parse(r.Id.Substring(3))
             ).DefaultIfEmpty().Max();

            // The conventiom for ids is rid01, rid02, etc
            string newId = id.ToString();
            int result;
            if (int.TryParse(newId, out result))
                return ("rId" + (result + 1));
            String guid = String.Empty;
            do
            {
                guid = Guid.NewGuid().ToString();
            } while (Char.IsDigit(guid[0]));
            return guid;
        }