ช่วยดูโค้ดให้หน่อยนะค่ะ ติดปัญหาเกี่ยวกับการใช้ SubString ค่ะ

private string GetNewMemberId()
        {
            string nextId = String.Format("{0:00}", 1) + DateTime.Now.ToString("MMyy") + String.Format("{0:0000}", 1);
            string currentId = lbMemberID.Text;
            if(currentId== null)
                currentId = String.Format("{0:00}", 0) + DateTime.Now.ToString("MMyy") + String.Format("{0:0000}", 0);
            int maxid = 0;
            int type = 0;
            int tototype = 0;
            if (currentId.Substring(2, 0) == DateTime.Now.ToString("MMyy"))
            {
                try
                {
                    tototype = Convert.ToInt32(currentId.Substring(1, 1));
                }
                catch { }

                if (type == comboBox1.SelectedIndex)
                    tototype = type;
            }

            if (currentId.Substring(0, 4) == DateTime.Now.ToString("MMyy"))
            {

                try
                {
                    maxid = Convert.ToInt32(currentId.Substring(6, 4));
                }
                catch { }

                if (maxid < 9999)
                    maxid++;
                else
                    maxid = 1;
            }

            nextId = String.Format("{0:00}", 1) + DateTime.Now.ToString("MMyy") + String.Format("{0:0000}", maxid);

            return nextId;
        }