C# Windows Form การ AutoNumber ไม่ให้เลขซ้ำกัน ต้องทำยังไงคะ

private void button2_Click(object sender, EventArgs e)
        {
                for (int i = 0; i < dataGridView1.Rows.Count; i++)
                {
                    tr = Conn.BeginTransaction();

                    sb.Remove(0, sb.Length);
                    sb.Append("INSERT INTO tableSet (AsId,AsName)");
                    sb.Append("VALUES (@Asid,@AsName)");
                    string sqlSave = sb.ToString();

                    cmd.CommandText = sqlSave;
                    cmd.CommandType = CommandType.Text;
                    cmd.Connection = Conn;
                    cmd.Transaction = tr;
                    cmd.Parameters.Clear();

                    cmd.Parameters.Add("@Asid", SqlDbType.VarChar).Value = dataGridView1.Rows[i].Cells[0].Value;
                    cmd.Parameters.Add("@AsName", SqlDbType.VarChar).Value = dataGridView1.Rows[i].Cells[1].Value;
                   
                    cmd.ExecuteNonQuery();
                    tr.Commit();
                    
                }
                MessageBox.Show("เพิ่มข้อมูลเรียบร้อยแล้ว", "ผลการทำงาน", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
        }

        private void button3_Click(object sender, EventArgs e)
        {
            int cellnum = 0;

            for (int i = 0; i < dataGridView1.Rows.Count; i++)
            {
                cellnum = cellnum + 1;
                dataGridView1.Rows[i].Cells[0].Value = txtID.Text + "000" + cellnum;                
            }
        }