สอบถามการเพิ่มข้อมูลต่อท้าย datagridview C# virtual studio , window form ค่ะ

               validatematpricetable();
          
                int SupID = (int)dgvsup.SelectedRows[0].Cells[0].Value;
                DataTable dt = new DataTable();
                SqlDataAdapter da1 = new SqlDataAdapter();
                SqlConnection conn = new SqlConnection();
                conn.ConnectionString = "Data Source=DAW-PC;Initial Catalog=A;Integrated Security=True";
                conn.Open();
                SqlCommand com = new SqlCommand();

                string sql;
                sql = " select Max(SLID)  from [SupplierLineItem] where @SupID = SupID ";

                com.CommandText = sql;
                com.Connection = conn;
                com.Parameters.AddWithValue("@SupID", SupID);

                int SLID = (int)com.ExecuteScalar() + 1;
                com.Parameters.AddWithValue("@SLID", SLID);
                dgvmatprice.DataSource = dt;
                object[] a = new object[6];
                 a[0] = SLID;
                 a[1] = cbmatname.SelectedValue;
                 a[2] = SupID;
                 a[3] = cbmatname.Text;
                 a[4] = txtmatminpack.Text;
                 a[5] = txtprice.Text;
                this.dgvmatprice.Rows.Add(a)

               validatematpricetable();
           
                int SupID = (int)dgvsup.SelectedRows[0].Cells[0].Value;
                DataTable dt = new DataTable();
                SqlDataAdapter da1 = new SqlDataAdapter();
                SqlConnection conn = new SqlConnection();
                conn.ConnectionString = "Data Source=DAW-PC;Initial Catalog=A;Integrated Security=True";
                conn.Open();
                SqlCommand com = new SqlCommand();

                string sql;
                sql = " select Max(SLID)  from [SupplierLineItem] where @SupID = SupID ";

                com.CommandText = sql;
                com.Connection = conn;
                com.Parameters.AddWithValue("@SupID", SupID);

                int SLID = (int)com.ExecuteScalar() + 1;
                com.Parameters.AddWithValue("@SLID", SLID);
                dgvmatprice.DataSource = dt;
                da1.SelectCommand = com;
                da1.Fill(dt);
                DataRow dr = dt.Rows[0];
                SLID = int.Parse(dr["SLID"].ToString());
                cbmatname.SelectedValue = dr["MatID"].ToString();
                SupID = int.Parse(dr["SupID"].ToString());
                txtmatminpack.Text = dr["Minpack"].ToString();
                txtprice.Text = dr["Priceperminpack"].ToString();
                conn.Close();