sorry.... use .Value
sql = "insert into Tb_Session_Structure ([Cmn_Minor_code],[Days],[Session1],[Session2],[Session3],[Session4])";
sql = sql + " values ('" + cb_Course_Code.Text + "',
'" + DGv_Session.Rows[i].Cells[0].Value.ToString() + "',
" + Convert.ToString(DGv_Session[1, i].Value == "true" ? 1 : 0) + ",
" + Convert.ToString(DGv_Session[2, i].Value == "true" ? 1 : 0) + ",
" + Convert.ToString(DGv_Session[3, i].Value == "true" ? 1:0) + ",
" + Convert.ToString(DGv_Session[4, i].Value == "true" ? 1 : 0) + ")";