مشکلت همون مشکل قبلی هست؟
کد رو تغییر ده به
   private void frmSalesInvoices_Load(object sender, EventArgs e)        {

            db = new LinqToTrazoDataContext();

            var query = db.FactorSaleTables;

            dgvSalesInvoi.DataSource = query;

            txtSalesDate.Text = (PerCale.GetYear(DateTime.Now) + "/" + PerCale.GetMonth(DateTime.Now)

                                 + "/" + PerCale.GetDayOfMonth(DateTime.Now)).ToString();

            s = txtSalesDate.Text.Replace("/", string.Empty);

            t = s.Substring(2, 4);

            //txtCodFactor.Text = t+i





            var firstRecord = query.Select(rf => rf.RowFact);





            if (firstRecord.Count() == 0)

                txtCodFactor.Text = t + 100;

            else

            {

                var lastRecord = query.Select(fn => fn.FactorNo);

                txtCodFactor.Text = (lastRecord.Max() + 1).ToString();

            }

            var list = db.CommoTables.ToList();

            cmbCommodityName.DataSource = list;

            //cmbCommodityName.ValueMember = "RowCommo";

            cmbCommodityName.DisplayMember = "CommodName";

            txtSaleCodCommodity.DataBindings.Add("Text", list, "CommodCod");

            txtNameCO.DataBindings.Add("Text", list, "CompanyName");

            //txtNumber.DataBindings.Add("Text", list, "CommodNumb");

            //txtSize.DataBindings.Add("Text", list, "CommodWeight");

            txtSize.DataBindings.Add("Text", list, "CommodSize");

            txtUnitPrice.DataBindings.Add("Text", list, "UnitPrice");

            txtTax.DataBindings.Add("Text", list, "Tax");

            txtOff.DataBindings.Add("Text", list, "Dicount");

        }