private void ddlState_SelectedIndexChanged(object sender, EventArgs e) { if (ddlState.SelectedValue.ToString() != null) { int stateId = Convert.ToInt32(ddlState.SelectedValue); CompanyBL companyBL = new CompanyBL(); List <City> cities = companyBL.GetCities(stateId); ddlCity.ValueMember = "ID"; ddlCity.DisplayMember = "Name"; ddlCity.DataSource = cities; } }