System.Windows.Forms.ListControl.SelectedValue.get 返回 null

目的是为了处理combox1选择值后自计算另一textbox1的值，但此时计算逻辑中的另一combox2的值为NULL故报错，此时只能

使用try catch处理错误

     private void getCoefficiet()
        {
            try
            {

                txt_coefficient.Text = new CoefficientService().coefficients().
                     Where(q => q.CustomerID.ToString() == cmb_custName.SelectedValue.ToString() & q.ScrapID.ToString() == cmb_scrapName.SelectedValue.ToString()).

                    Select(s => s.Data).ToList().FirstOrDefault().ToString();
            }
           
            //当cmb_scrapNaem的值为NULL时
            catch (Exception)
            {
                txt_coefficient.Text = "";
            }

        }