winform中comboBox控件加默认选项的问题

 #region  给下拉列表添加值
        public void BindEmployee()
        {
            DataTable dtItem;
            dtItem = NeonConn.CreateDataTable("select * from employee where  RestaurantID='" + DCLibrary.Current.RestaurantGuid + "'", "employee");
            DataRow dr = dtItem.NewRow();
            dr["EmployeeID"] = "-999";
            dr["DispName1"] = "--请选择--";
            dtItem.Rows.InsertAt(dr, 0);//指定起始位置插入
            cboemployee.DataSource = dtItem;
            cboemployee.DisplayMember = "DispName1";
            cboemployee.ValueMember = "EmployeeID";
        }