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";
}