ComboBox，三级联动菜单，新入门点小白，有些代码有待优化，大神勿喷

//当前窗体的Load事件
        private void provinceANDCity_Load(object sender, EventArgs e)
        {
            //连接字符串
            string strConn = "Data Source=.;Initial Catalog=Province;User Id=sa;Pwd=ok";
            //connection对象
            SqlConnection conn = new SqlConnection(strConn);
            //DataSet对象
            DataSet ds = new DataSet();
            //查询省份语句
            string sql = "SELECT * FROM province";
            SqlDataAdapter adapter = new SqlDataAdapter(sql,conn);
            adapter.Fill(ds,"province");
            //将省份的数据绑定到省份的下拉框中
            this.cboProvince.DataSource = ds.Tables["province"];
            this.cboProvince.ValueMember = "id";
            this.cboProvince.DisplayMember = "provinceName";

            //获取省份编号
            int id = Convert.ToInt32(this.cboProvince.SelectedValue);
            //根据省份编号查询城市
            sql = "SELECT * FROM City WHERE proId="+id;
            adapter = new SqlDataAdapter(sql,conn);
            adapter.Fill(ds,"City");
            //将查出来的城市绑定在城市下拉框中
            this.cboCity.DataSource = ds.Tables["City"];
            this.cboCity.ValueMember = "id";
            this.cboCity.DisplayMember = "cityName";

            //获取城市编号
            int cityId = Convert.ToInt32(this.cboCity.SelectedValue);
            //根据城市编号查询区县
            sql = "SELECT * FROM County WHERE cityId=" + cityId;
            adapter = new SqlDataAdapter(sql, conn);
            adapter.Fill(ds, "County");
            //将查出来的区县绑定在区县下拉框中
            this.cboCounty.DataSource = ds.Tables["County"];
            this.cboCounty.ValueMember = "id";
            this.cboCounty.DisplayMember = "countyName";

        }
        //省份下拉框的事件
        private void cboProvince_SelectionChangeCommitted(object sender, EventArgs e)
        {
            string strConn = "Data Source=.;Initial Catalog=Province;User Id=sa;Pwd=ok";
            SqlConnection conn = new SqlConnection(strConn);
            DataSet ds = new DataSet();
            //获取省份编号
            int id = Convert.ToInt32(this.cboProvince.SelectedValue);
            //根据省份编号查询城市
            string sql = "SELECT * FROM City WHERE proId="+id;
            SqlDataAdapter adapter = new SqlDataAdapter(sql, conn);
            adapter.Fill(ds, "City");
            //将查出来的城市绑定在城市下拉框中
            this.cboCity.DataSource = ds.Tables["City"];
            this.cboCity.ValueMember = "id";
            this.cboCity.DisplayMember = "cityName";
        }
        //城市下拉框的事件
        private void cboCity_SelectionChangeCommitted(object sender, EventArgs e)
        {
            string strConn = "Data Source=.;Initial Catalog=Province;User Id=sa;Pwd=ok";
            SqlConnection conn = new SqlConnection(strConn);
            DataSet ds = new DataSet();

            //获取城市编号
            int cityId = Convert.ToInt32(this.cboCity.SelectedValue);
            //根据城市编号查询区县
            string sql = "SELECT * FROM County WHERE cityId=" + cityId;
            SqlDataAdapter adapter = new SqlDataAdapter(sql, conn);
            adapter.Fill(ds, "County");
            //将查出来的区县绑定在区县下拉框中
            this.cboCounty.DataSource = ds.Tables["County"];
            this.cboCounty.ValueMember = "id";
            this.cboCounty.DisplayMember = "countyName";
        }