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";

        }