public class ChinaData
    {
        SqlConnection conn = null;
        SqlCommand cmd = null;

        public ChinaData()
        {
            conn = new SqlConnection("server=.;database=mydb;user=sa;pwd=123");
            cmd = conn.CreateCommand();
        }
        //查询方法 参数为父级代码
        public List<China> Select(string pcode)
        {
            List<China> lc = new List<China>();
            cmd.CommandText = "select *from ChinaStates where ParectAreaCode=@a";
            cmd.Parameters.Clear();
            cmd.Parameters.AddWithValue("@a",pcode);
            conn.Open();
            SqlDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                while(dr.Read())
                {
                    China c = new China();
                    c.AreaCode = dr["AreaCode"].ToString();
                    c.AreaName = dr["AreaName"].ToString();
                    c.ParentAreaCode = dr["ParentAreaCode"].ToString();
                    lc.Add(c);
                }
            }
            conn.Close();
            return lc;
        }
    }

 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            //窗口打开时，自动查询代码0001中国下的子级数据
            //绑定省
            comboBox1.DataSource = new ChinaData().Select("0001");
            comboBox1.DisplayMember = "AreaName";
            comboBox1.ValueMember = "AreaCode";
            //绑定市
            comboBox2.DataSource = new ChinaData().Select(comboBox1.SelectedValue.ToString ());
            comboBox2.DisplayMember = "AreaName";
            comboBox2.ValueMember = "AreaCode";
            //绑定区县
            comboBox3.DataSource = new ChinaData().Select(comboBox2.SelectedValue.ToString());
            comboBox3.DisplayMember = "AreaName";
            comboBox3.ValueMember = "AreaCode";
        }
    }

//省选中项改变时事件
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            //绑定市
            comboBox2.DataSource = new ChinaData().Select(comboBox1.SelectedValue.ToString());
            comboBox2.DisplayMember = "AreaName";
            comboBox2.ValueMember = "AreaCode";
        }
        //市选中项改变时事件
        private void comboBox2_SelectedIndexChanged_1(object sender, EventArgs e)
        {
            //绑定区县
            comboBox3.DataSource = new ChinaData().Select(comboBox2.SelectedValue.ToString());
            comboBox3.DisplayMember = "AreaName";
            comboBox3.ValueMember = "AreaCode";
        }