DropDownList

use master
go

if exists(select * from sys.databases where name='MyData')
drop database MyData
go

create database MyData
go

use MyData
go

if exists(select * from sys.objects where name='Provice')
drop table provice
go

create table Provice
(
 proid int primary key identity(1,1),
 proname varchar(100),
 upproid int
)
go

--select * from Provice
--go

insert into Provice(proname,upproid) values('河北',0)
insert into Provice(proname,upproid) values('保定',1)
insert into Provice(proname,upproid) values('张家界',1)

insert into Provice(proname,upproid) values('湖北',0)
insert into Provice(proname,upproid) values('荆州',4)
insert into Provice(proname,upproid) values('仙桃',4)

insert into Provice(proname,upproid) values('湖南',0)
insert into Provice(proname,upproid) values('长沙',7)
insert into Provice(proname,upproid) values('岳阳',7)
go

s

前台代码

<asp:DropDownList ID="ddlOne" runat="server" ontextchanged="ddlOne_TextChanged" onselectedindexchanged="Page_Load" AutoPostBack="true"></asp:DropDownList>            
<asp:DropDownList ID="ddlTwo" runat="server" onselectedindexchanged="Page_Load"></asp:DropDownList>

后台代码

 protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Share();
                CommondOne();
            }
        }

        public string str = "";
        public SqlConnection con;
        protected void Share()
        {
            str = ConfigurationManager.ConnectionStrings["ConStr"].ToString();
            con = new SqlConnection(str);
            con.Open();
        }

        protected void CommondOne()
        {
            SqlCommand cmd = new SqlCommand("select * from Provice where  upproid=0",con);
            SqlDataReader dr = cmd.ExecuteReader();

            ddlOne.Items.Clear();
            ddlOne.Items.Add(new ListItem("请选择省", ""));
            ddlTwo.Items.Add(new ListItem("请选择城市", ""));

            if (!dr.HasRows)
            {
                return;
            }

            while (dr.Read())
            {
                ddlOne.Items.Add(new ListItem(dr["proname"].ToString(), dr["proid"].ToString()));
            }
            dr.Close();
            con.Close();
        }

        protected void ddlOne_TextChanged(object sender, EventArgs e)
        {
            Share();
            CommondTwo();
        }

        protected void CommondTwo()
        {
            SqlCommand cmd = new SqlCommand("select * from Provice where upproid =" + Convert.ToInt32(ddlOne.SelectedValue), con);
            SqlDataReader dr = cmd.ExecuteReader();
            ddlTwo.Items.Clear();

            ddlTwo.Items.Add(new ListItem("请选择城市", ""));
            if (!dr.HasRows)
            {
                return;
            }

            while (dr.Read())
            {
                ddlTwo.Items.Add(new ListItem(dr["proname"].ToString(), dr["proid"].ToString()));
            }
            dr.Close();
            con.Close();
        }