asp下拉框联动

    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <div>
        <asp:Label runat="server" ID="lbl_Name"></asp:Label>
        <asp:TextBox runat="server" ID="txt_Name"></asp:TextBox>
        <asp:UpdatePanel runat="server">
            <ContentTemplate>
                <asp:DropDownList runat="server" ID="ddlProvince" AutoPostBack="true" OnSelectedIndexChanged="Drop_Province_Change">
                    <asp:ListItem Text="--请选择--" Value="-1"></asp:ListItem>
                    <asp:ListItem Text="浙江省" Value="0"></asp:ListItem>
                    <asp:ListItem Text="江苏省" Value="1"></asp:ListItem>
                </asp:DropDownList>
                <asp:DropDownList runat="server" ID="ddlCity" AutoPostBack="true" OnSelectedIndexChanged="Drop_City_Change">
                </asp:DropDownList>
                <asp:DropDownList runat="server" ID="ddlArea">
                </asp:DropDownList>
            </ContentTemplate>
        </asp:UpdatePanel>
        <asp:Button runat="server" ID="btnSave" Text="保存" OnClick="btn_Save_Click" />
    </div>
    </form>

protected void Drop_Province_Change(object sender, EventArgs e)
        {
            if (ddlProvince.SelectedValue == "-1")
            {
                Industry_Init(ddlCity);
            }
            else if (ddlProvince.SelectedValue == "0")
            {
                ddlCity.Items.Clear();
                ddlCity.Items.Add(new ListItem("--请选择--", "-1"));
                ddlCity.Items.Add(new ListItem("杭州", "0"));
                ddlCity.Items.Add(new ListItem("绍兴", "1"));
            }
            else if (ddlProvince.SelectedValue == "1")
            {
                ddlCity.Items.Clear();
                ddlCity.Items.Add(new ListItem("--请选择--", "-1"));
                ddlCity.Items.Add(new ListItem("苏州", "3"));
                ddlCity.Items.Add(new ListItem("无锡", "4"));
            }
            Industry_Init(ddlArea);
        }

        protected void Drop_City_Change(object sender, EventArgs e)
        {
            if (ddlCity.SelectedValue == "-1")
            {
                Industry_Init(ddlArea);
            }
            else if (ddlCity.SelectedValue == "0")
            {
                ddlArea.Items.Clear();
                ddlArea.Items.Add(new ListItem("--请选择--", "-1"));
                ddlArea.Items.Add(new ListItem("西湖区", "0"));
                ddlArea.Items.Add(new ListItem("下城区", "1"));
            }
            else if (ddlCity.SelectedValue == "0")
            {
                ddlArea.Items.Clear();
                ddlArea.Items.Add(new ListItem("--请选择--", "-1"));
                ddlArea.Items.Add(new ListItem("西湖区", "0"));
                ddlArea.Items.Add(new ListItem("下城区", "1"));
            }
            else if (ddlCity.SelectedValue == "1")
            {
                ddlArea.Items.Clear();
                ddlArea.Items.Add(new ListItem("--请选择--", "-1"));
                ddlArea.Items.Add(new ListItem("越城区", "0"));
                ddlArea.Items.Add(new ListItem("上虞", "1"));
            }
            else if (ddlCity.SelectedValue == "2")
            {
                ddlArea.Items.Clear();
                ddlArea.Items.Add(new ListItem("--请选择--", "-1"));
                ddlArea.Items.Add(new ListItem("苏州1区", "0"));
                ddlArea.Items.Add(new ListItem("苏州2区", "1"));
            }
            else if (ddlCity.SelectedValue == "3")
            {
                ddlArea.Items.Clear();
                ddlArea.Items.Add(new ListItem("--请选择--", "-1"));
                ddlArea.Items.Add(new ListItem("无锡1区", "0"));
                ddlArea.Items.Add(new ListItem("无锡2区", "1"));
            }
        }

        public void Industry_Init(DropDownList dropDown)
        {
            dropDown.Items.Clear();
            dropDown.Items.Add("--请选择--");
        }

        public void btn_Save_Click(object sender, EventArgs e)
        {
            var text = ddlProvince.SelectedItem.Text + ddlCity.SelectedItem.Text + ddlArea.SelectedItem.Text;
            ScriptManager.RegisterClientScriptBlock(this.Page, GetType(), "myscript", "alert('" + text + "'); ", true);
        }