asp.net dropdownlist控制textbox的值

我想让dropdownlist控制textbox的值：当dropdownlist的只改变时，texbox显示dropdownlist的选中值。不需要提交的那种！局部刷新实现的！  --------------------编程问答-------------------- updatepanel
   <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
    <div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
               <asp:DropDownList ID="ddlLB" runat="server" Width="15%" AutoPostBack="True" OnSelectedIndexChanged="ddlLB_SelectedIndexChanged">
               </asp:DropDownList>
                 </ContentTemplate>
        </asp:UpdatePanel>
        </div> 
 protected void ddlLB_SelectedIndexChanged(object sender, EventArgs e)
    {
        if(this.ddlLB.SelectedValue!=null)
        {
            string s=this.ddlLB.SelectedValue;
        }
    } --------------------编程问答-------------------- <asp:DropDownList ID="DropDownList1" runat="server" onclick="document.getElementById('TextBox1').value = document.getElementById('DropDownList1').value;">
...........
..............
</asp:DropDownList>

补充：.NET技术 ,  ASP.NET