C# Ajax toolkit DropDownList control Extender didn't display in textbox

I have textbox and listbox inside an update panel. My textbox is enabled with dropdownlistextender that its values is in some currencies like USD,MYR,Yen etc. But when I select value from this list, it doesnt show up in textbox. What should I do? When it display in textbox, it should be inserted into my created database. Thanks in forward.

 <asp:TextBox ID="txtCurrency" runat="server"></asp:Text开发者_如何学编程Box>
 <asp:DropDownExtender ID="txtCurrency_DropDownExtender" runat="server" 
     DropDownControlID="ListBox1" DynamicServicePath="" Enabled="True" 
     TargetControlID="txtCurrency">
 </asp:DropDownExtender>
 <asp:ListBox ID="ListBox1" runat="server" 
     onselectedindexchanged="ListBox1_SelectedIndexChanged">
     <asp:ListItem Selected="True"></asp:ListItem>             
     <asp:ListItem>MYR</asp:ListItem>
     <asp:ListItem>USD</asp:ListItem>
     <asp:ListItem>EURO</asp:ListItem>
     <asp:ListItem>POUND</asp:ListItem>
     <asp:ListItem>Singapore Dollar</asp:ListItem>
     <asp:ListItem>Yuan/Renmibi</asp:ListItem>
     <asp:ListItem>Japan Yen</asp:ListItem>
     <asp:ListItem></asp:ListItem>
 </asp:ListBox>