Dropdown Selected Value in TextBox using c#

See more:

ASP.NET

C#3.5

my dropdownlist in updatepanel and numaric textbox is outside the updatepanel
my designe is

<td style="width: 50%" align="left" valign="middle">                             
 <asp:UpdatePanel ID="UpdatePanel1" runat="server">
  <ContentTemplate>
<asp:DropDownList ID="ddlVendorName" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlVendorName_SelectedIndexChanged">
<asp:ListItem Text="Select" Value="0"></asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>

<td style="width: 50%">
<cc1:NumericTextBox ID="txtDiscount" runat="server" MaxLength="6"
TabIndex="1" AllowDecimal="True"></cc1:NumericTextBox>

</td>

In Page load I am Binding data to dropdownlist and
using dropdown selected value i am getting discount value from database and assign to dataset , from that dataset value assign to txtDiscount
in this ddlVendorName_SelectedIndexChanged event i have some code like
protected void ddlVendorName_SelectedIndexChanged(
DataSet ds = new MI.DAL.InventoryPurchases().GetVendorsPriceDetails(Convert.ToInt32(ddlVendorName.SelectedValue));
txtDeduction.Text = ds.Tables[0].Rows[0][0].ToString();

that value is coming but it is not sohwing at front end on browser .

pls help me ....