how to visible textbox when select data value field in c#

Dear All,

May i know how to visible textbox by select data value  field in c# ?

i have tried the following code , but it doesnt visible, kindly advise thank you

<asp:TableRow ID="TableRow17" runat="server" ForeColor="White"> <asp:TableHeaderCell BorderColor="Black" BorderWidth="1" HorizontalAlign="Left" BackColor="#0C0C25"> Item : </asp:TableHeaderCell> <asp:TableHeaderCell BorderColor="Black" BorderWidth="1" BackColor="ControlLight" HorizontalAlign="Left" ForeColor="Black"> <asp:DropDownList ID="WMItem" runat="server" RepeatDirection="Horizontal" AutoPostBack="True" OnSelectedIndexChanged="WMVendor_SelectedIndexChanged"></asp: ...