I want Editable dropdownlist in asp.net?

Hi ,
i want editable dropdownlist in asp.net for this i had done in the following way,
<form id="form1" runat="server">
        <div>
             <asp:TextBox Style="width: 120px;position:absolute;" ID="txtDisplay" runat="server"></asp:TextBox>
             <asp:DropDownList ID="ddSelect" runat="server" Width="140px">
                    <asp:ListItem Value="test1">test1</asp:ListItem>
                    <asp:ListItem Value="test2">test2</asp:ListItem>
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