Use C1ComboBoxExtender to display a resizable and editable text box/drop-down list on your HTML page.
C1ComboBoxExtender Markup
The following markup creates a C1ComboBoxExtender control. Set the TargetControlID property to attach the extender to a control.
<cc1:C1ComboBoxExtender ID="C1ComboBoxExtender1" runat="server" TargetControlID="DropDownList1">
</cc1:C1ComboBoxExtender>