The C1ListExtender control allows users to select one or more items from a list.
ListExtender Markup
The following markup creates a C1ListExtender control. Set the TargetControlID property to attach the extender to a control.
<cc1:C1ListExtender ID="Panel1_ListExtender" runat="server"
TargetControlID="Panel1">
</cc1:C1ListExtender>