图例：
注意，”请选择“ 的是个文本框
<asp:Label ID="TextLabel" runat="server" Text="请选择" Style="display: block; width: 300px; padding:2px; padding-right: 50px; font-family: Tahoma; font-size: 11px;" /><asp:Panel ID="DropPanel" runat="server" CssClass="ContextMenuPanel" Style="display :none; visibility: hidden;"><asp:LinkButton runat="server" ID="Option1" Text="董昊" CssClass="ContextMenuItem" OnClick="OnSelect" /><asp:LinkButton runat="server" ID="Option2" Text="比尔盖茨" CssClass="ContextMenuItem" OnClick="OnSelect" /><asp:LinkButton runat="server" ID="Option3" Text="二百五" CssClass="ContextMenuItem" OnClick="OnSelect" /> </asp:Panel><ajaxToolkit:DropDownExtender runat="server" ID="DDE" TargetControlID="TextLabel" DropDownControlID="DropPanel" />控件简介：
<ajaxToolkit:DropDownExtender runat="server" ID="DDE“TargetControlID="TextLabel"DropDownControlID="DropPanel"/>
TargetControlID –需要附加下拉列表的文本框ID
DropDownControlID –下拉列表（Panel）ID