ajaxToolkit:ModalPopupExtender可以让用户模拟新开一个窗口，以下演示，就是在模拟新开窗口作多项选项的功能。
 
演示例子中，是Repeater结合CheckBox进行多选，此功能实现，可以参考：https://www.jb51.net/article/33558.htm

选择的小图片，它将要被ajaxToolkit:ModalPopupExtender使用，因为需要设为服务控件，添加runat="server"：
复制代码 代码如下:
<img id="ab" runat="server" style="margin-left: 5px; margin-bottom: 30px;"
src="/UploadFiles/2021-04-02/ab.gif">
下面是Popup窗口的Html，不过Insus.NET已经省略了Repeater控件内的内容，你在应用时，需要添加的你自己的，还是就是btnSelected_Click事件。
复制代码 代码如下:
<asp:Panel ID="pnlPopupWindown" runat="server" Style="display: none; background-color: #ffffdd;
border-width: 3px; border-style: solid; border-color: Gray; padding: 3px; width: 500px;">
<asp:Panel ID="Panel3" runat="server" Style="float: left; margin-bottom: 5px; cursor: move;
background-color: #DDDDDD; border: solid 1px Gray; color: Black; height: 20px;
width: 475px; text-align: center; line-height: 20px;">
邮件地址列表
</asp:Panel>
<asp:Panel ID="Panel4" runat="server" Style="float: right; margin-bottom: 5px; background-color: #DDDDDD;
border: solid 1px Gray; color: Black; height: 20px; text-align: center; line-height: 20px;">
<asp:LinkButton ID="btnClose" runat="server" Style="margin-right: 4px; margin-left: 4px;"
OnClientClick="return false;" Text="×" ForeColor="Red" ToolTip="Close" />
</asp:Panel>
<div>
<asp:Panel ID="Panel1" runat="server" ScrollBars="Vertical" Height="198px" Width="100%"
BorderStyle="Solid" BorderWidth="1px" BorderColor="Gray">
<asp:Repeater ID="RepeaterEmailList" runat="server">
</asp:Repeater>
</asp:Panel>
<div style="height: 3px;">
</div>
<asp:Panel ID="Panel2" runat="server">
<asp:Button ID="btnSelected" runat="server" OnClick="btnSelected_Click" Text="插入"
CausesValidation="false" CssClass="button" />
</asp:Panel>
</div>
</asp:Panel>
<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="ab"
PopupControlID="pnlPopupWindown" BackgroundCssClass="modalBackground" CancelControlID="btnClose"
DropShadow="true" PopupDragHandleControlID="Panel3" />
<script type="text/javascript">
function setBodyHeightToContentHeight() {
document.body.style.height = Math.max(document.documentElement.scrollHeight, document.body.scrollHeight) + "px";
}
setBodyHeightToContentHeight();
$addHandler(window, "resize", setBodyHeightToContentHeight);
</script>

最后是CSS样式：
复制代码 代码如下:
/*Modal Popup*/
.modalBackground
{
background-color: Gray;
filter: alpha(opacity=70);
opacity: 0.7;
}