<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:Button ID="btnShowModal" runat="server" Text="显示弹出窗口" 
                   OnClick="btnShowModal_Click" />
        <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
                               TargetControlID="btnShowModal"
                               PopupControlID="pnlModal" 
                               BackgroundCssClass="modalBackground">
        </asp:ModalPopupExtender>
        
        <asp:Panel ID="pnlModal" runat="server" CssClass="modalPopup" style="display:none">
            <!-- 弹出窗口内容 -->
            <div>
                <h3>模式窗口标题</h3>
                <p>这里是弹出窗口的内容</p>
                <asp:Button ID="btnClose" runat="server" Text="关闭" 
                           OnClick="btnClose_Click" />
            </div>
        </asp:Panel>
    </ContentTemplate>
</asp:UpdatePanel>

protected void btnShowModal_Click(object sender, EventArgs e)
{
    // 显示模式窗口
    ModalPopupExtender1.Show();
}

protected void btnClose_Click(object sender, EventArgs e)
{
    // 隐藏模式窗口
    ModalPopupExtender1.Hide();
}

.modalBackground {
    background-color: Gray;
    filter: alpha(opacity=70);
    opacity: 0.7;
}

.modalPopup {
    background-color: #fff;
    border: 3px solid #ccc;
    padding: 20px;
    width: 300px;
}