vs.net控件updatePanel实现无刷新的方法

页面部分

<asp:scriptmanager id="scriptmanager1" runat="server">
    </asp:scriptmanager>
    <h2>
        使用vs控件updatepanel实现无刷新
    </h2>
    <p>
         <asp:updatepanel id="updatepanel1" rendermode="block" updatemode="conditional" runat="server">
        <contenttemplate>
            点击按钮从服务器获取当前时间<asp:textbox id="txttime" runat="server"></asp:textbox>
            <asp:button id="btn1" runat="server" text="button" onclick="btn1_click" />
        </contenttemplate>
        <triggers>
        <%--<asp:postbacktrigger controlid="btn1" />--%>
        <asp:asyncpostbacktrigger controlid="btn1"/>
        </triggers>
        </asp:updatepanel>
    </p>

后台代码

       protected void btn1_click(object sender, eventargs e)
        {
            txttime.text = datetime.now.tostring();
        }