control postback

  
    <form id="form1" runat="server">
        <asp:scriptmanager enablepartialrendering="true" id="scriptmanager1" runat="server">
        </asp:scriptmanager>

        <asp:updatepanel id="updatepanel1" runat="server">
          <contenttemplate>
            <asp:textbox id="textbox1" runat="server"></asp:textbox>
            <asp:button id="button2" runat="server" onclick="button2_click" text="btn 1" />
          </contenttemplate>
        </asp:updatepanel>
    </form>

    protected sub button2_click(byval sender as object, byval e as system.eventargs)
        textbox1.text = date.now.tostring
    end sub

    protected sub page_load(byval sender as object, byval e as system.eventargs) handles me.load
        textbox1.text = "load"
    end sub