linq query webcontrol.controls

i have 3 textbox controls page

<asp:textbox id="textbox1" runat="server" autopostback="true"
        ontextchanged="textbox_textchanged" tabindex="1">
<asp:textbox id="textbox2" runat="server" autopostback="true"
        ontextchanged="textbox_textchanged" tabindex="2">
<asp:textbox id="textbox3" runat="server" autopostback="true"
        ontextchanged="textbox_textchanged" tabindex="3">

and an eventuality handler

protected vacant textbox_textchanged(object sender, eventargs e)
{
    webcontrol changed_control = (webcontrol)sender;

    var next_controls = webcontrol control changed_control.parent.controls
                        where control.tabindex > changed_control.tabindex
                        orderby control.tabindex
                        name control;

    next_controls.defaultifempty(changed_control).first().focus();
}

the definition formula automatically name textbox unbroken tabindex after page post behind (see ). existence i accept invalidcastexception since it's dilettante ban system.web.ui.literalcontrol (webcontrol.controls contains indeed literalcontrols) system.web.ui.webcontrols.webcontrol.

i am prying illusory cgange aproach somehow accept operative solution? conclude you!