Placeholder is not updated by RadAjaxManager

PROBLEM

<asp:Placeholder> and <asp:Literal> controls and their content do not get updated by RadAjaxManager.

DESCRIPTION

RadAjaxManager looks for rendered content that it will wrap in an update panel. This means that the controls it AJAX-enables need to render on the page with the ID provided in the AjaxSettings.

Placeholders and LiteralControl instances do not render a wrapping element with their ID and so RadAjaxManager cannot get a proper handle on them. Sometimes this can also be true for user controls, depending on the concrete control.

You can get the same behavior for controls that initially had Visible="false" so they did not render.

SOLUTION

Wrap the content in an element that will render an HTML element with its ID. For example, an <asp:Panel> or <asp:Label>.

<telerik:RadAjaxManager runat="server" ID="RadAjaxManager1">
    <AjaxSettings>
        <telerik:AjaxSetting AjaxControlID="Button1">
            <UpdatedControls>
                <telerik:AjaxUpdatedControl ControlID="Panel1" />
                <%--instead of ControlID="PlaceHolder1"--%>
            </UpdatedControls>
        </telerik:AjaxSetting>
    </AjaxSettings>
</telerik:RadAjaxManager>
<asp:Button Text="partial rendering" runat="server" ID="Button1" />
<asp:Panel runat="server" ID="Panel1">
    <asp:PlaceHolder runat="server" ID="PlaceHolder1"></asp:PlaceHolder>
</asp:Panel>