Personalizando o HTML do UpdatePanel

Segue abaixo uma classe que tem como objetivo personalizar o HTML gerado a partir de um UpdatePanel.

namespace LeftSlipper {
    using System;
    using System.ComponentModel;
    using System.Web.UI;
    using Microsoft.Web.UI;

    public class CustomUpdatePanel : UpdatePanel {
        private string _cssClass;
        private HtmlTextWriterTag _tag = HtmlTextWriterTag.Div;

        [DefaultValue("")]
        [Description("Applies a CSS style to the panel.")]
        public string CssClass {
            get {
                return _cssClass ?? String.Empty;
            }
            set {
                _cssClass = value;
            }
        }

        // Hide the base class's RenderMode property since we don't use it
        [Browsable(false)]
        [EditorBrowsable(EditorBrowsableState.Never)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public new UpdatePanelRenderMode RenderMode {
            get {
                return base.RenderMode;
            }
            set {
                base.RenderMode = value;
            }
        }

        [DefaultValue(HtmlTextWriterTag.Div)]
        [Description("The tag to render for the panel.")]
        public HtmlTextWriterTag Tag {
            get {
                return _tag;
            }
            set {
                _tag = value;
            }
        }

        protected override void RenderChildren(HtmlTextWriter writer) {
            if (IsInPartialRendering) {
                // If the UpdatePanel is rendering in "partial" mode that means
                // it's the top-level UpdatePanel in this part of the page, so
                // it doesn't render its outer tag. We just delegate to the base
                // class to do all the work.
                base.RenderChildren(writer);
            }
            else {
                // If we're rendering in normal HTML mode we do all the new custom
                // rendering. We then go render our children, which is what the
                // normal control's behavior is.
                writer.AddAttribute(HtmlTextWriterAttribute.Id, ClientID);
                if (CssClass.Length > 0) {
                    writer.AddAttribute(HtmlTextWriterAttribute.Class, CssClass);
                }
                writer.RenderBeginTag(Tag);
                foreach (Control child in Controls) {
                    child.RenderControl(writer);
                }
                writer.RenderEndTag();
            }
        }
    }
}

Abaixo, a sua aplicação.

<%@ Register Namespace="LeftSlipper" TagPrefix="ls" %>
...
<ls:CustomUpdatePanel runat="server" ID="CustomUpdatePanel1"
    CssClass="greenPanel" Tag="B">
    <ContentTemplate>
        <asp:Label ID="Label1" runat="server"></asp:Label>
        <asp:Button ID="Button1" runat="server" Text="Button" />
    </ContentTemplate>
</ls:CustomUpdatePanel>

ls:CustomUpdatePanel runat="server" ID="CustomUpdatePanel1"
    CssClass="greenPanel" Tag="B">
    <ContentTemplate>
        <asp:Label ID="Label1" runat="server"></asp:Label>
        <asp:Button ID="Button1" runat="server" Text="Button" />
    </ContentTemplate>
</ls:CustomUpdatePanel>
And here's what got rendered with these settings (actual times may vary!):

<b id="CustomUpdatePanel1" class="greenPanel">
    <span id="Label1">12/6/2006 9:18:07 PM</span>
    <input type="submit" name="Button1" value="Button" id="Button1" />
</b>

Este mesmo princípio pode ser utilizado para modificar qualquer controle.

Fonte: Eilon Lipton's Blog