asp.Netページ出力前にhtml(render)を変更する

protected override void Render(HtmlTextWriter writer)
{
    StringWriter output = new StringWriter();
    base.Render(new HtmlTextWriter(output));
    //This is the rendered HTML of your page. Feel free to manipulate it.
    string outputAsString = output.ToString();

    writer.Write(outputAsString);
}