ASP.NET: Developing and Using a Custom Server Control

In general, when your control derives from WebControl and renders a single element, you should override the RenderContents method (and not the Render method) to render content within the control's tags. The Render method of WebControl invokes RenderContents after rendering the opening tag for the control and its style attributes. If you override the Render method to write contents, your control will lose the style-rendering logic that is built into the Render method of WebControl.