Tracing tracks and presents the execution details about a Web request. For trace data to be visible in a rendered page, you must enable tracing at the page or application level.
Tracing on a page is disabled by default. To enable tracing for a page, use the @ Page directive <% @ Page trace="true" %>. To enable tracing for an entire application, you must enable it in the application's configuration file, Web.config, which resides in the root directory of the application. For more information, see ASP.NET Tracing Overview.