ASP.NET supports a little known feature called “server-side comments” that you can use to completely disable code/controls/html in a page. Server-side comments in ASP.NET are delimited using a <%-- --%> syntax.

[http://weblogs.asp.net/scottgu/archive/2006/07/09/ Tip_2F00_Trick_3A00_-Using-Server-Side-Comments-with-ASP.NET-2.0-.aspx]