Server Side vs Client Side Comments in ASP.NET

Last week, I was asked to make a change to the name of a channel (folder) in an application using our Content Management Server (CMS). After I did this, I went to their home page and received the dreaded yellow error screen of ASP.net.

After making this change, the application was throwing a null reference exception from a user control that was within comments in the .aspx file. This user control was trying to do something with the old channel name, but was no longer able to find it.

But why was this an issue at all? After all, the code was commented. Yes it was, however it was using client side (HTML) comment “<!– HTML comment –>“, instead of an ASP.NET comment “<%– ASP.NET comment –%>“.

Because “<!– –>” is a client side comment, the browser does not display whatever is in the comment, however the server still handles this code and sends it to the browser. If you have things like server controls, users controls, etc in this type of comment, the server still runs these controls.

As an example, consider this simple aspx file:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title>Test Page</title>
</head>
<body>
  <!-- HTML Comment -->
  <%-- ASP.NET Comment --%>
  <p>Test</p>
</body>
</html>

When you do a view source in your browser on this page, you get:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Test Page</title>
</head>
<body>
  <!-- HTML Comment -->
  <p>Test</p>
</body>
</html>

Notice that the HTML comment does get sent to the browser, however the ASP.NET comment does not. The server ignores everything embedded in the the ASP.NET comment.

On the page I was looking at last week, this caused two major problems. First, we were sending code to the client that wasn’t really necessary. What was commented out was HTML code that the browser would not display, therefore it added a little to the download time for the page.

A bigger problem was this section of code had a couple of user controls in it, and these user controls took a few seconds to run each time this home page was hit. Therefore this was a very slow page to load, and the slowness was caused by the server generating an HTML comment which the user never saw. When I changed this, the load time for the page was greatly improved.

The moral of the story is simple. If you are writing a comment that should be seen in the HTML code, then go ahead and use the client side HTML comment code. However, in many cases, you don’t need/want the user to be able to see the comments. For these cases, you should use the ASP.NET comment code “<%– –%>”, which will cause the server to skip this code. In certain cases, this small change can have a dramatic effect on the performance of your web page.

For more information on this, see Scott Guthrie’s post: Tip/Trick: User Server Side Comments with ASP.NET 2.0

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