I found out the hard way that when using the trace on an ASP.Net page, the interaction of the trace observer is not totally separate from the page. I am working on a page where performance is a big issue, so I have been using Trace extensively to find the weak spots and shore them up. Essentially the page is searching both the Indexing Service and an Oracle database and finding where the two datasets intersect. Because the results are a grid of links, the user can click the links and then use the Back button to return to the results. Of course I was getting the dreaded “Warning: Page has Expired” message after using the back button. The page with the grid is the result of an HTTP post, so the post has expired. Clicking the Refresh button was the only way to get the data back initially. This is bad, as the page already was having performance problems, so rendering it again was not really an option. So I tried setting the page caching options to this:
<%@ OutputCache Duration=”120″ VaryByParam=”None” Location=”Client”%>
This should cause the page to cache on the client. No dice. No matter what I set, the http_cache_control header always reported “no-cache” in the trace results. I tried making the header changes in code. Again, no change. Still reporting “no-cache”. The simple solution to the problem is to turn off tracing on the page. Well, it turns out that with Trace=”true” in the @Page directive, the http_cache_control header is set to “no-cache” no matter what. When the Tracing is off, the “Warning: Page has Expired” message disappears and the page is back in control of setting the headers.
Also, a related tidbit I discovered when trying to solve this is that the page timeout is essentially set to infinity when compiling in debug mode.