Wednesday, February 16, 2011

Workaround For VS.NET ASP.NET Design Tab Not Rendering Controls

As of recent, most all of the projects I have been working on are in VS.NET 2010 using .NET Framework 4.0, and several are upgrades from .NET Framework 3.5 VS.NET 2008 solutions. I rarely use the 'Designer' tab to preview the generated controls of my ASP.NET pages, but in some instances I want to see the wizard of a control for configuration (i.e. Object Data Source Control). Not that there isn't code I couldn't just create by hand, but sometimes it is faster to have the code auto-generated for me. In this case I needed to have the control render in Design view.

On this note I will say I am not a big fan of the 'Design' tab to view auto-generated pages. I do not believe VS.NET uses the same rendering engine to show the auto-generated pages as is when the page is rendered in IIS. Therefore early on I found inconsistencies and nuances that showed visual differences between the two (design and run time), so I all but abandoned using the IDE 'Design' view. In fact I think one becomes a more proficient ASP.NET and web developer in general when you can begin to visualize the web pages during coding without needing a 'picture' to see after every line of code is written. Then run the page either in either the local dev server through VS.NET or in IIS to have it render properly, and then use any of the following to break down a page when needed: IE Developer Tools (IE), FireBug Lite (IE or FireFox), Chrome Developer Tools (Chrome).

Back to the problem at hand - I noticed that ALL of my pages (content pages of a single simple Master page) show the message "response is not available in this context" for all of my controls when accessing via the design view. I had tried deleting the page from my project, adding a new page, and copying back in the source and code; same error. I also tried cutting out controls 1 at a time, and clicking "Refresh" in the designer but that was not working either. Clearing all cached pages in the Temporary ASP.NET directories did fix the problem either.

Having all of my pages with the same issue, helped direct me to a class that all pages were inheriting from. In this class there is a method to Override the OnInit() method to run some common functionality. Within this method there was code to set properties on the HttpResponse object. Therein lies the problem. Making calls to manipulate the HttpResponse object and getting the error message: "response is not available in this context".

The fix is simple and straight forward. One way would be just to comment out the code, but this has its downside with no explanation needed. I wanted something more maintenance free. There was a better solution: if there is NO HttpContext avalaible, then do not attempt to access it, or the HttpResponse object. While in the VS.NET IDE and viewing the 'Design' tab, it makes sense that the HttpContext is not available and therefore any related code should be skipped.

'Do not attempt to access the HttpContext if it does not exist (i.e. in Design mode of IDE)
If HttpContext.Current IsNot Nothing Then
'Do not cache the web page
Response.CacheControl = "no-cache"
End If
One interesting thing I found in researching this issue was that cleaning the solution (which clears the \bin of all compiled code) and then viewing the page in the 'Design' tab with the above Response code only (no "If HttpContext.Current IsNot Nothing"), the page would display. As soon as I built the solution, and did not have the workaround above, I would go back to receiving the "response is not available in this context" error. So the IDE must be rendering the page differently based on if it has been compiled or not already, and possibly only showing the rendered page strictly on HTML and not on any precompiled code-behind in this situation. This is lower level details of the VS.NET IDE that I am unsure about. The main point here is to use the logic in the code example above to be more explicit about when that code will be ran, and when it will be skipped.


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