I wrote about why GDI+ is not supported in a service a couple of years ago but this is still a debated topic (or I should better say a misunderstood topic), then WPF (Windows Presentation Foundation) came into the game and it brought some more uncertainties with it.
Recently I worked on a custom application which basically was meant to made of a Windows Form client and a WCF Web Service used to get some data from a database, create a 3D image (a sort of chart) and send it to the client as a jpg image; the graphic part was done using classes such as Viewport3D, PerspectiveCamera, ModelVisual3D, RenderTargetBitmap and others taken from the System.Windows.Media.* and System.Drawing namespaces.
Everything was working fine as long as the project was being developed and debugged against the ASP.NET Development Server (Cassini), but when the project was deployed to IIS7 the image returned was missing the 3D part and the fancy transparency and shadow effects added through WPF classes, it merely had a blue background, nothing more.
The first thing I thought to, are the differences between Cassini and IIS: the former is a process (not a service), it runs under the credentials of the logged on user and also has Administrator rights as Visual Studio does (or should, to work properly). In IIS7 is really easy to test if this was a permission or missing user desktop issue, I changed the Identity property for the application pool to run it under my domain account and changed the Load User Profile to true to have access the my system variables, registry etc…
No luck.
I’ll save you the other tests I’ve done, what is important to know is that there actually is another difference between IIS and Cassini on Windows 2008 / Vista, or I should better say one of the differences I have mentioned before has an important implication on those platforms: services now run in an isolated session (Session 0) while processes run in the session assigned to the user logged on the machine:
Application Compatibility: Session 0 Isolation In Windows XP®, Windows Server® 2003, and earlier versions of the Windows® operating system, all services run in the same session as the first user who logs on to the console. This session is called Session 0. Running services and user applications together in Session 0 poses a security risk because services run at elevated privilege and therefore are targets for malicious agents that are looking for a means to elevate their own privilege levels. The Windows Vista® and Windows Server® 2008 operating systems mitigate this security risk by isolating services in Session 0 and making Session 0 non-interactive. In Windows Vista and Windows Server 2008, only system processes and services run in Session 0. The first user logs on to Session 1, and subsequent users log on to subsequent sessions. This approach means that services never run in the same session as users' applications and are therefore protected from attacks that originate in application code.
Application Compatibility: Session 0 Isolation
In Windows XP®, Windows Server® 2003, and earlier versions of the Windows® operating system, all services run in the same session as the first user who logs on to the console. This session is called Session 0. Running services and user applications together in Session 0 poses a security risk because services run at elevated privilege and therefore are targets for malicious agents that are looking for a means to elevate their own privilege levels.
The Windows Vista® and Windows Server® 2008 operating systems mitigate this security risk by isolating services in Session 0 and making Session 0 non-interactive. In Windows Vista and Windows Server 2008, only system processes and services run in Session 0. The first user logs on to Session 1, and subsequent users log on to subsequent sessions. This approach means that services never run in the same session as users' applications and are therefore protected from attacks that originate in application code.
Specific examples of affected driver classes include: Printer drivers, which are loaded by the spooler service All drivers authored with the User Mode Driver Framework (UMDF) because these drivers are hosted by a process in Session 0
Specific examples of affected driver classes include:
Application classes affected by this feature include: Services that create UI A service that tries to use window-message functions such as SendMessage and PostMessage to communicate with an application Applications creating globally named objects
Application classes affected by this feature include:
This can easily been verified adding the “Session ID” column in Task Manager: as you can see, the WinForm client (Test3DInService.exe) is running in Session 2, w3wp.exe is running in Session 0 (despite the fact that the process is running as my domain account and it has loaded the user profile) and finally WebDev.WebServer.exe (Cassini) is running in Session 2:
Even more important is this document: Impact of Session 0 Isolation on Services and Drivers in Windows Vista:
Because Session 0 is no longer a user session in Windows Vista, services that are running in Session 0 do not have access to the video driver in Windows Vista. This means that any attempt that a service makes to render graphics fails. In current builds of Windows Vista, querying the display resolution and color depth in Session 0 reports the correct results for the system up to a maximum of 1920x1200 at 32 bits per pixel (bpp).
Got it!
Well, almost, but we’re pretty close. I said the blue background was actually created, and other simple 2D shapes could be successfully rendered as well, so the statement “any attempt that a service makes to render graphic fails” is not fully correct, is should read as “any attempt that a service makes to render graphics that relies on the video driver fails”. Not all graphics need to interact with the video driver so generally speaking, GDI can be used (that’s a different set of libraries than GDI+) and “simple” graphic rendering should normally work fine (as if did in my sample with 2D graphics): the trick is never call (or try to call) into the video driver. This unfortunately means the fancy WPF shadows, transparencies and effects are off limits from a service. It is possible to use BIDs and BIDSection.
I could think to 3 possible solutions (in order of my preference) to this case, not all of them very nice but if nothing else can be done… if you have anything to add, feel free :
Carlo
Davvero interessante! Dopo 2 giorni di estenuanti ricerche sul perchè non è possibile fare il render di uno Xaml 3d, ho trovato con mio rammarico la motivazione.