What are the issues with running WPF across multiple AppDomains on one UI thread?

Question

We are looking at creating a WPF UI that runs across multiple AppDomains. One of the app domains would run the application while the remaining AppDomains would host a series of user controls and logic. The idea, of course, is to sandbox these User Controls and the logic away from the main application.

Here is an example of doing this using MAF/System.AddIn. What are some of the experiences other have had with this? How does this solution handle RoutedEvents/Commands that might occur inside one user control and do these get properly serialized across AppDomains? What about WPF resources? Can they be accessed across AppDomains seamlessly?

Answer 1

Old question, but nonetheless: You will need to have multiple UI threads - one per AppDomain. Create them like this:

        var thread = new Thread(() =>
        {
            var app = new Application();
            app.Run();
        });
        thread.Name = AppDomain.CurrentDomain.FriendlyName;
        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();

The biggest challenge is then that you cannot send FrameworkElements between AppDomains (they are not MarshalByRefObject), but you can use the FrameworkElementAdapters.ViewToContractAdapter() and ContractAdapterToView() methods to work around this limitation. See the FrameworkElementAdapters MSDN page for more details.

Then, once you have this in place, the biggest problem IMHO is that you cannot lay anything on top of the FrameworkElement from the "remote" domain (the classical "airspace problem"). See this thread for more info about this.

Answer 2

I answered a simular question here and edited it for WPF also, you can use an intersting property of how the compisition engine operate's to tail-coat a dispatcher Pump, into one of the rendering contexts. It's a really light weight option.

Also, I'm guessing you know about the enterprise library and unity?

There is a WPF application block so using that pattern is not too painful ;) But don't they say, no pain no gain?

There's also CAB (Composite UI Application Block), ties into unity. The WPF SDK folks have crafted a Silverlight & WPF platform. (a.k.a Prism).

Oh right, also, you asked about Resources? I prefer to load reasources manually in the Application class. One thing I've realized, say you have a ResourceDictionary in a sub-folder and you are loading up MergedDictionaries in that ResourceDictionary. So, if in your Application class, you load "my-res-dir/MergedDictionaryLoader.xaml" (by code or xaml), ALL FUTURE LOADS OF MERGEDDICTIONARIES ARE LOADED FROM "my-res-dir".

Sort of insane if you ask me, I would think that as the process current directory has not changed, you should specify "my-res-dir/foo.xaml" for all your additional directories. However this is not the case (I do not believe this is documented anywhere at least very well and should be considered a bug imho).

So remember, WPF resource dictionary loading is going to be based off of the directory from which the current XAML is in. So you specify Source="foo.xaml" from within your "my-res-dir/MergedDictionaryLoader.xaml". I've even played with the URI pack / absolute syntax, however I've never found that too be much more intuative.