~/Set property System.Windows.ResourceDictionary.DeferrableContent threw an exception error in WPF

20 December 2013

Ok so I’m working on a WPF launcher that loads a shell that loads a client. Three different appdomains. The launcher could load the shell. The shell could load the client. But when I tried to get the launcher to load the shell then the shell to load the client, I would get this lovely exception when the client instance was created:

Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception

Googling the exception gave a lot of talk about duplicate values in the resource dictionaries, which I burned a pile of time on. Then I noticed that the exception had an inner exception which had an inner exception and so on, and the final exception was about a UI component being created in a non-UI thread (InvalidOperationException, “The calling thread must be STA, because many UI components require this.”).

Turned out I was doing this in the shell, to keep its UI responsive:

var launchCompletedResetEvent = new AutoResetEvent(false);
var worker = new BackgroundWorker();
worker.DoWork += (s,e) => {
    ... // download and extract the client
};
worker.RunWorkerCompleted += (s,e) => {
    ... // launch the client
    launchCompletedResetEvent.Set();
};
worker.RunWorkerAsync();
launchCompletedResetEvent.WaitOne();

The launchCompletedResetEvent mutex is so the shell didn’t terminate before launching the client.

This was launching the client in a non-UI thread. For some reason this seems to have worked when running the shell directly but not when running the shell through the launcher. Maybe when I ran the shell directly I was bypassing the download step and launching the client on the main thread. Actually that makes sense.

I just had to move the client launch to after the WaitOne to get everything working:

var launchCompletedResetEvent = new AutoResetEvent(false);
var worker = new BackgroundWorker();
worker.DoWork += (s,e) => {
    ... // download and extract the client
};
worker.RunWorkerCompleted += (s,e) => {
    launchCompletedResetEvent.Set();
};
worker.RunWorkerAsync();
launchCompletedResetEvent.WaitOne();
... // launch the client