Dynamic Application-level resources are not dynamic when hosted in ElementHost

Question

I'm hosting a WPF UserControl in a WinForms container. Now, I want to be able to theme/skin the UserControl. To do this, I've got several resource dictionaries that define the "skins." When my app starts up I create a "new System.Windows.Application()" so that Application.Current exists. To change the theme the old skin is removed and a new skin is merged into the Application level resource dictionary at runtime. However, this does not change any of the dyanamically referenced resources in the UserControl. I tried this in a straight WPF application and it worked just fine. Am I missing something, or is it not possible to do this at all? By the way, if I add a skin into the application resources before the UserControl is initialized it will work but I cannot change the skin after that.

To repo this in the most basic way:

Create a new WinForms application. Add a WPF UserControl to the app. This is simple enough:

<UserControl ...>
   <Grid>
      <Button
         Background="{DynamicResource ButtonBG}"/>
   </Grid>
</UserControl>

Create two ResourceDictionaries, White.xaml and Black.xaml (or whatever) that have a SolidColorBrush with the key ButtonBG with respective color. In Form1.cs, add two Buttons and an ElementHost. Set the child of the ElementHost to an instance of the UserControl we just created. Wire up the buttons to events that swap the skin:

private void White_Click(object sender, EventArgs e)
{
   Application.Current.Resources.MergedDictionaries[0] = 
      (ResourceDictionary)Application.LoadComponent(
         new Uri(@"\WpfThemes;component\White.xaml", UriKind.Relative)));
}

private void Black_Click(object sender, EventArgs e)
{
   Application.Current.Resources.MergedDictionaries[0] = 
      (ResourceDictionary)Application.LoadComponent(
         new Uri(@"\WpfThemes;component\Black.xaml", UriKind.Relative)));
}

In Program.cs, ensure that Application.Current exists and set the initial skin:

[STAThread]
static void Main()
{
   new System.Windows.Application();

   Application.Current.Resources.MergedDictionaries[0] =
      (ResourceDictionary)Application.LoadComponent(
         new Uri(@"\WpfThemes;component\White.xaml", UriKind.Relative)));

   ...
}

Now, when the White button is clicked I would expect the button in the UserControl to turn white and when the Black button is clicked I would expect the button to turn black. This does not happen, however.

Does anyone know why? Is there a solution?

Edit: Idea: Perhaps, if there's a way to force re-evaluation of DynamicResources when the theme changes, that would work.

Thanks, Dusty

Answer 1

I think this may be an overlooked issue in the WPF framework.

From what I can tell via Reflector, it appears that when the Application resource dictionary is catastrophically changed (a change that will likely have wide ranging effects like adding, removing, or replacing a skin), there is code that loops over all of the Windows in the application and forces them to re-evaluate their DynamicResources. However, other elements that I would consider top-level in WPF like ElementHosts do not get the same treatment. This leads to the behavior that I'm experiencing.

My workaround to this issue is to manually go through all of my ElementHosts individually and add, remove, or replace the skin ResourceDictionary file. It's not perfect, but it gets the job done.

Answer 2

Another workaround would be to create a dummy window and specify the content of the elementhost as content. If you look into the Application and check how it handles changes of resourcedictionaries, you see that it only notifies windows..

The only thing you should remind is to never show the window (-> exception), and to close it when disposing the elementhost, so the application can shutdown properly.