Prism (V2) RegionManager fails to recognize non-WPF applications

In the recent change of Prism (V2), the way how the Region Manager detects that it’s running in a Design Mode or not, has been changed. This has ultimately broke all my Prism plug-ins that were hosted by non-WPF applications. One of the examples of hosts for such a plug-in, is an Office AddIn.

It took me about 6 hours to track down this problem and, to my amazement, I’ve found the place where it broke:

private static bool RegionManager::IsInDesignMode(DependencyObject element)
{
    // Due to a known issue in Cider, GetIsInDesignMode attached property value is not enough to know if it's in design mode.
    return DesignerProperties.GetIsInDesignMode(element) || Application.Current == null
        || Application.Current.GetType() == typeof(Application);
}

The reason is that for the non-WPF application the Application.Current is NULL !!!

The solution

1. 1. Create an empty class that will inherit from System.Windows.Applicatoin. (Name doesn’t matter):

   public class MyApp : System.Windows.Application 	{ 	}

    

2. At the point of entry to a plug-in execute the following code:

   if (System.Windows.Application.Current == null)
   {
       // create the Application object
       new MyApp();
   }

This is it – now you have an Application.Current that is not null and it’s not equal to typeof(Application).

If, by any chance, you’d like to merge your application resources – do the following:

 // merge in your application resources
System.Windows.Application.Current.Resources.MergedDictionaries.Add(
    System.Windows.Application.LoadComponent(
        new Uri("MyLibrary;component/Resources/MyResourceDictionary.xaml",
        UriKind.Relative)) as ResourceDictionary);  

For more information check DR.WPF’s article (http://www.drwpf.com/blog/Home/tabid/36/EntryID/10/Default.aspx) that beautifully explains the reasons and workarounds for the problem, described above.