Marc Melvin’s Blog

    static public class ExtensionHelpers
    {
        static public void AddAssemblyResource(this Application app, string assemblyName, string path)
        {
            if (!UriParser.IsKnownScheme("pack"))
                UriParser.Register(new GenericUriParser(GenericUriParserOptions.GenericAuthority), "pack", -1);
 
            ResourceDictionary dict = new ResourceDictionary();
            Uri uri = new Uri("/" + assemblyName + ";component/" + path, UriKind.Relative);
            dict.Source = uri;
            app.Resources.MergedDictionaries.Add(dict);
        }
    }

App.Current.AddAssemblyResource("My.Assembly.Name", "Path/To/Resource.xaml");