Since Application.Current
was null in my case, I’ve ended up using this:
var myResourceDictionary = new ResourceDictionary();
myResourceDictionary.Source =
new Uri("/DllName;component/Resources/MyResourceDictionary.xaml",
UriKind.RelativeOrAbsolute);
and then getting the specified key I needed by using
myResourceDictionary["KeyName"] as TypeOfItem
(source)