Question

 public static IEnumerable<UIElement> Traverse(this UIElementCollection source)
    {
        source.OfType<Grid>().SelectMany(v => Traverse(v.Children));
        //This is the top level.
        foreach (UIElement item in source)
        {
            yield return item;
        }
    }

This never returns anything recursively. I have been around the houses. The Linq chain should call back into the function/extension method but never does. The line does nothing as far as I can tell!

Answer 1

 public static IEnumerable<UIElement> Traverse(this UIElementCollection source)
    {
        //This is the top level.
        foreach (UIElement item in source.OfType<Grid>().SelectMany(v => Traverse(v.Children)).Concat(source.Cast<UIElement>()))
        {
            yield return item;
        }
    }

This has the desired result, not sure it is optimal though!