Using reflection is not needed (and a bad idead IMHO) because the framework has already utility classes for this (but they're not obvious to find :-).

Here is an answer based on this article: Enumerate Bindings and the LocalValueEnumerator Structure

    public static IEnumerable<DependencyProperty> EnumerateDependencyProperties(this DependencyObject obj)
    {
        if (obj != null)
        {
            LocalValueEnumerator lve = obj.GetLocalValueEnumerator();
            while (lve.MoveNext())
            {
                yield return lve.Current.Property;
            }
        }
    }

Here is another answer based on this other article: Getting list of all dependency/attached properties of an Object that uses MarkupWriter.GetMarkupObjectFor Method.

    public static IEnumerable<DependencyProperty> EnumerateDependencyProperties(object element)
    {
        if (element != null)
        {
            MarkupObject markupObject = MarkupWriter.GetMarkupObjectFor(element);
            if (markupObject != null)
            {
                foreach (MarkupProperty mp in markupObject.Properties)
                {
                    if (mp.DependencyProperty != null)
                        yield return mp.DependencyProperty;
                }
            }
        }
    }

    public static IEnumerable<DependencyProperty> EnumerateAttachedProperties(object element)
    {
        if (element != null)
        {
            MarkupObject markupObject = MarkupWriter.GetMarkupObjectFor(element);
            if (markupObject != null)
            {
                foreach (MarkupProperty mp in markupObject.Properties)
                {
                    if (mp.IsAttached)
                        yield return mp.DependencyProperty;
                }
            }
        }
    }