c# - Difference between Attached and non-Attached Dependency Properties in Silverlight

Question

Okay Stackers, I've spent a good couple of hours on this question, and I want to know if anybody has a definitive answer.
For all the research I've done, I can't find ANY difference between .Register and .RegisterAttached in Silverlight. Now, before you jump the gun and tell me that .RegisterAttached is used for attaching a DP to another class, try implementing an Attached Dependency Property using DependencyProperty.Register(). I have found not a single difference, and so I am at a loss as to what the difference is.
Furthermore, in my specific case, I'm attempting to extend the functionality of the Grid class, and want to give it some extra properties. As such, I've tried listing passing both typeof(Grid) and typeof(FluidLayoutManager) (the implementing class) as the ownerType parameter and it also seems to make very little difference... (I believe it does make a difference when I pass two custom classes from the same namespace. However when passing a Microsoft defined class vs. a custom class, I always have it showing up in the XAML as a DP of the custom class.)
Any clarification on this topic would be much appreciated, as I'm sitting here scratching my head, and wondering if there is any difference at all, or if Microsoft is just screwing with me once again.

Answer 1

Given the discussions flowing in comments I will try to do this one in plain English:

The main difference between Attached Dependency Properties and Dependency Properties (and therefore between .Register and .RegisterAttached) is that RegisterAttached allows the value to be assigned to any dependency object whereas Register only allows it to be attached to the class passed as the ownerType parameter.

As Haris Hasan mentions (deep in the comment thread), your example is using the only type allowed (i.e. CustomControl) and does not show you that the Attached version can be assigned to ANY dependency object.

e.g. you can do this with your Attached Dependency Property (but not a plain DP):

<Grid local:AttacherClass.ADP1="1" x:Name="LayoutRoot" Background="White">
</Grid>

The best reference for ADPs I can find is this one: http://msdn.microsoft.com/en-us/library/ms749011.aspx

We used ADPs as the basis of a localisation system, so that translations could be parasited onto objects during load rather than using horrendously long bindings. Couldn't do that with DPs

Update:

I would also like to clarify that the parent limitation applies to XAML based use of the attribute. From code the parent limitation apparently does not apply.