Get the value of an AttachedProperty of a UI item not (yet) displayed

I'm trying to do the following using MVVM, Binding and AttachedProperty

• Create a ViewObj (System.Windows.Controls.Control derived class)
  • The ViewObj has 1 AttachedProperty named "Order" (OrderProperty) declared in a class named View.
  • The attached property is bound on a property of the ViewModel in the xaml
• Create the ViewModel
• Set the ViewModel as DataContext of the ViewObj

Before the ViewObj is displayed/rendered/etc.

• Get the order in code doing var order = View.GetOrder(ViewObj)

• The ViewObj is displayed and is showing the bound va开发者_开发问答lue ...

If the AttachedProperty is a value and not a binding expression, the value returned by View.GetOrder(ViewObj) is the good one and not the default one.

Any ideas?

EDIT:

I forced the databinding expression to be evaluated using the BindingExpression class. I discovered that the BindingExpression.Status was set to Unattached which seems to explain why it is not working.

I think the binding is attached when the element is attached to the visual tree.

But ... that do not help me a lot with my problem ...

I discovered that (in my case at least), the Binding was Unattached, but the DataContext was set.

So I decided to get the DataContext (the ViewModel) and to work with it.

Any others suggestions are welcome.