How do I get the width and height of my viewbox

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C#

WPF

Hello everyone,
I need to get the dynamical Height and Width of my Viewbox
The hierarchy of my xaml is like this:

<Grid>
 <DockPanel>
  <Border>
   <ViewBox/>
  </border>
 </DockPanel>
</Grid>

The Viewbox Width is on "Auto" and the Height is binded to a variable in my Controller.
The Class hierarchy is like this:
Xaml <--> Controller
(the xaml.cs is mostly ignored)

Now my problem is that even in the direct code-behind (xaml.cs), I get 0 as ActualWidth and ActualHeight. So I don't even get the standart Width and Height not to mention the dynamical one if I change the dimension of my mainwindow.

If Code-Snippets or information are missing. Please tell me.

Regards
Richard

What I have tried:

I tried to get the Dimensions with in the Code-Behind:
Height/Width (Only get the standard Height and NaN for Width)
ActualHeight/ActualWidth (both are 0)
RenderSize.Height/RenderSize.Width (both are 0)