Set Binding.FallbackValue depending on DesignMode and RuntimeMode

Hi I try to set a custom class to handle the FallbackValue depending on the
Design- and RuntimeMode.

The basic Idea:
We build Xaml to include in other Xamls.
For example an Adress.xaml that contains Name, Surname, City etc.
This Xaml we "recycle" (include) in other Xamls.
For examle into a Xaml to show patients, into a Xaml to show employers, and
so on
(all these "Persons" have an Adresse)

The DataContext is defined Outside in the "Root Xaml".
If we now want design the lowest Xaml (Adress.xaml) in Blend there is no
Datacontext defined.
To simulate datas we set the FallbackValue.

This works fine at designtime.
At runtime it works also.
But not if a datafield in the database contains a DbNull value.
In this case our "DesignTime" Value is shown.

In the attached VB sample I try to set the FallBackValue depending of the
Design- or RuntimeMode.

What I don't now is in what kind af way I have to set the return Value of my
"MyFallbackValueSource" Class
Override the ToString Function dons't help.

How can tell me how can return a value?
I my sample i return a String, can I also retrun numbers?

Thanks for support

Best regards
Horst