Your Path
called "Forme" has its geometry defined using the Path Mini-Language right?
This type of Geometry cannot be share by multiple elements.
The work-around is store the path data as a string in a ResourceDictionary
accessible to both your "Forme" element and "bouton1" then assign it using StaticResource
. Something like:-
<StackPanel>
<StackPanel.Resources>
<sys:String x:Key="MyPath">M 10,100 C 10,300 300,-200 300,100</sys:String>
</StackPanel.Resources>
<Button x:Name="btn" Content="Button" Height="150" Clip="{StaticResource MyPath}" />
<Path Data="{StaticResource MyPath}" Stroke="Black" StrokeThickness="2" />
</StackPanel>
The painful downside is that the VS2010 designer doesn't grasp this and therefore doesn't apply the path. You would need to run the app to visually see the results.