Hey everyone I have tried to sum up and come with the basic question and my idea which does not work so far :S
Basicly my question is: The user adds elements togethe
Geometry.Parse (and as far as I can tell, it's back-end StreamGeometry) is indeed missing from the Windows Phone platform, but according to this page: http://msdn.microsoft.com/en-us/library/ms752293(v=vs.110).aspx
WPF provides two classes that provide mini-languages for describing geometric paths: StreamGeometry and PathFigureCollection.
PathFigureCollection is available for Windows Phone, so this looks like the place to check:
http://msdn.microsoft.com/en-us/library/windowsphone/develop/system.windows.media.pathfigure(v=vs.105).aspx
Now you just need to be able to create PathGeometry from the XAML-style markup. It looks like there are examples here for generating a Path from the XAML syntax:
Convert XAML PathGeometry to WPF PathGeometry
Windows Phone 7: How to parse Bezier Path string like in XAML?
Basically something like
string data = "M 100,200 C 100,25 400,350 400,175 H 280";
Path path = XamlReader.Load(" ") as Path;