As a quick solution, I added Dispose calls in our code to force the destructor to run in main thread

I was going to suggest to follow the using pattern:

StreamGeometry geom = new StreamGeometry();

using (StreamGeometryContext ctx = geom.Open())

{

ctx.BeginFigure(...);

ctx.LineTo(...);

ctx.LineTo(...);

}

return geom;

It should automatically call Dispose.

I know, that is my quick fix.

But the problem is still there, the Finalizers in .net is executed in some GC thread. That must be handled correctly.