var area = figure switch 
  {
      Line _      => 0,
      Rectangle r => r.Width * r.Height,
      Circle c    => c.Radius * 2.0 * Math.PI,
      _           => throw new UnknownFigureException(figure)
  };

So more and more of F# is making it's way to C#. Between this and better tuple literals, I'm very happy about this!

(I still need to get around to learning F# for real though, I really love it's brevity.)