Polyline pl = new Polyline ();
pl.Points = new System.Windows.Point[] { new Point (0, 0), new Point (0, 200), new Point (200, 200), new Point (200, 0), new Point (0, 0) };
pl.Stroke = new SolidColorBrush (Color.FromRgb (70, 70, 70));
pl.StrokeThickness = 7;
pl.StrokeLineJoin = PenLineJoin.Round;

Children.Add (pl);