Code

PointF MiddlePoint(DiagramLink link)
{
	var points = link.ControlPoints;
	if (link.SegmentCount % 2 == 0)
		return points[points.Count / 2];

	return link.Shape == LinkShape.Bezier ?
		Utilities.GetBezierPt(points, link.SegmentCount / 2, 0.5f) :
		InternalUtils.MidPoint(points[points.Count / 2 - 1], points[points.Count / 2]);
}

private void diagram_DrawLink(object sender, DrawLinkEventArgs e)
{
	if (!e.Shadow)
	{
		var point = MiddlePoint(e.Link);
		e.Graphics.FillEllipse(Brushes.Red, point.X - 2, point.Y - 2, 4, 4);
	}
}