public double RotationAngle {get; set;}

{
  // retrieve the curve's properties
  EllipticArcBuilderEx builder = new EllipticArcBuilderEx(ellipticArcSegment);
  MapPoint startPt = builder.StartPoint;
  MapPoint endPt = builder.EndPoint;
  Coordinate2D centerPt = builder.CenterPoint;
  bool isCircular = builder.IsCircular;
  bool isMinor = builder.IsMinor;
  double startAngle = builder.StartAngle;
  double endAngle = builder.EndAngle;
  double centralAngle = builder.CentralAngle;
  double rotationAngle = builder.RotationAngle;
  ArcOrientation orientation = builder.Orientation;
}