public double RotationAngle {get;}

{
  // retrieve the curve's control points
  EllipticArcSegment arc = EllipticArcBuilderEx.CreateEllipticArcSegment(ellipticArcSegment);
  MapPoint startPt = arc.StartPoint;
  MapPoint endPt = arc.EndPoint;
  Coordinate2D centerPt = arc.CenterPoint;
  bool isCircular = arc.IsCircular;
  bool isMinor = arc.IsMinor;
  bool isCounterClockwise = arc.IsCounterClockwise;
  bool isCurve = arc.IsCurve;
  double len = arc.Length;
  double ratio = arc.MinorMajorRatio;

  double semiMajorAxis, semiMinorAxis;
  // get the axes
  arc.GetAxes(out semiMajorAxis, out semiMinorAxis);
  // or use the properties
  // semiMajorAxis = arc.SemiMajorAxis;
  // semiMinorAxis = arc.SemiMinorAxis;

  double startAngle, centralAngle, rotationAngle;
  // or use QueryCoords to get complete information
  arc.QueryCoords(out centerPt, out startAngle, out centralAngle, out rotationAngle, out semiMajorAxis, out semiMinorAxis);

  // use properties to get angle information
  //double endAngle = arc.EndAngle;
  //centralAngle = arc.CentralAngle;
  //rotationAngle = arc.RotationAngle;
  //startAngle = arc.StartAngle;
}