public double RotationAngle {get; set;}

// retrieve the curve's properties
EllipticArcBuilderEx builder = new EllipticArcBuilderEx(arcSegment);
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;