public double StartAngle {get; set;}

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // retrieve the curve's control points
  using (EllipticArcBuilder builder = new EllipticArcBuilder(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;
    esriArcOrientation orientation = builder.Orientation;
  }
});