public void GetAxes( 
   out double semiMajorAxis,
   out double semiMinorAxis
)

// Construct an elliptic arc centered at (1,1), startAngle = 0, centralAngle = PI/2, 
// rotationAngle = 0, semiMajorAxis = 1, minorMajorRatio = 0.5.
// Use a builder convenience method or use a builder constructor.

Coordinate2D centerPt = new Coordinate2D(1, 1);

// Builder convenience methods don't need to run on the MCT.
EllipticArcSegment circularArc = EllipticArcBuilder.CreateEllipticArcSegment(centerPt, 0, Math.PI / 2, 0, 1, 0.5);

double semiMajor;
double semiMinor;
circularArc.GetAxes(out semiMajor, out semiMinor);
// semiMajor = 1, semiMinor = 0.5

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (EllipticArcBuilder cab = new EllipticArcBuilder(centerPt, 0, Math.PI / 2, 0, 1, 0.5))
  {
    // do something with the builder

    EllipticArcSegment otherCircularArc = cab.ToSegment();
  }
});