public void QueryCoords( 
   out Coordinate2D centerPt,
   out double startAngle,
   out double centralAngle,
   out double rotationAngle,
   out double semiMajorAxis,
   out double semiMinorAxis
)

// Construct a circle with center at (-1,-1), radius = 2, and oriented clockwise.
// Use a builder convenience method or use a builder constructor.

Coordinate2D centerPtCoord = new Coordinate2D(-1, -1);

// Builder convenience methods don't need to run on the MCT.
EllipticArcSegment circle = EllipticArcBuilder.CreateEllipticArcSegment(centerPtCoord, 2, esriArcOrientation.esriArcClockwise);
// circle.IsCircular = true
// circle.IsCounterClockwise = false
// circle.IsMinor = false

double startAngle, rotationAngle, centralAngle, semiMajor, semiMinor;
Coordinate2D actualCenterPt;
circle.QueryCoords(out actualCenterPt, out startAngle, out centralAngle, out rotationAngle, out semiMajor, out semiMinor);

// semiMajor = 2.0
// semiMinor = 2.0
// startAngle = PI/2
// centralAngle = -2*PI
// rotationAngle = 0
// endAngle = PI/2

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (EllipticArcBuilder builder = new EllipticArcBuilder(centerPtCoord, 2, esriArcOrientation.esriArcClockwise))
  {
    // do something with the builder

    EllipticArcSegment otherCircle = builder.ToSegment();
  }
});