public EllipticArcBuilderEx( 
   MapPoint startPt,
   MapPoint endPt,
   Coordinate2D interiorPt,
   SpatialReference spatialReference
)

// Construct a circular arc from (2, 1) to (1, 2) with interior pt (1 + sqrt(2)/2, 1 + sqrt(2)/2).
// Use a builderEx convenience method or use a builderEx constructor.

MapPoint fromPt = MapPointBuilderEx.CreateMapPoint(2, 1);
MapPoint toPt = MapPointBuilderEx.CreateMapPoint(1, 2);
Coordinate2D interiorPt = new Coordinate2D(1 + Math.Sqrt(2) / 2, 1 + Math.Sqrt(2) / 2);

// BuilderEx convenience methods don't need to run on the MCT.
EllipticArcSegment circularArc = EllipticArcBuilderEx.CreateCircularArc(fromPt, toPt, interiorPt);

// BuilderEx constructors don't need to run on the MCT.
EllipticArcBuilderEx eab = new EllipticArcBuilderEx(fromPt, toPt, interiorPt);
// do something with the builder

EllipticArcSegment anotherCircularArc = eab.ToSegment();

//Must be on QueuedTask.Run(() => { ...

//Build geometry
Coordinate2D center = new Coordinate2D(4.5, 4);
var eabCir = new EllipticArcBuilderEx(center, 0.5, ArcOrientation.ArcClockwise);
var cir = eabCir.ToSegment();

var poly = PolygonBuilderEx.CreatePolygon(
  PolylineBuilderEx.CreatePolyline(cir, AttributeFlags.AllAttributes));

//Set symbolology, create and add element to layout
CIMTextSymbol sym = SymbolFactory.Instance.ConstructTextSymbol(
                ColorFactory.Instance.GreenRGB, 10, "Arial", "Regular");
string text = "Circle, circle, circle";

GraphicElement cirTxtElm = ElementFactory.Instance.CreateTextGraphicElement(
  container, TextType.CircleParagraph, poly, sym, text, "New Circle Text", false);