public static EllipticArcSegment CreateCircle( 
   Coordinate2D centerPt,
   double radius,
   ArcOrientation orientation,
   SpatialReference spatialReference
)

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

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

// Builder convenience methods don't need to run on the MCT.
EllipticArcSegment circle = EllipticArcBuilderEx.CreateCircle(centerPtCoord, 2, ArcOrientation.ArcClockwise);
// 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

// BuilderEx constructors need to run on the MCT.
EllipticArcBuilderEx builder = new EllipticArcBuilderEx(centerPtCoord, 2, ArcOrientation.ArcClockwise);
EllipticArcSegment otherCircle = builder.ToSegment();

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

//Build geometry
Coordinate2D center = new Coordinate2D(2, 4);
EllipticArcSegment circle_seg = EllipticArcBuilderEx.CreateCircle(
  new Coordinate2D(2, 4), 0.5, ArcOrientation.ArcClockwise, null);
var circle_poly = PolygonBuilderEx.CreatePolygon(PolylineBuilderEx.CreatePolyline(circle_seg));

//PolylineBuilderEx.CreatePolyline(cir, AttributeFlags.AllAttributes));
//Set symbolology, create and add element to layout
CIMStroke outline = SymbolFactory.Instance.ConstructStroke(
  ColorFactory.Instance.BlackRGB, 2.0, SimpleLineStyle.Dash);

CIMPolygonSymbol circleSym = SymbolFactory.Instance.ConstructPolygonSymbol(
  ColorFactory.Instance.RedRGB, SimpleFillStyle.Solid, outline);
SymbolFactory.Instance.ConstructPolygonSymbol(null,
  SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.RedRGB, 2));

var circleGraphic = GraphicFactory.Instance.CreateSimpleGraphic(circle_poly, circleSym);

//Make an element to add to GraphicsLayer or Layout
//var elemInfo = new ElementInfo() { Anchor = Anchor.CenterPoint };
//GraphicElement cirElm = ElementFactory.Instance.CreateGraphicElement(
//  container, circleGraphic, "New Circle", true, elemInfo);