public CIMPolygonSymbol ConstructPolygonSymbol( 
   CIMColor color,
   SimpleFillStyle fillStyle,
   CIMStroke outline
)

//Create a simple 2D rectangle graphic and apply simple fill and
//outline symbols.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2D envelope geometry
  Coordinate2D rec_ll = new Coordinate2D(1.0, 4.75);
  Coordinate2D rec_ur = new Coordinate2D(3.0, 5.75);
  //At 2.x - Envelope rec_env = EnvelopeBuilder.CreateEnvelope(rec_ll, rec_ur);
  Envelope rec_env = EnvelopeBuilderEx.CreateEnvelope(rec_ll, rec_ur);

  //Set symbolology, create and add element to layout
  CIMStroke outline = SymbolFactory.Instance.ConstructStroke(
    ColorFactory.Instance.BlackRGB, 5.0, SimpleLineStyle.Solid);
  CIMPolygonSymbol polySym = SymbolFactory.Instance.ConstructPolygonSymbol(
    ColorFactory.Instance.GreenRGB, SimpleFillStyle.DiagonalCross, outline);

  //At 2.x - GraphicElement recElm =
  //           LayoutElementFactory.Instance.CreateRectangleGraphicElement(
  //                                                  layout, rec_env, polySym);
  //         recElm.SetName("New Rectangle");
  //
  GraphicElement recElm = ElementFactory.Instance.CreateGraphicElement(
    container, rec_env, polySym, "New Rectangle");
  //Or use Predefined shape
  GraphicElement recElm2 = ElementFactory.Instance.CreatePredefinedShapeGraphicElement(
                            container, PredefinedShape.Rectangle, rec_env, polySym, 
                            "New Rectangle2");
});

CIMStroke outline = SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.BlueRGB, 2.0, SimpleLineStyle.Solid);
CIMPolygonSymbol fillWithOutline = SymbolFactory.Instance.ConstructPolygonSymbol(ColorFactory.Instance.RedRGB, SimpleFillStyle.Solid, outline);

CIMPolygonSymbol fillWithoutOutline = SymbolFactory.Instance.ConstructPolygonSymbol(ColorFactory.Instance.RedRGB, SimpleFillStyle.Solid, null);