public CIMSymbol Symbol {get; set;}

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

//Build geometry
List<Coordinate2D> plCoords = new List<Coordinate2D>();
plCoords.Add(new Coordinate2D(1, 8.5));
plCoords.Add(new Coordinate2D(1.66, 9));
plCoords.Add(new Coordinate2D(2.33, 8.1));
plCoords.Add(new Coordinate2D(3, 8.5));
Polyline linePl = PolylineBuilderEx.CreatePolyline(plCoords);

//Reference a line symbol in a style
var ProjectStyles = Project.Current.GetItems<StyleProjectItem>();
StyleProjectItem style = ProjectStyles.First(x => x.Name == "ArcGIS 2D");
var symStyle = style.SearchSymbols(StyleItemType.LineSymbol, "Line with 2 Markers")[0];
CIMLineSymbol lineSym = symStyle.Symbol as CIMLineSymbol;
lineSym.SetSize(20);

//Set symbolology, create and add element to layout
//CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);
ElementFactory.Instance.CreateGraphicElement(
  container, linePl, lineSym, "New Line");

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

//Build geometry
Coordinate2D coord2D = new Coordinate2D(2.0, 10.0);

//Reference a point symbol in a style
StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>()
         .FirstOrDefault(item => item.Name == "ArcGIS 2D");
SymbolStyleItem symStyleItm = stylePrjItm.SearchSymbols(
                      StyleItemType.PointSymbol, "City Hall")[0];
CIMPointSymbol pointSym = symStyleItm.Symbol as CIMPointSymbol;
pointSym.SetSize(50);

var elemInfo = new ElementInfo()
{
  CustomProperties = new List<CIMStringMap>() {
     new CIMStringMap() { Key = "Key1", Value = "Value1"},
     new CIMStringMap() { Key = "Key2", Value = "Value2"}
   },
  Anchor = Anchor.TopRightCorner,
  Rotation = 45.0
};

var graphic = GraphicFactory.Instance.CreateSimpleGraphic(
                              coord2D.ToMapPoint(), pointSym);

ElementFactory.Instance.CreateGraphicElement(
  container, graphic, "New Point", true, elemInfo);

SymbolStyleItem symbolItem = null;
CIMSymbol symbol = await QueuedTask.Run<CIMSymbol>(() =>
{
  return symbolItem.Symbol;
});

//Note: call within QueuedTask.Run()
CIMSymbol symbol = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 1.0, SimpleMarkerStyle.Circle);
  //You can generate a swatch for a text symbols also.
  var si = new SymbolStyleItem()
  {
      Symbol = symbol,
      PatchHeight = 64,
      PatchWidth = 64
  };
  return si.PreviewImage;

public Task SetFeatureLayerSymbolFromStyleItemAsync(
           FeatureLayer ftrLayer, SymbolStyleItem symbolItem)
{
  if (ftrLayer == null || symbolItem == null)
    throw new System.ArgumentNullException();

  return QueuedTask.Run(() =>
  {
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = ftrLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
      return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbol = symbolItem.Symbol;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbol.SetRealWorldUnits(ftrLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbol.MakeSymbolReference();
    //Update the feature layer renderer
    ftrLayer.SetRenderer(currentRenderer);
  });
}

  // var map = MapView.Active.Map;
  // if (map == null)
  //        return;
  // var pointFeatureLayer =
  //       map.GetLayersAsFlattenedList()
  //          .OfType<FeatureLayer>()
  //         .Where(fl => fl.ShapeType == esriGeometryType.esriGeometryPoint);
  //   await ApplySymbolToFeatureLayerAsync(pointFeatureLayer.FirstOrDefault(), "Fire Station");

  public Task ApplySymbolToFeatureLayerAsync(FeatureLayer featureLayer, string symbolName)
  {
    return QueuedTask.Run(async () =>
    {
      //Get the ArcGIS 2D System style from the Project
      var arcGIS2DStyle =
Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 2D");

      //Search for the symbolName style items within the ArcGIS 2D style project item.
      var items = await QueuedTask.Run(() =>
      arcGIS2DStyle.SearchSymbols(StyleItemType.PointSymbol, symbolName));

      //Gets the CIMSymbol
      CIMSymbol symbol = items.FirstOrDefault().Symbol;

      //Get the renderer of the point feature layer
      CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

      //Set symbol's real world setting to be the same as that of the feature layer
      symbol.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

      //Apply the symbol to the feature layer's current renderer
      renderer.Symbol = symbol.MakeSymbolReference();

      //Appy the renderer to the feature layer
      featureLayer.SetRenderer(renderer);
    });
  }