public CIMPolygonSymbol ConstructPolygonSymbol()

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

  //Note: Must be on QueuedTask.Run

  //Build 2D envelope geometry
  Coordinate2D rec_ll = new Coordinate2D(1.0, 4.75);
  Coordinate2D rec_ur = new Coordinate2D(3.0, 5.75);

  Envelope rec_env = EnvelopeBuilderEx.CreateEnvelope(rec_ll, rec_ur);

  //Set symbology, 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);


  GraphicElement recElm = ElementFactory.Instance.CreateGraphicElement(
    layout, rec_env, polySym, "New Rectangle");
  //Or use Predefined shape
  GraphicElement recElm2 = ElementFactory.Instance.CreatePredefinedShapeGraphicElement(
                            layout, PredefinedShape.Rectangle, rec_env, polySym,
                            "New Rectangle2");
}

/// <summary>
/// Create a procedural symbol that can be applied to a polygon building footprint layer
/// ![ProceduralSymbol](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-procedural.png)        
/// Note: The rule package used in this method can be obtained from the Sample Data included in the arcgis-pro-sdk-community-samples repository.
/// </summary>    
{
  //Note: Run withing QueuedTask
  string _rulePkgPath = @"C:\Data\RulePackages\Venice_2014.rpk";
  //Polygon symbol to hold the procedural layer
  var proceduralSymbol = SymbolFactory.Instance.ConstructPolygonSymbol();

  //Array of layers to hold a procedural symbol layer
  CIMSymbolLayer[] proceduralSymbolLyr =
        {
            new CIMProceduralSymbolLayer()
            {
                PrimitiveName = "Venice Rule package 2014",
                RulePackage = _rulePkgPath,
                RulePackageName = "Venice_2014",
            }
  };
  proceduralSymbol.SymbolLayers = proceduralSymbolLyr;
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = proceduralSymbol.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}