CIMStroke ConstructStroke( 
   CIMColor color,
   double width,
   SimpleLineStyle lineStyle
)

/// <summary>
/// Create a polygon symbol with a diagonal cross hatch fill. <br/>
/// ![PolygonSymbolDiagonalCrossHatch](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/polygon-diagonal-crosshatch.png)
/// </summary>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateDiagonalCrossPolygonAsync()
{
  return QueuedTask.Run<CIMPolygonSymbol>(() =>
  {
    var trans = 50.0;//semi transparent
    CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(0, 0, 0, trans), 2.0, SimpleLineStyle.Solid);

    //Stroke for the fill
    var solid = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(255, 0, 0, trans), 1.0, SimpleLineStyle.Solid);

    //Mimic cross hatch
    CIMFill[] diagonalCross =
              {
                new CIMHatchFill() {
                    Enable = true,
                    Rotation = 45.0,
                    Separation = 5.0,
                    LineSymbol = new CIMLineSymbol() { SymbolLayers = new CIMSymbolLayer[1] { solid } }
                },
                new CIMHatchFill() {
                    Enable = true,
                    Rotation = -45.0,
                    Separation = 5.0,
                    LineSymbol = new CIMLineSymbol() { SymbolLayers = new CIMSymbolLayer[1] { solid } }
                }
      };
    List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
      {
                outline
      };
    foreach (var fill in diagonalCross)
      symbolLayers.Add(fill);
    return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  });
}

/// <summary>
/// Create a polygon symbol using the ConstructHatchFill method . <br/>
/// ![PolygonSymbolDiagonalCrossHatch](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/ConstructHatchFill.png)
/// </summary>
/// <returns></returns>
private static Task<CIMPolygonSymbol> CreateHatchFillPolygonAsync()
{
  return QueuedTask.Run<CIMPolygonSymbol>(() =>
  {
    CIMStroke lineStroke = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(51, 51, 51, 60), 4, SimpleLineStyle.Solid);
    //gradient
    var hatchFill = SymbolFactory.Instance.ConstructHatchFill(lineStroke, 45, 6, 0);

    List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
    {
      hatchFill
    };
    return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  });
}

/// <summary>
/// Create a polygon symbol with a gradient color fill. <br/>
/// ![PolygonSymbolGradientColor](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/polygon-gradient-color.png)
/// </summary>
/// <remarks>
/// 1. Create a solid colored stroke with 50% transparency
/// 1. Create a fill using gradient colors red through green
/// 1. Apply both the stroke and fill as a symbol layer array to the new PolygonSymbol
/// </remarks>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateGradientFillAsync()
{
  return QueuedTask.Run<CIMPolygonSymbol>(() =>
  {
    var trans = 50.0;//semi transparent
    CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(0, 0, 0, trans), 2.0, SimpleLineStyle.Solid);
    //Mimic cross hatch
    CIMFill solidColorHatch =
           new CIMGradientFill()
       {
         ColorRamp = ColorFactory.Instance.ConstructColorRamp(ColorRampAlgorithm.LinearContinuous,
                                  ColorFactory.Instance.RedRGB, ColorFactory.Instance.GreenRGB)
       };
    List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
      {
                outline,
                solidColorHatch
      };

    return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  });
}

/// <summary>
/// Create a polygon symbol using the ConstructGradientFill method. Constructs a gradient fill using the specified color ramp. <br/>
/// ![PolygonSymbolColorRamp](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolColorRamp.png)
/// </summary>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateGradientColorRampPolygonAsync()
{
  return QueuedTask.Run<CIMPolygonSymbol>(() =>
  {
    //outine
    CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(49, 49, 49), 2.0, SimpleLineStyle.Solid);

    //gradient fill using a color ramp
    var gradientFill = SymbolFactory.Instance.ConstructGradientFill(GetColorRamp(), GradientFillMethod.Linear);

    List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
      {
                outline,
                gradientFill
      };
    return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  });

}