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

/// <summary>
/// Create a polygon symbol with a diagonal cross hatch fill. <br/>
/// ![PolygonSymbolDiagonalCrossHatch](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-diagonal-crosshatch.png)
/// </summary>
{
  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 =
    [
      outline, .. diagonalCross
    ];
  //This is the polygon symbol with a diagonal cross hatch fill
  CIMPolygonSymbol diagonalCrossHatchFillSymbol = new()
  {
    SymbolLayers = [.. symbolLayers]
  };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = diagonalCrossHatchFillSymbol.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}

/// <summary>
/// Create a polygon symbol using the ConstructHatchFill method . <br/>
/// ![PolygonSymbolDiagonalCrossHatch](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/ConstructHatchFill.png)
/// </summary>
{
  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()
  {
    hatchFill
  };
  //This is the polygon symbol with a diagonal cross hatch fill
  CIMPolygonSymbol crossHatchPolygonSymbol = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = crossHatchPolygonSymbol.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}

/// <summary>
/// Create a polygon symbol with a dash dot fill. <br/>
/// ![PolygonSymbolDashDot](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-dash-dot.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  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 dashDot = SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.RedRGB, 1.0, SimpleLineStyle.DashDotDot);
  //Mimic cross hatch
  CIMFill[] solidColorHatch =
        {

           new CIMHatchFill()
          {
              Enable = true,
              Rotation = 0.0,
              Separation = 2.5,
              LineSymbol = new CIMLineSymbol(){SymbolLayers = new CIMSymbolLayer[1] {dashDot } }
          },
           new CIMSolidFill()
          {
              Enable = true,
              Color = ColorFactory.Instance.CreateRGBColor(255, 255, 0)
          },
};
  List<CIMSymbolLayer> symbolLayers = [outline, .. solidColorHatch];
  //This is the polygon symbol with a dash dot fill
  CIMPolygonSymbol dashDotFillPolygon = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = dashDotFillPolygon.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}

/// <summary>
/// Create a polygon symbol with a gradient color fill. <br/>
/// ![PolygonSymbolGradientColor](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-gradient-color.png)
/// 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
/// </summary>
{
  //Note: Run withing QueuedTask
  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
    };

  CIMPolygonSymbol gradientColorFillPolygon = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = gradientColorFillPolygon.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}

/// <summary>
/// Create a polygon symbol using the ConstructGradientFill method. Constructs a gradient fill using the specified color ramp. <br/>
/// ![PolygonSymbolColorRamp](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolColorRamp.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  //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
    };
  CIMPolygonSymbol gradientColorRampSymbol = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = gradientColorRampSymbol.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);

  //Helper method to get a color ramp from the "ArcGIS Colors" style.
  static CIMColorRamp GetColorRamp()
  {
    //Get a ColorRamp
    StyleProjectItem style =
            Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS Colors");
    var colorRampList = style.SearchColorRamps("Heat Map 4 - Semitransparent");

    CIMColorRamp colorRamp = colorRampList[0].ColorRamp;
    return colorRamp;
  }
}