public CIMFill ConstructHatchFill( 
   CIMStroke lineStroke,
   double angle,
   double separation,
   double offset
)

/// <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() };
  });
}