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() };
    });
}