public override string ToJson( 
   JsonSerializationSettings settings
)

{
  // Note: must be called on the QueuedTask
  {
    //On the QueuedTask
    //Place a line symbol using the extent's lower left and upper right corner.
    var extent = MapView.Active.Extent;
    //get the lower left corner of the extent
    var pointFromCoordinates = new Coordinate2D(extent.XMin, extent.YMin);
    //get the upper right corner of the extent
    var pointToCoordinates = new Coordinate2D(extent.XMax, extent.YMax);
    List<Coordinate2D> points = new List<Coordinate2D> { pointFromCoordinates, pointToCoordinates };
    //create the polyline
    var lineSegment = PolylineBuilderEx.CreatePolyline(points);

    //specify a symbol
    var line_symbol = SymbolFactory.Instance.ConstructLineSymbol(
                          ColorFactory.Instance.GreenRGB);
    //create a CIMGraphic 
    var graphic = new CIMLineGraphic()
    {
      Symbol = line_symbol.MakeSymbolReference(),
      Line = lineSegment,
    };
    graphicsLayer.AddElement(graphic);
  }
}