public class CIMLineGraphic : CIMShapeGraphic, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

//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);