//These methods must be called within the lambda passed to QueuedTask.Run
var markerTriangle = SymbolFactory.Instance.ConstructMarker(ColorFactory.Instance.RedRGB, 12, SimpleMarkerStyle.Triangle);
markerTriangle.Rotation = -90; // or -90
markerTriangle.MarkerPlacement = new CIMMarkerPlacementOnLine() { AngleToLine = true, RelativeTo = PlacementOnLineRelativeTo.LineEnd };
var lineSymbolWithArrow = new CIMLineSymbol()
{
SymbolLayers = new CIMSymbolLayer[2] { markerTriangle,
SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.RedRGB, 2)
}
};