public PlacementOnLineRelativeTo RelativeTo {get; set;}

{
      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)
}
      };
  }