public MapPoint ObserverPoint {get; set;}

var losParams = new LineOfSightParams();
losParams.ObserverPoint = observerPoint;
losParams.TargetPoint = targetPoint;

// add offsets if appropriate
// losParams.ObserverHeightOffset = observerOffset;
// losParams.TargetHeightOffset = targerOffset;

// set output spatial reference
losParams.OutputSpatialReference = MapView.Active.Map.SpatialReference;

LineOfSightResult results = null;
try
{
  if (tinLayer.CanGetLineOfSight(losParams))
    results = tinLayer.GetLineOfSight(losParams);
}
catch (Exception ex)
{
  // log exception message
}

if (results != null)
{
  bool targetIsVisibleFromObserverPoint = results.IsTargetVisibleFromObserverPoint;
  //These properties are not used. They will always be false
  // results.IsTargetVisibleFromVisibleLine;
  // results.IsTargetVisibleFromInvisibleLine;


  if (results.VisibleLine != null)
    MapView.Active.AddOverlay(results.VisibleLine, visibleLineSymbol.MakeSymbolReference());
  if (results.InvisibleLine != null)
    MapView.Active.AddOverlay(results.VisibleLine, invisibleLineSymbol.MakeSymbolReference());
  if (results.ObstructionPoint != null)
    MapView.Active.AddOverlay(results.ObstructionPoint, obstructionPointSymbol.MakeSymbolReference());
}