public IElement[] Elements {get;}

{
  ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
  {
    //check if the container is a graphics layer - could be a Layout (or even map view)
    if (args.Container is ArcGIS.Desktop.Mapping.GraphicsLayer graphicsLayer)
    {
      //get the total selection count for the container
      var count = args.Elements.Count();
      //Check count - could have been an unselect or clearselect
      if (count > 0)
      {
        //this is a selection or add to selection
        var elems = graphicsLayer.GetSelectedElements();
        //TODO process the selection...
      }
      else
      {
        //This is an unselect or clear selection
        //TODO process the unselect or clear select
      }
    }
  });
}