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
    }
  }
});

//Report the event args when an element is added to the layout.

//At 2.x - ArcGIS.Desktop.Layouts.Events.ElementAddedEvent.Subscribe((args) => 
//{
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("ElementAddedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});

ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  string elemNames = "null";
  if (args.Elements?.Count() > 0)
  {
    elemNames = string.Join(",", args.Elements.Select(e => e.Name).ToList());
  }
  if (args.Hint == ElementEventHint.ElementAdded)
  {
    System.Diagnostics.Debug.WriteLine($"ElementEventHint.ElementAdded: {elemNames}");
  }
});

//Report the event args when an element is removed from the layout.

//At 2.x - ArcGIS.Desktop.Layouts.Events.ElementRemovedEvent.Subscribe((args) => 
//{
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("ElementRemovedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});

ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  string elemNames = "null";
  if (args.Elements?.Count() > 0)
  {
    elemNames = string.Join(",", args.Elements.Select(e => e.Name).ToList());
  }
  if (args.Hint == ElementEventHint.ElementRemoved)
  {
    System.Diagnostics.Debug.WriteLine($"ElementEventHint.ElementRemoved: {elemNames}");
  }
});

//Report the event args when an element's placement properties are changed.

//At 2.x - ArcGIS.Desktop.Layouts.Events.ElementsPlacementChangedEvent.Subscribe((args) => 
//{
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("ElementsPlacementChangedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});

ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  string elemNames = "null";
  if (args.Elements?.Count() > 0)
  {
    elemNames = string.Join(",", args.Elements.Select(e => e.Name).ToList());
  }
  if (args.Hint == ElementEventHint.PlacementChanged)
  {
    System.Diagnostics.Debug.WriteLine($"ElementEventHint.PlacementChanged: {elemNames}");
  }
});

//Report the event args when an element's style is changed.

//At 2.x - ArcGIS.Desktop.Layouts.Events.ElementStyleChangedEvent.Subscribe((args) => 
//{
//  var elements = args.Elements;
//  string elemNames = "null";
//  if (elements != null && elements.Count() > 0)
//  {
//    elemNames = string.Join(",", elements.Select(e => e.Name).ToList());
//  }
//  System.Windows.MessageBox.Show("ElementStyleChangedEvent:" +
//  Environment.NewLine +
//  "   arg Elements: " + elemNames.ToString() +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});
ArcGIS.Desktop.Layouts.Events.ElementEvent.Subscribe((args) =>
{
  string elemNames = "null";
  if (args.Elements?.Count() > 0)
  {
    elemNames = string.Join(",", args.Elements.Select(e => e.Name).ToList());
  }
  if (args.Hint == ElementEventHint.StyleChanged)
  {
    System.Diagnostics.Debug.WriteLine($"ElementEventHint.StyleChanged: {elemNames}");
  }
});