public ArcGISPortal ActivePortal {get;}

{
  ArcGIS.Desktop.Core.Events.ActivePortalChangedEvent.Subscribe((args) =>
  {
    var active_uri = args.ActivePortal?.PortalUri.ToString();
    //etc
  });

  ArcGIS.Desktop.Core.Events.ArcGISPortalAddedEvent.Subscribe((args) =>
  {
    var added_portal = args.Portal;
    //etc
  });

  ArcGIS.Desktop.Core.Events.ArcGISPortalRemovedEvent.Subscribe((args) =>
  {
    var old_uri = args.RemovedPortalUri;
    //etc
  });

  ArcGIS.Desktop.Core.Events.PortalSignOnChangedEvent.Subscribe((args) =>
  {
    var portal = args.Portal;
    var isSignedOn = args.IsSignedOn;
    //etc
  });
}