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
});