public string CurrentID {get;}

{
  ArcGIS.Desktop.Framework.Events.ActiveToolChangedEvent.Subscribe((args) =>
    {
      string prevTool = args.PreviousID;
      string newTool = args.CurrentID;
    });
}

{
  ArcGIS.Desktop.Framework.Events.ActiveToolChangedEvent.Unsubscribe((args) =>
  {
    string prevTool = args.PreviousID;
    string newTool = args.CurrentID;
  });
}