public string PreviousID {get;}

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

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