public string PreviousID {get;}

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