Part of the difficulty is that there is no synchronous method for ending calibration. If the user (or the OS) wants to close the application during calibration, I would have to try to cancel the quit event, call StopCalibrationAsync with a callback, and finally close the application in the callback. To me, trying to cancel quit events to clean up asynchronously seems like bad practice; a quit event should be interpreted as a demand to clean up synchronously. Thus, a resource such as the eye tracker should be disposable quickly and synchronously during application tear-down.
For me, the issue is arising with an EyeTracker that is created in the constructor of App (my Application subclass), accessed from a Page subclass that listens for eye tracking data events and calls eye tracking calibration methods, and disposed in App’s OnExit method.