Detect when C# app has gone to sleep (SystemEvents.PowerModeChanged not being called)

Question

The PowerModeChanged event seems problematic (many questions about it), but none of the answers that I find are helping me.

I've got a C#/WPF Windows client application running .NET Core 3.1 (not a service, nothing fancy, just a plain GUI desktop app). I have things that happen on timers, and need to know when I've missed a timer or two because the laptop went to sleep (or suspended or whatever it's doing).

Here's what I have in my main window, I'm grabbing every event I can find:

private void OnLoaded(object sender, RoutedEventArgs e)
{
    DispatcherTimer timer = new DispatcherTimer();
    timer.Tick += (a, b) => LiveTimer();
    timer.Interval = new TimeSpan(0, 0, 10);
    timer.Start();

    SystemEvents.PowerModeChanged += OnPowerModeChanged;
    SystemEvents.SessionSwitch += OnSessionSwitch;
}

protected override void OnDeactivated(EventArgs e)
{
    Debug.WriteLine("MainWindow.OnDeactivated()");
    Log.Info(this, "Main window deactivated");
    base.OnDeactivated(e);
}

protected override void OnStateChanged(EventArgs e)
{
    Debug.WriteLine("MainWindow.OnStateChanged()");
    Log.Info(this, "Main window state changed [WindowState=" + WindowState + "]");
    base.OnStateChanged(e);
}

private void OnSessionSwitch(object sender, SessionSwitchEventArgs e)
{
    Debug.WriteLine("MainWindow.OnSessionSwitch(sender=" + sender + ", e [Reason=" + e.Reason + "])");
    Log.Info(this, "Session switched because " + e.Reason);
}

private void OnPowerModeChanged(object sender, PowerModeChangedEventArgs e)
{
    Debug.WriteLine("MainWindow.OnPowerModeChanged(sender=" + sender + ", e [Mode=" + e.Mode + "])");
    Log.Info(this, "Power mode changed to " + e.Mode);
}

private void LiveTimer()
{
   Debug.WriteLine("MainWindow.LiveTimer() at " + DateTime.Now.ToString("HH:mm:ss.fff"));
}

So if I run this application on my laptop and wait for the screen to lock (5 minutes) and for the laptop to go to sleep (10 minutes), here's what I see in the debug console:

MainWindow.LiveTimer() at 10:07:17.711
(this happens every 10 seconds)
MainWindow.LiveTimer() at 10:12:20.593

MainWindow.OnSessionSwitch(sender=Microsoft.Win32.SystemEvents, e [Reason=SessionLock])

MainWindow.LiveTimer() at 10:12:30.614
(this happens every 10 seconds)
MainWindow.LiveTimer() at 10:17:24.173

(and now it goes to sleep)
(and then I wake it up)

MainWindow.LiveTimer() at 10:31:28.519

MainWindow.OnSessionSwitch(sender=Microsoft.Win32.SystemEvents, e [Reason=SessionUnlock])

MainWindow.LiveTimer() at 10:31:38.716
(and now we're back to firing every 10 seconds)

Basically, I'm getting absolutely no kind of notification that the laptop is going to sleep and I'm not going to get my timer messages for a while.

How does my app discover that it's gone to sleep and been re-awakened?

Answer 1

Is there a reason you need to go that route of figuring out when the computer sleeps, wakes up, etc., if you just want to measure the time difference between the current run and the last run?

i.e. Couldn't you just calculate the time difference between now and the previous run, whenever a timer run starts?

Pseudo-code:

private DateTime? previousRunTimestamp;

private void TimerTick()
{
    var now = DateTime.Now;
    var elapsedTimeSinceLastRun = previousRunTimestamp.HasValue ? now - previousRunTimestamp.Value : TimeSpan.Zero;
    previousRunTimestamp = now;
    
    const int secondsBetweenRuns = 10;
    var numberOfRunsMissed = (int)Math.Floor(elapsedTimeSinceLastRun.TotalSeconds / secondsBetweenRuns);

    // Do something about it when numberOfRunsMissed > 0

    // ...
}