Alarm clock application in .Net

Question

I'm not really writing an alarm clock application, but it will help to illustrate my question.

Let's say that I have a method in my application, and I want this method to be called every hour on the hour (e.g. at 7:00 PM, 8:00 PM, 9:00 PM etc.). I could create a Timer and set its Interval to 3600000, but eventually this would drift out of sync with the system clock. Or I could use a while() loop with Thread.Sleep(n) to periodically check the system time and call the method when the desired time is reached, but I don't like this either (Thread.Sleep(n) is a big code smell for me).

What I'm looking for is some method in .Net that lets me pass in a future DateTime object and a method delegate or event handler, but I haven't been able to find any such thing. I suspect there's a method in the Win32 API that does this, but I haven't been able to find that, either.

Solution 1

Or, you could create a timer with an interval of 1 second and check the current time every second until the event time is reached, if so, you raise your event.

You can make a simple wrapper for that :

public class AlarmClock
{
    public AlarmClock(DateTime alarmTime)
    {
        this.alarmTime = alarmTime;

        timer = new Timer();
        timer.Elapsed += timer_Elapsed;
        timer.Interval = 1000;
        timer.Start();

        enabled = true;
    }

    void  timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        if(enabled && DateTime.Now > alarmTime)
        {
            enabled = false;
            OnAlarm();
            timer.Stop();
        }
    }

    protected virtual void OnAlarm()
    {
        if(alarmEvent != null)
            alarmEvent(this, EventArgs.Empty);
    }


    public event EventHandler Alarm
    {
        add { alarmEvent += value; }
        remove { alarmEvent -= value; }
    }

    private EventHandler alarmEvent;
    private Timer timer;
    private DateTime alarmTime;
    private bool enabled;
}

Usage:

AlarmClock clock = new AlarmClock(someFutureTime);
clock.Alarm += (sender, e) => MessageBox.Show("Wake up!");

Please note the code above is very sketchy and not thread safe.

Solution 2

Interesting, I've actually come across a very similar issue and went looking for a method in the .Net framework that would handle this scenario. In the end, we ended up implementing our own solution that was a variation on a while loop w/ Thread.Sleep(n) where n gets smaller the closer you get to the desired target time (logarithmically actually, but with some reasonable thresholds so you're not maxing the cpu when you get close to the target time.) Here's a really simple implementation that just sleeps half the time between now and the target time.

class Program
{
    static void Main(string[] args)
    {
        SleepToTarget Temp = new SleepToTarget(DateTime.Now.AddSeconds(30),Done);
        Temp.Start();
        Console.ReadLine();
    }

    static void Done()
    {
        Console.WriteLine("Done");
    }
}

class SleepToTarget
{
    private DateTime TargetTime;
    private Action MyAction;
    private const int MinSleepMilliseconds = 250;

    public SleepToTarget(DateTime TargetTime,Action MyAction)
    {
        this.TargetTime = TargetTime;
        this.MyAction = MyAction;
    }

    public void Start()
    {
        new Thread(new ThreadStart(ProcessTimer)).Start();
    }

    private void ProcessTimer()
    {
        DateTime Now = DateTime.Now;

        while (Now < TargetTime)
        {
            int SleepMilliseconds = (int) Math.Round((TargetTime - Now).TotalMilliseconds / 2);
            Console.WriteLine(SleepMilliseconds);
            Thread.Sleep(SleepMilliseconds > MinSleepMilliseconds ? SleepMilliseconds : MinSleepMilliseconds);
            Now = DateTime.Now;
        }

        MyAction();
    }
}

Solution 3

You could simply reset the timer duration each time it fires, like this:

// using System.Timers;

private void myMethod()
{
    var timer = new Timer { 
        AutoReset = false, Interval = getMillisecondsToNextAlarm() };
    timer.Elapsed += (src, args) =>
    {
        // Do timer handling here.

        timer.Interval = getMillisecondsToNextAlarm();
        timer.Start();
    };
    timer.Start();
}

private double getMillisecondsToNextAlarm()
{
    // This is an example of making the alarm go off at every "o'clock"
    var now = DateTime.Now;
    var inOneHour = now.AddHours(1.0);
    var roundedNextHour = new DateTime(
        inOneHour.Year, inOneHour.Month, inOneHour.Day, inOneHour.Hour, 0, 0);
    return (roundedNextHour - now).TotalMilliseconds;
}

Solution 4

I know it's a bit of an old question, but I came across this when I was looking for an answer to something else. I thought I'd throw my two cents in here, since I recently had this particular issue.

Another thing you can do is schedule the method like so:

/// Schedule the given action for the given time.
public async void ScheduleAction ( Action action , DateTime ExecutionTime )
{
    try
    {
        await Task.Delay ( ( int ) ExecutionTime.Subtract ( DateTime.Now ).TotalMilliseconds );
        action ( );
    }
    catch ( Exception )
    {
        // Something went wrong
    }
}

Bearing in mind it can only wait up to the maximum value of int 32 (somewhere around a month), it should work for your purposes. Usage:

void MethodToRun ( )
{
    Console.WriteLine ("Hello, World!");
}

void CallingMethod ( )
{
    var NextRunTime = DateTime.Now.AddHours(1);
    ScheduleAction ( MethodToRun, NextRunTime );
}

And you should have a console message in an hour.

Solution 5

You could create an Alarm class which has a dedicated thread which goes to sleep until the specified time, but this will use the Thread.Sleep method. Something like:

/// <summary>
/// Alarm Class
/// </summary>
public class Alarm
{
    private TimeSpan wakeupTime;

    public Alarm(TimeSpan WakeUpTime)
    {
        this.wakeupTime = WakeUpTime;
        System.Threading.Thread t = new System.Threading.Thread(TimerThread) { IsBackground = true, Name = "Alarm" };
        t.Start();
    }

    /// <summary>
    /// Alarm Event
    /// </summary>
    public event EventHandler AlarmEvent = delegate { };

    private void TimerThread()
    {
        DateTime nextWakeUp = DateTime.Today + wakeupTime;
        if (nextWakeUp < DateTime.Now) nextWakeUp = nextWakeUp.AddDays(1.0);

        while (true)
        {
            TimeSpan ts = nextWakeUp.Subtract(DateTime.Now);

            System.Threading.Thread.Sleep((int)ts.TotalMilliseconds);

            try { AlarmEvent(this, EventArgs.Empty); }
            catch { }

            nextWakeUp = nextWakeUp.AddDays(1.0);
        }
    }
}

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Comments

[MusiGenesis]

Yeah, but that's also not what I'm looking for, sorry.

[RichardOD]

What you are looking for does not exist.

[Coincoin]

Sorry but, what are you looking for then?

[RichardOD]

I think MusiGenesis really wants a job scheduler.

[Admin]

You could calculate the interval rather than check every second.

[Coincoin]

@JonB: As explained in the question, he wants something that doesn't drift.

[MusiGenesis]

This wouldn't solve the problem, unfortunately. The drift problem is because the Timer is inaccurate over long durations (Stopwatch has the same problem).

[MusiGenesis]

@Coincoin: this approach may be the only way to achieve this in Windows, but I was hoping for at least a Win32 API call that does this (like NetScheduleJobAdd mentioned by xcud, although that function runs a command at the scheduled time instead of calling a particular method in a particular running process).

[JDunkerley]

Indeed. More a prototype of what you could do. Better to use a Timer like Jacob describes.

[Jacob]

I'd never heard that before. What are you basing that inaccuracy claim on?

[MusiGenesis]

@Jacob: don't take my word for it - try it yourself. Start a timer (any variety) and let it run for a day or two, and see how far off it gets.

[AMissico]

I don't get this "drift" problem. I used a System.Timers.Timer with an elapsed time of five seconds within a service and did not have any drift over the four months I used it. I know this for a fact becasue the service wrote to the event log on every interval, and I have events every five seconds like clockwork, which was expected.

[MusiGenesis]

TimeOfDay is a VB.Net-only thing, and the While loop with a DoEvents() call will max out your processor while this thing is running.

[Cyclone]

Didn't know that, but now I do. The original code had the loop sleep for 15 seconds, but since you didn't want that I left it out.

[MusiGenesis]

@AMissico: Did your service only start the Timer once and then write to the event log on each Elapsed call, or was your service periodically re-started by Windows? I just tested this again, and my Timer (with an Interval of 5000) drifted off from the system time by a full second after only 12 minutes.

[Matt Wilko]

Why you would want Application.DoEvents in a method running in a thread (or even at all) is beyond me