How might I schedule a C# Windows Service to perform a task daily?

I wouldn't use Thread.Sleep(). Either use a scheduled task (as others have mentioned), or set up a timer inside your service, which fires periodically (every 10 minutes for example) and check if the date changed since the last run:

private Timer _timer;
private DateTime _lastRun = DateTime.Now.AddDays(-1);

protected override void OnStart(string[] args)
{
    _timer = new Timer(10 * 60 * 1000); // every 10 minutes
    _timer.Elapsed += new System.Timers.ElapsedEventHandler(timer_Elapsed);
    _timer.Start();
    //...
}


private void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{
    // ignore the time, just compare the date
    if (_lastRun.Date < DateTime.Now.Date)
    {
        // stop the timer while we are running the cleanup task
        _timer.Stop();
        //
        // do cleanup stuff
        //
        _lastRun = DateTime.Now;
        _timer.Start();
    }
}

Check out Quartz.NET. You can use it within a Windows service. It allows you to run a job based on a configured schedule, and it even supports a simple "cron job" syntax. I've had a lot of success with it.

Here's a quick example of its usage:

// Instantiate the Quartz.NET scheduler
var schedulerFactory = new StdSchedulerFactory();
var scheduler = schedulerFactory.GetScheduler();

// Instantiate the JobDetail object passing in the type of your
// custom job class. Your class merely needs to implement a simple
// interface with a single method called "Execute".
var job = new JobDetail("job1", "group1", typeof(MyJobClass));

// Instantiate a trigger using the basic cron syntax.
// This tells it to run at 1AM every Monday - Friday.
var trigger = new CronTrigger(
    "trigger1", "group1", "job1", "group1", "0 0 1 ? * MON-FRI");

// Add the job to the scheduler
scheduler.AddJob(job, true);
scheduler.ScheduleJob(trigger);

A daily task? Sounds like it should just be a scheduled task (control panel) - no need for a service here.

Does it have to be an actual service? Can you just use the built in scheduled tasks in the windows control panel.

The way I accomplish this is with a timer.

Run a server timer, have it check the Hour/Minute every 60 seconds.

If it's the right Hour/Minute, then run your process.

I actually have this abstracted out into a base class I call OnceADayRunner.

Let me clean up the code a bit and I'll post it here.

    private void OnceADayRunnerTimer_Elapsed(object sender, ElapsedEventArgs e)
    {
        using (NDC.Push(GetType().Name))
        {
            try
            {
                log.DebugFormat("Checking if it's time to process at: {0}", e.SignalTime);
                log.DebugFormat("IsTestMode: {0}", IsTestMode);

                if ((e.SignalTime.Minute == MinuteToCheck && e.SignalTime.Hour == HourToCheck) || IsTestMode)
                {
                    log.InfoFormat("Processing at: Hour = {0} - Minute = {1}", e.SignalTime.Hour, e.SignalTime.Minute);
                    OnceADayTimer.Enabled = false;
                    OnceADayMethod();
                    OnceADayTimer.Enabled = true;

                    IsTestMode = false;
                }
                else
                {
                    log.DebugFormat("Not correct time at: Hour = {0} - Minute = {1}", e.SignalTime.Hour, e.SignalTime.Minute);
                }
            }
            catch (Exception ex)
            {
                OnceADayTimer.Enabled = true;
                log.Error(ex.ToString());
            }

            OnceADayTimer.Start();
        }
    }

The beef of the method is in the e.SignalTime.Minute/Hour check.

There are hooks in there for testing, etc. but this is what your elapsed timer could look like to make it all work.

As others already wrote, a timer is the best option in the scenario you described.

Depending on your exact requirements, checking the current time every minute may not be necessary. If you do not need to perform the action exactly at midnight, but just within one hour after midnight, you can go for Martin's approach of only checking if the date has changed.

If the reason you want to perform your action at midnight is that you expect a low workload on your computer, better take care: The same assumption is often made by others, and suddenly you have 100 cleanup actions kicking off between 0:00 and 0:01 a.m.

In that case you should consider starting your cleanup at a different time. I usually do those things not at clock hour, but at half hours (1.30 a.m. being my personal preference)

I would suggest that you use a timer, but set it to check every 45 seconds, not minute. Otherwise you can run into situations where with heavy load, the check for a particular minute is missed, because between the time the timer triggers and the time your code runs and checks the current time, you might have missed the target minute.

For those that found the above solutions not working, it's because you may have a this inside your class, which implies an extension method which, as the error message says, only makes sense on a non-generic static class. Your class isn't static. This doesn't seem to be something that makes sense as an extension method, since it's acting on the instance in question, so remove the this.