Make a .net System.Timers.Timer fire immediately

I am using a Timer to run an event periodically on a reasonably long interval (2 minutes). This is working fine. However I would like the event to fire immediately when the timer is created (instead of waiting 2 minutes).

Note that I can't do this just by calling the method, since it takes some time to run and would block the application. I need the timer to fire as normal and run the event in a separate thread.

The best way I can think of doing this at the moment is subclassing the timer and creating a TriggerManually method that would do something like this:

• Turn auto reset off
• Set the interval to 1ms
• Enable the tim开发者_如何学Goer

This would trigger the elapsed event straight away, and I could put all the settings back to normal.

Seems a bit roundabout though. Is there a better way to do it?

Couldn't you just call your event handler for the elapsed event manually?

Even if you were expecting it to execute on a thread pool thread, you could invoke it.

class Blah
{
    private Timer mTimer;

    public Blah()
    {
        mTimer = new Timer(120000);

        ElapsedEventHandler handler = new ElapsedEventHandler(Timer_Elapsed);
        mTimer.Elapsed += handler;
        mTimer.Enabled = true;

        //Manually execute the event handler on a threadpool thread.
        handler.BeginInvoke(this, null, new AsyncCallback(Timer_ElapsedCallback), handler);
    }

    private static void Timer_Elapsed(object source, ElapsedEventArgs e)
    {
        //Do stuff...
    }

    private void Timer_ElapsedCallback(IAsyncResult result)
    {
        ElapsedEventHandler handler = result.AsyncState as ElapsedEventHandler;
        if (handler != null)
        {
            handler.EndInvoke(result);
        }
    }
}

I liked Rob Cooke's answer, so I built a small EagerTimer class that subclasses System.Timers.Timer and adds this functionality. (With hints from these articles)

I know I could use System.Threading.Timer instead, but this is simple and works well in my application.

EagerTimer

/// <summary>
// EagerTimer is a simple wrapper around System.Timers.Timer that
// provides "set up and immediately execute" functionality by adding a
// new AutoStart property, and also provides the ability to manually
// raise the Elapsed event with RaiseElapsed.
/// </summary>
public class EagerTimer : Timer
{
    public EagerTimer()
        : base() { }

    public EagerTimer(double interval)
        : base(interval) { }

    // Need to hide this so we can use Elapsed.Invoke below
    // (otherwise the compiler complains)
    private event ElapsedEventHandler _elapsedHandler;
    public new event ElapsedEventHandler Elapsed
    {
        add { _elapsedHandler += value; base.Elapsed += value; }
        remove { _elapsedHandler -= value; base.Elapsed -= value; }
    }

    public new void Start()
    {
        // If AutoStart is enabled, we need to invoke the timer event manually
        if (AutoStart)
        {
            this._elapsedHandler.BeginInvoke(this, null, new AsyncCallback(AutoStartCallback), _elapsedHandler); // fire immediately
        }

        // Proceed as normal
        base.Start();
    }

    private void AutoStartCallback(IAsyncResult result)
    {
        ElapsedEventHandler handler = result.AsyncState as ElapsedEventHandler;
        if (handler != null) handler.EndInvoke(result);
    }

    // Summary:
    //     Gets or sets a value indicating whether the EagerTimer should raise
    //     the System.Timers.Timer.Elapsed event immediately when Start() is called,
    //     or only after the first time it elapses. If AutoStart is false, EagerTimer behaves
    //     identically to System.Timers.Timer.
    //
    // Returns:
    //     true if the EagerTimer should raise the System.Timers.Timer.Elapsed
    //     event immediately when Start() is called; false if it should raise the System.Timers.Timer.Elapsed
    //     event only after the first time the interval elapses. The default is true.
    [Category("Behavior")]
    [DefaultValue(true)]
    [TimersDescription("TimerAutoStart")]
    public bool AutoStart { get; set; }

    /// <summary>
    /// Manually raises the Elapsed event of the System.Timers.Timer.
    /// </summary>
    public void RaiseElapsed()
    {
        if (_elapsedHandler != null)
            _elapsedHandler(this, null);
    }
}

Unit Tests

[TestClass]
public class Objects_EagerTimer_Tests
{
    private const int TimerInterval = 10; // ms

    private List<DateTime> _timerFires = new List<DateTime>();
    private DateTime _testStart;

    [TestInitialize]
    public void TestSetup()
    {
        _timerFires.Clear();
        _testStart = DateTime.Now;
    }

    [TestMethod]
    public void Objects_EagerTimer_WithAutoStartDisabled()
    {
        // EagerTimer should behave as a normal System.Timers.Timer object
        var timer = new EagerTimer(TimerInterval);
        timer.AutoReset = false;
        timer.Elapsed += timerElapsed;
        timer.Start();

        // Wait (not enough time for first interval)
        Thread.Sleep(5);
        Assert.IsFalse(_timerFires.Any());

        // Wait a little longer
        Thread.Sleep(TimerInterval);
        Assert.AreEqual(1, _timerFires.Count);
    }

    [TestMethod]
    public void Objects_EagerTimer_WithAutoStartEnabled()
    {
        // EagerTimer should fire immediately on Start()
        var timer = new EagerTimer(TimerInterval);
        timer.AutoReset = false;
        timer.AutoStart = true;
        timer.Elapsed += timerElapsed;
        timer.Start();

        // Wait (not enough time for first interval)
        Thread.Sleep(5);
        Assert.IsTrue(_timerFires.Any());

        // Wait a little longer, now it will have fired twice
        Thread.Sleep(TimerInterval);
        Assert.AreEqual(2, _timerFires.Count);
    }

    [TestMethod]
    public void Objects_EagerTimer_WhenRaisingManually()
    {
        // EagerTimer should fire immediately on Start()
        var timer = new EagerTimer(TimerInterval);
        timer.AutoReset = false;
        timer.AutoStart = false;
        timer.Elapsed += timerElapsed;

        Assert.IsFalse(_timerFires.Any());
        timer.RaiseElapsed();
        Assert.IsTrue(_timerFires.Any());
    }

    private void timerElapsed(object sender, ElapsedEventArgs e) {
        _timerFires.Add(DateTime.Now);
    }
}

Could you use a System.Threading.Timer instead ? It has a constructor that lets you choose the interval as well as the delay (which can be set to 0 to begin immediately). http://msdn.microsoft.com/en-us/library/2x96zfy7.aspx