Recipe 15.11 Configuring a Timer

Problem

You have one of the following timer configuration needs:

• You want to use a timer to call a timer callback method at a fixed time after the timer object has been created. Once this callback method has been called the first time, you want to call this same callback method at a specified interval (this interval might be different from the time interval between the creation of the timer and the first time the timer callback method is called).

• You want to use a timer to call a timer callback method immediately upon creation of the System.Threading.Timer object, after which the callback method is called at a specified interval.

• You want to use a timer to call a timer callback method one time only.

• You have been using a System.Threading.Timer object and need to change the intervals at which its timer callback method is called.

Solution

To fire a System.Threading.Timer after an initial delay, and then at a specified period after that, use the System.Threading.Timer constructor to set up different times for the initial and following callbacks:

using System;
using System.Threading;

public class TestTimers
{
    public static int count = 0;
    public static Timer timerRef = null;

    public static void Main( )
    {
        TimerCallback callback = new TimerCallback(TimerMethod);

        // Create a timer that waits one half second, then invokes 
        // the callback every second thereafter.
        Timer timer = new Timer(callback, null,500, 1000);

        // store a reference to this timer so the callback can use it
        timerRef = timer;

        // The main thread does nothing until the timer is disposed.
        while(timerRef != null)
            Thread.Sleep(0);
        Console.WriteLine("Timer example done.");
    }

    static void TimerMethod(Object state)
    {
        count++;
        if(count == 5)
        {
            timerRef.Dispose( );
            timerRef = null;
        }
    }
}

The previous method showed how to fire the callback after 500 milliseconds. To fire the initial callback immediately, change the value to zero:

// Create a timer that doesn't wait, then invokes 
// the callback every second thereafter.
Timer timer = new Timer(callback, null,0, 1000);

To have the timer call the callback only once, change the constructor to pass Timeout.Infinite for the callback interval. You also have to change the current scheme that waits for five callbacks before disposing of the timer to do it the first time. If you didn't do this, the program would hang since the Main function is still waiting for the timer to have Dispose called, but the fifth callback will never trigger the Dispose call:

        // Create a timer that waits for half a second, then is disposed
        Timer timer = new Timer(callback, null,500, Timeout.Infinite);

// Also change this...to
    static void TimerMethod(Object state)
    {
            timerRef.Dispose( );
            timerRef = null;
    }

To change the interval of a running System.Threading.Timer, call the Change method specifying the delay before the next callback and the new callback interval, like this:

static void TimerMethod(Object state)
{
    count++;
    if(count == 5)
    {
        timerRef.Change(1000,2000);
    }
    if(count == 10)
    {
        timerRef.Dispose( );
        timerRef = null;
    }
}

This code now checks for the fifth callback and changes the interval from one second to two seconds. The sixth callback will happen one second after, and then callbacks through ten will happen two seconds apart.

Discussion

One item to be aware of when using System.Threading.Timers and TimerCallbacks is that they are serviced from the ThreadPool. This means that if you have other work being farmed out to the ThreadPool in your application, it could be contending with the Timer callbacks for an available worker thread. The basic timer is enough to serve the earlier scenarios, but if you are doing UI work and want to use timers, you should investigate the System.Windows.Forms.Timer class. If you are doing server work, you might also want to look at System.Timers.Timer as well. Both of these classes add events for when the timers are disposed and when the timer "ticks"; they also add properties that expose the settings.

See Also

See the "System.Threading.Timer Class," "TimerCallback Delegate," "System.Windows.Forms.Timer Class," and "System.Timers.Timer" topics in the MSDN documentation.