DispatcherTimer for displaying seconds - wpf

Question

I am trying to display a simple second counter. I have a dispatchertimer with a tick interval of 1 second and a textbox which I update in the tick handler with the current amount of seconds. There is a tiny amount of work in the tick handler, namely a call to 'tostring()' on some ints.
My issue is that the seconds are slower than they should be. Even if I set the interval to 100 milliseconds and make a check when elapsed, it is still slower than it should be. (over the course of a minute it is roughly 6 seconds slow).
Can anyone point me in the right direction for displaying a second counter that is accurate?
EDIT: Some code here (in .xaml.cs). It is taken from an example which works fine. The difference is that I am setting the Text property of a TextBox, rather than a Value property of another control.
...
this.timer.Interval = TimeSpan.FromMilliseconds(100);
...
private void OnDispatcherTimer_Tick(object sender, EventArgs e) {
if (this.currentValue > TimeSpan.Zero) {
this.currentValue = this.currentValue.Value.Subtract(TimeSpan.FromMilliseconds(100));
} else {
// stop timer etc
}
this.seconds.Text = this.currentValue.Value.Seconds.ToString();
}

Answer 1

Your way of keeping track of time is flawed. You're incrementing a counter each time the timer ticks, but there's no guarantee your timer will execute every 100 ms. And even if it did, you'd have to take into account the execution time of your code. Therefore, no matter what you do, your counter will drift.
What you must do is storing the date at which you started your counter. Then, every time the timer ticks, you compute the number of seconds that have elapsed:
private DateTime TimerStart { get; set; }
private void SomePlaceInYourCode()
{
this.TimerStart = DateTime.Now;
// Create and start the DispatcherTimer
}
private void OnDispatcherTimer_Tick(object sender, EventArgs e) {
var currentValue = DateTime.Now - this.TimerStart;
this.seconds.Text = currentValue.Seconds.ToString();
}

Answer 2

If you care about precise time a dispatchertimer is not good choice.
I thing you should separate counting seconds(time) and displaying on screen.
Use a System.Threading.Timer and use Dispatcher.BeginInvoke() in Timer callback.
simple example:
public partial class MainPage : PhoneApplicationPage
{
private DateTime _startDate;
private int _secondDuration;
private Timer _timer;
// Constructor
public MainPage()
{
InitializeComponent();
_startDate = DateTime.Now;
_secondDuration = 0;
_timer= new Timer(timerCallback, null, 0, 10);
}
private void timerCallback(object state)
{
var now = DateTime.Now;
if (now > _startDate + TimeSpan.FromSeconds(1))
{
_secondDuration += 1;
_startDate = now;
Dispatcher.BeginInvoke(() => { Counter.Text = _secondDuration.ToString(); });
}
}
}
After every 10 milisecond timer checks for one second has elapsed and print to textbox elapsed seconds
or you can do this like:
public partial class MainPage : PhoneApplicationPage
{
private Timer _timer;
private int _secondDuration;
// Constructor
public MainPage()
{
InitializeComponent();
_timer = new Timer(timerCallback, null, 0, 1000);
}
private void timerCallback(object state)
{
_secondDuration += 1;
Dispatcher.BeginInvoke(() => { Counter.Text = _secondDuration.ToString(); });
}
}