WPF Dispatchertimer tick increments, - wpf

Question

I am making a countdown clock using dispatchertimer and timespan in WPF.
I have a button to start the countdown and a button to stop the countdown. When I hit the start button again it has doubled the interval from one second to two seconds. I display the counting in a textbox. What is wrong?
The code is simple:
public partial class MainWindow : Window
{
private DispatcherTimer DPtimerA;
private TimeSpan timeA;
public MainWindow()
{
InitializeComponent();
DPtimerA = new DispatcherTimer();
DPtimerA.Interval = new TimeSpan(0, 0, 1);
timeA = TimeSpan.FromSeconds(21);
}
private void DPtimerA_Tick(object sender, EventArgs e)
{
timeA = timeA.Add(TimeSpan.FromSeconds(-1));
txtClockA.Text = timeA.ToString("c");
if (timeA == TimeSpan.Zero) DPtimerA.Stop();
}
private void btnStartA_Click(object sender, RoutedEventArgs e)
{
DPtimerA.Tick += DPtimerA_Tick;
DPtimerA.Start();
}
private void btnStopA_Click(object sender, RoutedEventArgs e)
{
DPtimerA.Stop();
}
}
}

Answer 1

Change it like this:
private void btnStartA_Click(object sender, RoutedEventArgs e)
{
DPtimerA.Tick -= DPtimerA_Tick;
DPtimerA.Tick += DPtimerA_Tick;
DPtimerA.Start();
}

Answer 2

Make DispatcherTimer a readonly field that you initialize and hook up an event handler to once:
public partial class MainWindow : Window
{
private readonly DispatcherTimer DPtimerA =
new DispatcherTimer() { Interval = new TimeSpan(0, 0, 1) };
private TimeSpan timeA = TimeSpan.FromSeconds(21);
public MainWindow()
{
InitializeComponent();
DPtimerA.Tick += DPtimerA_Tick;
}
private void DPtimerA_Tick(object sender, EventArgs e)
{
timeA = timeA.Add(TimeSpan.FromSeconds(-1));
txtClockA.Text = timeA.ToString("c");
if (timeA == TimeSpan.Zero)
DPtimerA.Stop();
}
private void btnStartA_Click(object sender, RoutedEventArgs e)
{
DPtimerA.Start();
}
private void btnStopA_Click(object sender, RoutedEventArgs e)
{
DPtimerA.Stop();
}
}