The time is displayed every 5ms, and the difference between displaying "fff" is not 5, it How is it calculated?

Question

I want to display the time every 5ms, and 'fff' is used in wpf to represent ms.

Why is the fff difference in the code of new TimeSpan(0, 0, 0, 0, 5); displayed result not 5? How is it calculated?

Xaml:

<Grid>
        <ListBox Height="140" HorizontalAlignment="Left" Margin="18,31,0,0" 
                 Name="listBox1" VerticalAlignment="Top" Width="308" />
        <Button x:Name="btn" Click="btn_Click" Height="50" Content=" stop"/>
    </Grid>

Codebehind:

using System;
using System.Windows;
using System.Windows.Input;
using System.Windows.Threading;
namespace WpfApp1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
         DispatcherTimer timer = new DispatcherTimer();
        public bool What { get; set; }
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("Start");

            timer.Interval = new TimeSpan(0, 0, 0, 0, 5);
            timer.Tick += new EventHandler(dispatcherTimer_Tick);

            timer.Start();
        }
        private void dispatcherTimer_Tick(object sender, EventArgs e)
        {
            listBox1.Items.Add(DateTime.Now.Hour.ToString() + ":" +
                DateTime.Now.Minute.ToString() + ":" + DateTime.Now.Second.ToString() + ":"+DateTime.Now.Millisecond.ToString());

            CommandManager.InvalidateRequerySuggested();
            listBox1.Items.MoveCurrentToLast();
            listBox1.SelectedItem = listBox1.Items.CurrentItem;
            listBox1.ScrollIntoView(listBox1.Items.CurrentItem);
        }

        private void btn_Click(object sender, RoutedEventArgs e)
        {
            timer.Stop();
        }
       
    }
}

The result:

Update :

  public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            CancellationTokenSource tokenSource = new CancellationTokenSource();
           Task timerTask = RunPeriodically(sendRequest, TimeSpan.FromMilliseconds(num), tokenSource.Token);
            
        }
        private void sendRequest()
        {
            k++;
            datas.Add(DateTime.Now );
       
        }
        async Task RunPeriodically(Action action, TimeSpan interval, CancellationToken token)
        {
            while (k<7)
            {
                action();
                await Task.Delay(interval, token);
            }
        }

Answer 1

When you set the Interval of the timer to 5 milliseconds, the timer will not be executed every 5 milliseconds. The timer doesn't have that high resolution.

From the documentation:

Timers are not guaranteed to execute exactly when the time interval occurs, but they are guaranteed to not execute before the time interval occurs. This is because DispatcherTimer operations are placed on the Dispatcher queue like other operations. When the DispatcherTimer operation executes is dependent on the other jobs in the queue and their priorities.

---

Task.Delay also uses a timer internally. The resolution is again not high enough for 5ms intervals.

From the documentation:

This method depends on the system clock. This means that the time delay will approximately equal the resolution of the system clock if the millisecondsDelay argument is less than the resolution of the system clock, which is approximately 15 milliseconds on Windows systems.