WPF DispatcherTimer update periodically

<Window x:Class="WpfApp38.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp38"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition />
        </Grid.RowDefinitions>
        <TextBlock x:Name="_timeTb" HorizontalAlignment="Center" FontSize="20" Margin="8"/>
        <Ellipse Margin="10" Stroke="Black" StrokeThickness="2"
                 x:Name="_e1" Grid.Row="1"/>
    </Grid>
</Window> 


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Reflection;
using System.Windows.Threading;

namespace WpfApp38
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        DispatcherTimer _timer = new DispatcherTimer();
        int _counter;
        Random _rnd = new Random();
        public MainWindow()
        {
            InitializeComponent();
            DispatcherTimerUpdate();
        }

        private void DispatcherTimerUpdate()
        {
            _timer.Interval = TimeSpan.FromSeconds(0.5);
            _timer.Tick += _timer_Tick;
            _timer.Start();
        }

        private void _timer_Tick(object sender, EventArgs e)
        {
            _timeTb.Text = DateTime.Now.ToLongTimeString();
            if(++_counter==4)
            {
                var brushes=typeof(Brushes).GetProperties(BindingFlags.Public | BindingFlags.Static);
                _e1.Fill = (Brush)brushes[_rnd.Next(brushes.Length)].GetValue(null);
                _counter = 0;
            } 
        }
    }
}