WPF WriteableBitmap DispatcherTimer WritePixels BackBufferStride

<Window x:Class="WpfApp46.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:WpfApp46"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Image x:Name="_img"/>
    </Grid>
</Window>


//cs
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.Windows.Threading;

namespace WpfApp46
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        WriteableBitmap _bmp;
        DispatcherTimer _timer;
        Random _rnd = new Random();
        int[] _pixel = { 255 };

        public MainWindow()
        {
            InitializeComponent();
            _bmp = new WriteableBitmap(100, 100, 0, 0, PixelFormats.Gray8, null);
            _img.Source= _bmp;
            _timer=new DispatcherTimer();
            _timer.Interval = TimeSpan.FromMilliseconds(200);
            _timer.Tick += delegate
            {
                int x = _rnd.Next(_bmp.PixelWidth);
                int y=_rnd.Next(_bmp.PixelHeight);
                _bmp.WritePixels(new Int32Rect(x, y, 1, 1), _pixel, _bmp.BackBufferStride, 0);
            };
            _timer.Start();
        }
    }
}