WPF使用WriteableBitmap更新图像

<Window x:Class="WpfApp2.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:WpfApp2"
        mc:Ignorable="d"
        Title="MainWindow" Height="400" Width="400">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Button x:Name="Btn" Content="更新图像" Click="BtnClkUpdateImage"></Button>
        <Image Name="Img" Grid.Row="1"/>
    </Grid>
</Window>

using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace WpfApp2
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        private WriteableBitmap _wbBitmap;
        int width = 1000;
        int height = 1000;
        int length = 0;    
        short[] dates;
        public MainWindow()
        {
            InitializeComponent();
            length = width * height;
            _wbBitmap = new WriteableBitmap(width, height, 96, 96, PixelFormats.Gray16, null);
            Img.Source = _wbBitmap;
            dates = new short[length];
        }
        public void ShowImage(short[] rawData)
        {
            unsafe
            {
                _wbBitmap.Lock();
                Marshal.Copy(rawData, 0, _wbBitmap.BackBuffer, length);
                _wbBitmap.AddDirtyRect(new System.Windows.Int32Rect(0, 0, width, height)); 
                _wbBitmap.Unlock();
            }
        }
        private void BtnClkUpdateImage(object sender, RoutedEventArgs e)
        {
            //随机产生图像数据
            Random ran;
            for (int i = 0; i < length; i++) {
                ran = new Random();
                dates[i] = (short)ran.Next(65535);
            }
            Btn.IsEnabled = false;
            Task task = Task.Run(() =>
            {
                while (true)
                {
                    Array.Reverse(dates);// 逆转数组
                    Img.Dispatcher.Invoke(new Action(() => 
                    {
                        ShowImage(dates);
                    }));
                    Thread.Sleep(1);
                }
            });
        }
    }
}