关于WPF？WriteableBitmap类直接操作像素点的问题（wpf objectdataprovider）奔走相告

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
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.Drawing;
using System.Drawing.Drawing2D;

namespace WpfApp1
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
public void Button_Click(object sender, RoutedEventArgs e)
WriteableBitmap wb=new WriteableBitmap((int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight, 96, 96, PixelFormats.Pbgra32, null);
wb.Lock();
Bitmap backBitmap=new Bitmap((int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight, wb.BackBufferStride, System.Drawing.Imaging.PixelFormat.Format32bppArgb, wb.BackBuffer);
Int32Rect rect=new Int32Rect(0, 0, (int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight);
byte[] pixels=new byte[(int)imgGrid.ActualWidth * (int)imgGrid.ActualHeight * wb.Format.BitsPerPixel / 8];
Random rand=new Random();
for (int y=0; y < wb.PixelHeight; y++)
{
for (int x=0; x < wb.PixelWidth; x++)
{
int alpha=0;
int red=0;
int green=0;
int blue=0;
if ((x % 5==0) || (y % 7==0))
{
red=(int)((double)y / wb.PixelHeight * 255);
green=rand.Next(100, 255);
blue=(int)((double)x / wb.PixelWidth * 255);
alpha=255;
}
else
red=(int)((double)x / wb.PixelWidth * 255);
blue=(int)((double)y / wb.PixelHeight * 255);
alpha=50;
int pixeloffset=(x + y * wb.PixelWidth) * wb.Format.BitsPerPixel / 8;
pixels[pixeloffset]=(byte)blue;
pixels[pixeloffset + 1]=(byte)green;
pixels[pixeloffset + 2]=(byte)red;
pixels[pixeloffset + 3]=(byte)alpha;
}
int stride=(wb.PixelWidth * wb.Format.BitsPerPixel) / 8;
wb.WritePixels(rect, pixels, stride, 0);
}
wb.Unlock();
backBitmap.Dispose();
backBitmap=null;
img.Source=wb;
}
}