WriteableBitmap类 问题 个人实在搞不定，不知问题出在哪了

[zwb65300297]

我模仿深蓝一个WPF剪切图片的方法 使用WriteableBitmap类
部分源码：
void bi1_ImageOpened(object sender, RoutedEventArgs e)
{
img2.Source = sender as BitmapImage;

writeableBitmap.Render(img2, null);
writeableBitmap.Invalidate();
this.img.Source = writeableBitmap;
EquipPart(Equipment, 8, 34, 150, 150);

this.img11.Source = PartImage[0, 1][0, 0];
}
此时this.img.Source = writeableBitmap;
正常显示
下来开始剪切图片：EquipPart方法
public ImageSource[,] EquipPart(int[] Equipment, int rowNum, int colNum, int singleWidth, int singleHeight)
{

ImageSource[,] bitmap = new ImageSource[rowNum, colNum];

WriteableBitmap wb1 = new WriteableBitmap(singleWidth, singleHeight);
for (int i = 0; i < rowNum; i++)
{

for (int j = 0; j < colNum; j++)
{
wb1.Render(this.img, new TranslateTransform() { X = -1 * rowNum * singleWidth, Y = -1 * rowNum * singleHeight });
wb1.Invalidate();
bitmap[i, j] =(ImageSource) wb1;


}

}

return bitmap;

此时this.img11.Source = PartImage[0, 1][0, 0];
不能正常显示 实在不知道问题出在哪了
往大家帮帮忙。。。