imageItem在c中返回null#

我有一行代码要从这个项目中提取图像数据,但代码会跳过,因为imageItem=null。造成这种情况的可能原因是什么?

foreach (Layer layer in level.Layers)
{
    foreach (Item item in layer.Items)
    {
        ImageItem imageItem = item as ImageItem;
        if (imageItem != null)
        {
            Texture2D texture = imageItem.Texture;
            imageItemList[iImageItemNum].itemTextureDataList[0] =
                     new Color[imageItem.Texture.Width * imageItem.Texture.Height];
            imageItem
                .Texture
                .GetData(imageItemList[iImageItemNum]
                .itemTextureDataList[0]);
            iImageItemNum++;
        }
    }
}