c# – 列出异常“参数无效”.

List<Image> _Images = new List<Image>();
    int currIndex = 0;

    private void btnAdd_Click(object sender,EventArgs e)
    {
        using (OpenFileDialog dialog = new OpenFileDialog())
        {
            dialog.Filter = "All Images|*.jpg;*.jpeg;*.png;*.bmp;*.gif";
            if (dialog.ShowDialog() == DialogResult.Cancel) 
                return;
            _Images.Add(Image.FromFile(dialog.FileName));
            currIndex = _Images.Count - 1;
            picBox.Image = _Images[currIndex];
        }
    }

    private void btnNext_Click(object sender,EventArgs e)
    {
        if (currIndex + 1 >= _Images.Count)
            return;
        picBox.Image = _Images[++currIndex];
    }

    private void btnBack_Click(object sender,EventArgs e)
    {
        if (currIndex - 1 < 0)
            return;
        picBox.Image = _Images[--currIndex]; 
    }

在我将两个图像添加到该列表后,当我按下返回按钮btnBack_Click时,我得到了此异常：参数无效.
当我第一次添加图像时,为什么它才起作用picBox.Image = _Images [currIndex];然后当我试图从索引中获取图像时,它给了我那个例外吗？

注意：我没有使用ImageList,因为据我所知,它有ImageSize,它对所有图像都是常量.

那怎么能让它运作起来呢？

更新：
它现在有效,当我改变List< Image>列出<流>

picBox.Image = Image.FromStream(_Images[--currIndex]);