保存图像时GDI异常

Question

这是我的代码：

protected void Resize(object sender, EventArgs e)
{

    img3.Src = img1.Src;

    img3.Width = Int32.Parse(imgWidth.Text);
    img3.Height = Int32.Parse(imgHeight.Text);

    String saveDir = "~/Images/rect/";
    Image rimg = Image.FromFile(Server.MapPath(img3.Src));
    Rectangle section = new Rectangle(new Point(0, 0), new Size(Int32.Parse(imgw.Value), Int32.Parse(imgh.Value)));
    Bitmap b = new Bitmap(rimg, section.Size);
    Graphics gr = Graphics.FromImage(b);
    b.Save(Server.MapPath(saveDir + "rec" + DateTime.Now.ToString("ddMMyyyyhhmmss") + ".jpg"));
}

错误在这一行：

b.Save(Server.MapPath(saveDir + "rec" + DateTime.Now.ToString("ddMMyyyyhhmmss") + ".jpg"));

错误是：

  

`System.Runtime.InteropServices.ExternalException：一般错误   发生在GDI +。