C#中怎样将Graphics转换为Stream类型？

System.Drawing.Bitmap bmp = new Bitmap((int)size.Width, (int)size.Height,
PixelFormat.Format24bppRgb);

using (Graphics g = Graphics.FromImage(bmp))
{
// 加载背景
g.DrawImage(
System.Drawing.Image.FromFile(sBkImgFile),
0, 0, size.Width, size.Height);
g.DrawString(sCode,
f,
Brushes.Black,
new RectangleF(0, 0, size.Width, size.Height),
sf);

}
using (System.IO.FileStream fs = new System.IO.FileStream("c:\\1.bmp"))
{
bmp.Save(fs, System.Drawing.Imaging.ImageFormat.Bmp);
}

Graphics 本身不能转，因为 Graphics 本身是非实体类型