c# Bitmap转Byte[]

            Bitmap bitmap = qrEncoder.Encode(content); //二维码生成的bitmap数据
            MemoryStream stream = new MemoryStream();
            bitmap.Save(stream, ImageFormat.Png);

            byte[] rtn = new byte[stream.Length];
            stream.Seek(0, SeekOrigin.Begin);
            stream.Read(rtn, 0, Convert.ToInt32(stream.Length));
            stream.Close();