C#快速获取JPG图片大小及英寸分辨率

        public static int getJpgSize(string FileName, out Size JpgSize, out float Wpx, out float Hpx)
        {//C#快速获取JPG图片大小及英寸分辨率
            JpgSize = new Size(0, 0);
            Wpx = 0; Hpx = 0;
            int rx = 0;
            if (!File.Exists(FileName)) return rx;
            FileStream F_Stream = File.OpenRead(FileName);
            int ff = F_Stream.ReadByte();
            int type = F_Stream.ReadByte();
            if (ff != 0xff || type != 0xd8)
            {//非JPG文件
                F_Stream.Close();
                return rx;
            }
            long ps = 0;
            do
            {
                do
                {
                    ff = F_Stream.ReadByte();
                    if (ff < 0) //文件结束
                    {
                        F_Stream.Close();
                        return rx;
                    }
                } while (ff != 0xff);

                do
                {
                    type = F_Stream.ReadByte();
                } while (type == 0xff);

                //MessageBox.Show(ff.ToString() + "," + type.ToString(), F_Stream.Position.ToString());
                ps = F_Stream.Position;
                switch (type)
                {
                    case 0x00:
                    case 0x01:
                    case 0xD0:
                    case 0xD1:
                    case 0xD2:
                    case 0xD3:
                    case 0xD4:
                    case 0xD5:
                    case 0xD6:
                    case 0xD7:
                        break;
                    case 0xc0: //SOF0段
                        ps = F_Stream.ReadByte() * 256;
                        ps = F_Stream.Position + ps + F_Stream.ReadByte() - 2; //加段长度

                        F_Stream.ReadByte(); //丢弃精度数据
                        //高度
                        JpgSize.Height = F_Stream.ReadByte() * 256;
                        JpgSize.Height = JpgSize.Height + F_Stream.ReadByte();
                        //宽度
                        JpgSize.Width = F_Stream.ReadByte() * 256;
                        JpgSize.Width = JpgSize.Width + F_Stream.ReadByte();
                        //后面信息忽略
                        if (rx!=1 && rx < 3) rx = rx + 1;
                        break;
                    case 0xe0: //APP0段
                        ps = F_Stream.ReadByte() * 256;
                        ps = F_Stream.Position + ps + F_Stream.ReadByte() - 2; //加段长度

                        F_Stream.Seek(5,SeekOrigin.Current); //丢弃APP0标记(5bytes)
                        F_Stream.Seek(2,SeekOrigin.Current); //丢弃主版本号(1bytes)及次版本号(1bytes)
                        int units=F_Stream.ReadByte(); //X和Y的密度单位,units=0：无单位,units=1：点数/英寸,units=2：点数/厘米

                        //水平方向(像素/英寸)分辨率
                        Wpx=F_Stream.ReadByte()*256;
                        Wpx=Wpx+F_Stream.ReadByte();
                        if (units == 2) Wpx = (float)(Wpx * 2.54); //厘米变为英寸
                        //垂直方向(像素/英寸)分辨率
                        Hpx = F_Stream.ReadByte() * 256;
                        Hpx = Hpx + F_Stream.ReadByte();
                        if (units == 2) Hpx = (float)(Hpx * 2.54); //厘米变为英寸
                        //后面信息忽略
                        if (rx!=2 && rx<3) rx = rx + 2;
                        break;

                    default: //别的段都跳过////////////////
                        ps = F_Stream.ReadByte() * 256;
                        ps = F_Stream.Position + ps + F_Stream.ReadByte() - 2; //加段长度
                        break;
                }
                if (ps + 1 >= F_Stream.Length) //文件结束
                {
                    F_Stream.Close();
                    return rx;
                }
                F_Stream.Position = ps; //移动指针
            } while (type != 0xda); // 扫描行开始
            F_Stream.Close();
            return rx;
        }