C#解析DWG文件并实现BMP图像预览

C#读取DWG文件

struct BITMAPFILEHEADER
        {
            public short bfType;
            public int bfSize;
            public short bfReserved1;
            public short bfReserved2;
            public int bfOffBits;
        }
        public static System.Drawing.Image GetDwgImage(string FileName)
        {
            if (!(File.Exists(FileName)))
            {
                throw new FileNotFoundException("文件没有被找到");
            }
            
            FileStream DwgF=null;   //文件流
            int PosSentinel;   //文件描述块的位置
            BinaryReader br=null;   //读取二进制文件
            int TypePreview;   //缩略图格式
            int PosBMP;    //缩略图位置 
            int LenBMP;    //缩略图大小
            short biBitCount; //缩略图比特深度 
            BITMAPFILEHEADER biH; //BMP文件头，DWG文件中不包含位图文件头，要自行加上去
            byte[] BMPInfo;    //包含在DWG文件中的BMP文件体
            MemoryStream BMPF = new MemoryStream(); //保存位图的内存文件流
            BinaryWriter bmpr = new BinaryWriter(BMPF); //写二进制文件类
            System.Drawing.Image myImg = null;
            try

            {

                DwgF = new FileStream(FileName, FileMode.Open, FileAccess.Read); //文件流

                br = new BinaryReader(DwgF);
                DwgF.Seek(13, SeekOrigin.Begin); //从第十三字节开始读取
                PosSentinel = br.ReadInt32();   //第13到17字节指示缩略图描述块的位置
                DwgF.Seek(PosSentinel + 30, SeekOrigin.Begin);   //将指针移到缩略图描述块的第31字节
                TypePreview = br.ReadByte();   //第31字节为缩略图格式信息，2 为BMP格式，3为WMF格式
                if (TypePreview == 1)
                {
                }
                else if (TypePreview == 2 || TypePreview == 3)
                {
                    PosBMP = br.ReadInt32(); //DWG文件保存的位图所在位置
                    LenBMP = br.ReadInt32(); //位图的大小
                    DwgF.Seek(PosBMP + 14, SeekOrigin.Begin); //移动指针到位图块
                    biBitCount = br.ReadInt16(); //读取比特深度
                    DwgF.Seek(PosBMP, SeekOrigin.Begin); //从位图块开始处读取全部位图内容备用
                    BMPInfo = br.ReadBytes(LenBMP); //不包含文件头的位图信息
                    br.Close();
                    DwgF.Close();
                    biH.bfType = 19778; //建立位图文件头
                    if (biBitCount < 9)
                    {
                        biH.bfSize = 54 + 4 * (int)(Math.Pow(2, biBitCount)) + LenBMP;
                    }
                    else
                    {
                        biH.bfSize = 54 + LenBMP;