我在网上找了方法
 struct BITMAPFILEHEADER
        {
            public short bfType;
            public int bfSize;
            public short bfReserved1;
            public short bfReserved2;
            public int bfOffBits; 
        }

        public Image GetDwgImage(string FileName)
        {
            if (!(File.Exists(FileName)))
            {
                throw new FileNotFoundException("文件没有被找到");
            }
            FileStream DwgF;  //文件流   
            int PosSentinel;  //文件描述块的位置   
            BinaryReader br;  //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
            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(LenBMP, 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;
                    }
                    biH.bfReserved1 = 0; //保留字节   
                    biH.bfReserved2 = 0; //保留字节   
                    biH.bfOffBits = 14 + 40 + 1024; //图像数据偏移   
                    //以下开始写入位图文件头   
                    bmpr.Write(biH.bfType); //文件类型   
                    bmpr.Write(biH.bfSize);  //文件大小   
                    bmpr.Write(biH.bfReserved1); //0   
                    bmpr.Write(biH.bfReserved2); //0   
                    bmpr.Write(biH.bfOffBits); //图像数据偏移   
                    bmpr.Write(BMPInfo); //写入位图   
                    BMPF.Seek(0, SeekOrigin.Begin); //指针移到文件开始处   
                    myImg = Image.FromStream(BMPF); //创建位图文件对象   
                    bmpr.Close();
                    BMPF.Close();
                }
                return myImg;
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message);
            }
        }   实现不了

有没有人做过呀。。。。