ASP.NET中如何检测一个图片是否是真实图片 防范病毒上传

主要是用来判断客户端上传的图片是否为真实的图片，以防病毒侵入，保证上传的文件的安全。

主要代码如下：

需要引用

/// <summary>        /// 上传头像        /// </summary>        /// <param name="userId">用户编号</param>        /// <returns>Json(-1表示系统异常，-2表示文件不合法)</returns>        [HttpPost]         public JsonResult UploadAvatar(string userId)        {            //上传头像            string folderPath = "/upload/avatar/";            //判断路径是否存在            if (!Directory.Exists(folderPath))                Directory.CreateDirectory(folderPath);//创建文件路径            HttpPostedFileBase uploadFile = Request.Files["avatars"];            if (uploadFile != null)            {                string oriFileName = uploadFile.FileName;//原始文件名                string fileName = userId + "_" + oriFileName;                uploadFile.SaveAs(Server.MapPath(folderPath + fileName));                FileStream fs = new FileStream(Server.MapPath(folderPath + fileName), FileMode.Open, FileAccess.Read);                BinaryReader reader = new BinaryReader(fs);                string fileClass;                byte buffer;                byte[] b = new byte[2];                buffer = reader.ReadByte();                b[0] = buffer;                fileClass = buffer.ToString();                buffer = reader.ReadByte();                b[1] = buffer;                fileClass += buffer.ToString();                reader.Close();                fs.Close();                if (fileClass == "255216" || fileClass == "7173" || fileClass == "6677" || fileClass == "13780")                {                    //255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar                     //Response.Write("图片可用");                     //保存到数据库中                }                else                {                                       //Response.Write("图片非法");                     FileInfo f = new FileInfo(Server.MapPath(folderPath + fileName));                    f.Delete(); //删除文件                    return Json(-2, JsonRequestBehavior.AllowGet);                }                return Json(Server.HtmlEncode(folderPath + fileName), JsonRequestBehavior.AllowGet);            }            return Json(-1, JsonRequestBehavior.AllowGet);        }