C#检测上传文件真正类型的方法

本文实例讲述了C#检测上传文件真正类型的方法。分享给大家供大家参考。具体分析如下：

对于用户上传的文件如果只是根据扩展名判断，很容易上传上来可执行文件，这是非常危险的，这段代码可以在服务器端检测上传文件的真实类型。

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
 void Alert(string s)
 {
  Page.ClientScript.RegisterStartupScript(Page.GetType(), "js", "alert('" + s + "')", true);
 }
 protected void Button1_Click(object sender, EventArgs e)
 {
  saveFile();
 }
 protected String saveFile()
 {
  String MaxSize = "1024";
  //最大文件大小
  int imgMaxSize = Convert.ToInt32(MaxSize) * 1024 * 1024;
  HttpPostedFile imgFile = FuImg.PostedFile;
  if (imgFile == null || FuImg.FileName == "")
  {
   Alert("请选择文件。");
   return "";
  }
  String dirPath = Server.MapPath("~/");
  string saveUrl = Page.ResolveUrl("~/");
  if (!System.IO.Directory.Exists(dirPath))
  {
   Alert("上传目录不存在。");
   return "";
  }
  String fileName = imgFile.FileName;
  String fileExt = System.IO.Path.GetExtension(fileName).ToLower();
  if (imgFile.InputStream == null || imgFile.InputStream.Length > imgMaxSize)
  {
   Alert("上传文件大小超过限制。");
   return "";
  }
  //验证文件格式
  String fpath = IsAllowedExtension(imgFile);
  if ("" == fpath)
  {
   Alert("图片格式不正确。");
   return "";
  }
  String ymd = DateTime.Now.ToString("yyyyMMdd", System.Globalization.DateTimeFormatInfo.InvariantInfo);
  dirPath += ymd + "/";
  saveUrl = saveUrl + ymd + "/";
  //判断目录是否存在
  if (!System.IO.Directory.Exists(dirPath))
  {
   //创建目录
   System.IO.Directory.CreateDirectory(dirPath);
  }
  String newFileName = Guid.NewGuid().ToString() + fileExt;
  //图片名字
  String filePath = dirPath + newFileName;
  System.IO.File.Move(fpath, filePath);
  String fileUrl = saveUrl + newFileName;
  Img.ImageUrl = fileUrl;
  //ImageUrl = saveUrl + newFileName;
  return fileUrl;
 }
 public String IsAllowedExtension(HttpPostedFile f)
 {
  String newFile = Server.MapPath("~/" + System.Guid.NewGuid().ToString("D") + ".tmp");
  f.SaveAs(newFile);
  System.IO.FileStream fs = new System.IO.FileStream(newFile, System.IO.FileMode.Open, System.IO.FileAccess.Read);
  System.IO.BinaryReader r = new System.IO.BinaryReader(fs);
  string fileclass = "";
  byte buffer;
  buffer = r.ReadByte();
  fileclass = buffer.ToString();
  buffer = r.ReadByte();
  fileclass += buffer.ToString();
  r.Close();
  fs.Close();
  /* 文件扩展名说明
  *7173    gif
  *255216   jpg
  *13780    png
  *6677    bmp
   */
  Dictionary<String, String> ftype = new Dictionary<string, string>();
  //添加允许的文件类型
  ftype.Add("7173", "gif");
  ftype.Add("255216", "jpg");
  ftype.Add("13780", "png");
  ftype.Add("6677", "bmp");
  if (ftype.ContainsKey(fileclass))
  {
   return newFile;
  }
  else
  {
   System.IO.File.Delete(newFile);
   return "";
  }
 }
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
</head>
<body>
 <form id="form1" runat="server">
 <asp:FileUpload ID="FuImg" runat="server" />
 <asp:Button ID="Button1" runat="server"
 OnClick="Button1_Click" Text="上传测试" />
 <asp:Image ID="Img" runat="server" />
 </form>
</body>
</html>

希望本文所述对大家的C#程序设计有所帮助。