asp.net three effective ways to judge uploaded file types

  This article mainly introduces the use of ASP.net to determine the type of upload file three ways, the need for friends can refer to the following

First, the security is relatively low, the text file 1.txt to 1.jpg can be uploaded, but its implementation is easy to understand, implementation is also simple, so many online or take this approach.     Code as follows: Boolean fileok = false;  String path = Server.MapPath ("~/images/"); //Determine if file   if (F Ileupload1.hasfile)   { //Get file extension and convert to lowercase   string fileextension = System.IO.Path.GetExtension ( Fileupload1.filename). ToLower (); //qualification can only upload jpg and gif pictures   string[] allowextension = {". jpg", ". gif"}; //to the type of uploaded file   Int J = 0;  for (int i = 0; i < allowextension.length i++)   {  if (fileextension = Allowextension[i]) & nbsp {  FileOK = true;  return; }  else  {  j++; } }  if (J > 0)   {&nbs P Response.Write ("<script>alert (' incorrect file format ');</script>");  return; } }  else  {   Response.Write (' You have not selected file ');</script>;  return; } //If the extension meets the criteria, Then upload   if (fileok)   {  FileUpload1.PostedFile.SaveAs (path + FileuploaD1. FileName);  Response.Write ("<script>alert (' uploaded success ');</script>"); }    II, does not detect file suffixes but detects file MIME content types.   Code is as follows: Boolean fileok = false;  String path = Server.MapPath ("~/images/"); //Determine if the file   if is selected (FILEUPL Oad1. HasFile)   { //Get file MIME content type   String type = this. FileUpload1.PostedFile.ContentType.ToLower ();  if (type. Contains ("image")//the MIME type of the picture is "image/xxx", here only determines whether the picture.   {  FileOK = true; }  else  {  ("Response.Write (' malformed ') <script>alert > "); } }  else  {  Response.Write (" You have not selected file ') <script>alert ");  } //If the extension meets the criteria, upload   if (fileok)   {  FileUpload1.PostedFile.SaveAs (path + Fileupload1.filename);  Response.Write ("<script>alert (' upload success ');</script>"); }    Three , can realize the true sense of file type judgment   code as follows: try  { //Determine if the file has been selected   if (fileupload1.hasfile)   {  if ( IsallowedexTension (FILEUPLOAD1))   {  String path = Server.MapPath ("~/images/");  FileUpload1.PostedFile.SaveAs ( Path + fileupload1.filename);  Response.Write ("<script>alert (' upload success ');</script>"); }  else  {  Response.Write ("<script>alert (' You can only upload jpg or gif images ');</script>"; }  else  {  Response.Write ("You have not selected file");</script>; } }  catch ( Exception error)   {  Response.Write (error. ToString ()); }  #endregion  } //True to determine the key function of the file type   public static bool Isallowedextension ( FileUpload hifile)   {  System.IO.FileStream fs = new System.IO.FileStream (hifile. Postedfile.filename, System.IO.FileMode.Open, System.IO.FileAccess.Read);  System.IO.BinaryReader r = new System.IO.BinaryReader (FS);  string fileclass = ""; //Here's the bit length to be specifically judged .  byte buffer;  try  Buffer = R.readbyte ();  fileclass = buffer. Tostring();  buffer = R.readbyte ();  Fileclass + = buffer. ToString (); }  catch  { }  r.close ();  fs. Close ();  if (Fileclass = = "255216" | | | fileclass = = "7173")//Description 255216 is jpg;7173 is gif;6677 is bmp,13780 is EXE, 8297 is rar  {  return true; }  else  {  return false; } }