Is there a way to check if a file is in use

Question

I m writing a program in C  that needs to repeatedly access 1 image file  Most of the time it works  but if my computer s running fast  it will try to access the file before it s been saved back to the filesystem and throw an error    quot File in use by another process quot   I would like to find a way around this  but all my Googling has only yielded creating checks by using exception handling  This is against my religion  so I was wondering if anyone has a better way of doing it

User · Answer

You can suffer from a thread race condition on this which there are documented examples of this being used as a security vulnerability. If you check that the file is available, but then try and use it you could throw at that point, which a malicious user could use to force and exploit in your code.

Your best bet is a try catch / finally which tries to get the file handle.

try
{
   using (Stream stream = new FileStream("MyFilename.txt", FileMode.Open))
   {
        // File/Stream manipulating code here
   }
} catch {
  //check here why it failed and ask user to retry if the file is in use.
}

User · Answer

Updated NOTE on this solution  Checking with FileAccess ReadWrite will fail for Read-Only files so the solution has been modified to check with FileAccess Read  While this solution works because trying to check with FileAccess Read will fail if the file has a Write or Read lock on it  however  this solution will not work if the file doesn t have a Write or Read lock on it  i e  it has been opened  for reading or writing  with FileShare Read or FileShare Write access   ORIGINAL  I ve used this code for the past several years  and I haven t had any issues with it   Understand your hesitation about using exceptions  but you can t avoid them all of the time   protected virtual bool IsFileLocked FileInfo file        try               using FileStream stream   file Open FileMode Open  FileAccess Read  FileShare None                         stream Close                        catch  IOException                  the file is unavailable because it is            still being written to           or being processed by another thread           or does not exist  has already been processed          return true               file is not locked     return false

User · Answer

Try and move copy the file to a temp dir  If you can  it has no lock and you can safely work in the temp dir without getting locks  Else just try to move it again in x seconds

User · Answer

retry possibility    somecode here  try       using FileStream stream   file Open FileMode Open  FileAccess Read  FileShare None                 stream Close                write or open your file here   catch  IOException        DialogResult dialogResult   MessageBox Show  quot This file is opened by you or another user  Please close it and press retry  n quot   expFilePath   quot File Locked quot   MessageBoxButtons RetryCancel       if  dialogResult    DialogResult Retry                goto retry possibility            else if  dialogResult    DialogResult Cancel                  do nothing

User · Answer

I once needed to upload PDFs to an online backup archive   But the backup would fail if the user had the file open in another program  such as PDF reader    In my haste  I attempted a few of the top answers in this thread but could not get them to work   What did work for me was trying to move the PDF file to its own directory   I found that this would fail if the file was open in another program  and if the move were successful there would be no restore-operation required as there would be if it were moved to a separate directory   I want to post my basic solution in case it may be useful for others  specific use cases   string str path and name   str path          str filename  FileInfo fInfo   new FileInfo str path and name   bool open elsewhere   false  try       fInfo MoveTo str path and name     catch  Exception ex        open elsewhere   true     if  open elsewhere          handle case

User · Answer

the only way I know of is to use the Win32 exclusive lock API which isn t too speedy  but examples exist   Most people  for a simple solution to this  simply to try catch sleep loops

User · Answer

I m interested to see if this triggers any WTF reflexes  I have a process which creates and subsequently launches a PDF document from a console app  However  I was dealing with a frailty where if the user were to run the process multiple times  generating the same file without first closing the previously generated file  the app would throw an exception and die  This was a rather frequent occurrence because file names are based on sales quote numbers   Rather than failing in such an ungraceful manner  I decided to rely on auto-incremented file versioning   private static string WriteFileToDisk byte   data  string fileName  int version   0        try               var versionExtension   version  gt  0       version 000     string Empty          var filePath   Path Combine AppDomain CurrentDomain BaseDirectory     fileName  versionExtension  pdf            using  var writer   new FileStream filePath  FileMode Create                         writer Write data  0  data Length                     return filePath            catch  IOException                return WriteFileToDisk data  fileName    version             Probably some more care can be given to the catch block to ensure I m catching the correct IOException s   I ll probably also clear out the app storage on startup since these files are intended to be temporary anyways   I realize this goes beyond the scope of the OP s question of simply checking if the file is in use but this was indeed the problem I was looking to solve when I arrived here so perhaps it will be useful to someone else

User · Answer

Use this to check if a file is locked   using System IO  using System Runtime InteropServices  internal static class Helper   const int ERROR SHARING VIOLATION   32  const int ERROR LOCK VIOLATION   33   private static bool IsFileLocked Exception exception        int errorCode   Marshal GetHRForException exception   amp    1  lt  lt  16  - 1       return errorCode    ERROR SHARING VIOLATION    errorCode    ERROR LOCK VIOLATION     internal static bool CanReadFile string filePath          Try-Catch so we dont crash the program and can check the exception     try             The  using  is important because FileStream implements IDisposable and            using  will avoid a heap exhaustion situation when too many handles             are left undisposed          using  FileStream fileStream   File Open filePath  FileMode Open  FileAccess ReadWrite  FileShare None                 if  fileStream    null  fileStream Close       This line is me being overly cautious  fileStream will never be null unless an exception occurs    and I know the  using  does it but its helpful to be explicit - especially when we encounter errors - at least for me anyway                      catch  IOException ex              THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED            if  IsFileLocked ex                    do something  eg File Copy or present the user with a MsgBox - I do not recommend Killing the process that is locking the file             return false                      finally             return true        For performance reasons I recommend you read the file content in the same operation  Here are some examples   public static byte   ReadFileBytes string filePath        byte   buffer   null      try               using  FileStream fileStream   File Open filePath  FileMode Open  FileAccess ReadWrite  FileShare None                         int length    int fileStream Length      get file length             buffer   new byte length                 create buffer             int count                                actual number of bytes read             int sum   0                              total number of bytes read                 read until Read method returns 0  end of the stream has been reached              while   count   fileStream Read buffer  sum  length - sum    gt  0                  sum    count      sum is a buffer offset for next reading              fileStream Close      This is not needed  just me being paranoid and explicitly releasing resources ASAP                     catch  IOException ex                  THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED            if  IsFileLocked ex                            do something                       catch  Exception ex                  finally                 return buffer     public static string ReadFileTextWithEncoding string filePath        string fileContents   string Empty      byte   buffer      try               using  FileStream fileStream   File Open filePath  FileMode Open  FileAccess ReadWrite  FileShare None                         int length    int fileStream Length      get file length             buffer   new byte length                 create buffer             int count                                actual number of bytes read             int sum   0                              total number of bytes read                 read until Read method returns 0  end of the stream has been reached              while   count   fileStream Read buffer  sum  length - sum    gt  0                                sum    count      sum is a buffer offset for next reading                            fileStream Close      Again - this is not needed  just me being paranoid and explicitly releasing resources ASAP                Depending on the encoding you wish to use - I ll leave that up to you             fileContents   System Text Encoding Default GetString buffer                       catch  IOException ex                  THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED            if  IsFileLocked ex                            do something                       catch  Exception ex                  finally                  return fileContents     public static string ReadFileTextNoEncoding string filePath        string fileContents   string Empty      byte   buffer      try               using  FileStream fileStream   File Open filePath  FileMode Open  FileAccess ReadWrite  FileShare None                         int length    int fileStream Length      get file length             buffer   new byte length                 create buffer             int count                                actual number of bytes read             int sum   0                              total number of bytes read                 read until Read method returns 0  end of the stream has been reached              while   count   fileStream Read buffer  sum  length - sum    gt  0                                 sum    count      sum is a buffer offset for next reading                            fileStream Close      Again - this is not needed  just me being paranoid and explicitly releasing resources ASAP              char   chars   new char buffer Length   sizeof char    1               System Buffer BlockCopy buffer  0  chars  0  buffer Length               fileContents   new string chars                       catch  IOException ex                  THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED            if  IsFileLocked ex                            do something                       catch  Exception ex                  finally                  return fileContents      Try it out yourself   byte   output1   Helper ReadFileBytes   c  temp test txt    string output2   Helper ReadFileTextWithEncoding   c  temp test txt    string output3   Helper ReadFileTextNoEncoding   c  temp test txt

User · Answer

I use this workaround  but i have a timespan between when i check the file locking with IsFileLocked function and when i open the file  In this timespan some other thread can open the file  so i will get IOException   So  i added extra code for this  In my case i want load XDocument           XDocument xDoc   null           while  xDoc    null                        while  IsFileBeingUsed  interactionXMLPath                                 Logger WriteMessage Logger LogPrioritet Warning   Deserialize can not open XML file  is being used by another process  wait                       Thread Sleep 100                             try                               xDoc   XDocument Load  interactionXMLPath                             catch                               Logger WriteMessage Logger LogPrioritet Error   Load working                                   What do you think  Can i change some thing  Maybe i did not have to use IsFileBeingUsed function at all   Thanks

User · Answer

static bool FileInUse string path                try                       using  FileStream fs   new FileStream path  FileMode OpenOrCreate                                 fs CanWrite                           return false                    catch  IOException ex                        return true                   string filePath    C   Documents And Settings  yourfilename   bool isFileInUse   isFileInUse   FileInUse filePath       Then you can do some checking if  isFileInUse     Console WriteLine  File is in use    else    Console WriteLine  File is not in use      Hope this helps

User · Answer

In my experience  you usually want to do this  then  protect  your files to do something fancy and then use the  protected  files  If you have just one file you want to use like this  you can use the trick that s explained in the answer by Jeremy Thompson  However  if you attempt to do this on lots of files  say  for example when you re writing an installer   you re in for quite a bit of hurt   A very elegant way this can be solved is by using the fact that your file system will not allow you to change a folder name if one of the files there it s being used  Keep the folder in the same file system and it ll work like a charm   Do note that you should be aware of the obvious ways this can be exploited  After all  the files won t be locked  Also  be aware that there are other reasons that can result in your Move operation to fail  Obviously proper error handling  MSDN  can help out here   var originalFolder     c  myHugeCollectionOfFiles      your folder name here var someFolder   Path Combine originalFolder        Guid NewGuid   ToString  N      try       Directory Move originalFolder  someFolder           Use files   catch    TODO  proper exception handling          Inform user  take action   finally       Directory Move someFolder  originalFolder       For individual files I d stick with the locking suggestion posted by Jeremy Thompson

User · Answer

You can return a task which gives you a stream as soon as it becomes available  It s a simplified solution  but it is a good starting point  It s thread safe   private async Task lt Stream gt  GetStreamAsync         try               return new FileStream  sample mp3   FileMode Open  FileAccess Write             catch  IOException                await Task Delay TimeSpan FromSeconds 1            return await GetStreamAsync              You can use this stream as usual   using  var stream   await FileStreamGetter GetStreamAsync          Console WriteLine stream Length

User · Answer

The accepted answers above suffer an issue where if file has been opened for writing with a FileShare Read mode or if the file has a Read-Only attribute the code will not work  This modified solution works most reliably  with two things to keep in mind  as true for the accepted solution also     It will not work for files that has been opened with a write share mode This does not take into account threading issues so you will need to lock it down or handle threading issues separately    Keeping the above in mind  this checks if the file is either locked for writing or locked to prevent reading   public static bool FileLocked string FileName        FileStream fs   null       try                  NOTE  This doesn t handle situations where file is opened for writing by another process but put into write shared mode  it will not throw an exception and won t show it as write locked         fs   File Open FileName  FileMode Open  FileAccess ReadWrite  FileShare None      If we can t open file for reading and writing then it s locked by another process for writing           catch  UnauthorizedAccessException     https   msdn microsoft com en-us library y973b725 v vs 110  aspx                  This is because the file is Read-Only and we tried to open in ReadWrite mode  now try to open in Read only mode         try                       fs   File Open FileName  FileMode Open  FileAccess Read  FileShare None                     catch  Exception                        return true     This file has been locked  we can t even open it to read                     catch  Exception                return true     This file has been locked           finally               if  fs    null              fs Close              return false

User · Answer

Would something like this help  var fileWasWrittenSuccessfully   false  while  fileWasWrittenSuccessfully    false        try               lock  new Object                          using  StreamWriter streamWriter   new StreamWriter  quot filepath txt quot    true                                 streamWriter WriteLine  quot text quot                                     fileWasWrittenSuccessfully   true            catch  Exception

User · Answer

Aside from working 3-liners and just for reference  If you want the full blown information - there is a little project on Microsoft Dev Center   https   code msdn microsoft com windowsapps How-to-know-the-process-704839f4  From the Introduction      The C  sample code developed in  NET Framework 4 0 would help in   finding out which is the process that is having a lock on a file    RmStartSession function which is included in rstrtmgr dll has been   used to create a restart manager session and according to the return   result a new instance of Win32Exception object is created  After   registering the resources to a Restart Manager session via   RmRegisterRescources function  RmGetList function is invoked to check   what are the applications are using a particular file by enumerating   the RM PROCESS INFO array    It works by connecting to the  Restart Manager Session       The Restart Manager uses the list of resources registered with the session to   determine which applications and services must be shut down and restarted    Resources can be identified by filenames  service short names  or   RM UNIQUE PROCESS structures that describe running applications    It might be a little overengineered for your particular needs    But if that is what you want  go ahead and grab the vs-project

User · Answer

Perhaps you could use a FileSystemWatcher and watch for the Changed event   I haven t used this myself  but it might be worth a shot  If the filesystemwatcher turns out to be a bit heavy for this case  I would go for the try catch sleep loop

User · Answer

Just use the exception as intended   Accept that the file is in use and try again  repeatedly until your action is completed   This is also the most efficient because you do not waste any cycles checking the state before acting   Use the function below  for example  TimeoutFileAction      gt    System IO File etc     return null         Reusable method that times out after 2 seconds  private T TimeoutFileAction lt T gt  Func lt T gt  func        var started   DateTime UtcNow      while   DateTime UtcNow - started  TotalMilliseconds  lt  2000                try                       return func                                          catch  System IO IOException exception                          ignore  or log somewhere if you want to                     return default T

User · Answer

Here is some code that as far as I can best tell does the same thing as the accepted answer but with less code       public static bool IsFileLocked string file                try                       using  var stream   File OpenRead file                   return false                    catch  IOException                        return true                            However I think it is more robust to do it in the following manner       public static void TryToDoWithFileStream string file  Action lt FileStream gt  action           int count  int msecTimeOut                FileStream stream   null          for  var i   0  i  lt  count    i                        try                               stream   File OpenRead file                   break                            catch  IOException                                Thread Sleep msecTimeOut                                   action stream

User · Answer

You can use my library for accessing files from multiple apps   You can install it from nuget  Install-Package Xabe FileLock  If you want more information about it check https   github com tomaszzmuda Xabe FileLock  ILock fileLock   new FileLock file   if fileLock Acquire TimeSpan FromSeconds 15   true         using fileLock                   file operations here           fileLock Acquire method will return true only if can lock file exclusive for this object  But app which uploading file must do it in file lock too  If object is inaccessible metod returns false

[c#] Is there a way to check if a file is in use?

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