I have a WPF app, which opens and edits XML files. Currently, the app can be launched multiple times, and several instances can have the same file open. I need to lock the files such that, when one is open, it won't let another instance of the app open the file. I have tried using the FileShare.None attribute when opening the file, as well as FileStream.Lock(), but for some reason, these fail to prevent a separate instance of the app from opening it.
EDIT: Relevant code
try
{
FileStream iStream = File.Open(fileName, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
iStream.Lock(0, iStream.Length);
// DO STUFF WITH FILE HERE
}
catch (System.IO.IOException ioException)
{
// Raise exception to higher level, where application will terminate.
throw (ioException);
}
You need to keep file open all the time you are "editing" it from the moment you start editing till closing (assuming you have separate processes for each instance of your app).
Your code looks like you are opening file inside on method and likely close it inside this method either by using "using" as recommended for short file operations or just letting GC to close it. As result you lock file for some time, but release it soon enough for other instances to be able to open it again.
Note that if your application implements some sort of single instance approach this locking may not be enough as all of the open operations will be executed from the same process.
Related
Let's say I have contents of an executable (or a bat script, doesn't matter) in memory and want to run it as a new process. That is easy.
File.WriteAllBytes(filePath, contents);
// gap
Process.Start(filePath)
But I want to make sure that the executed file is not tampered by any other process. And there is a gap between file creation and execution. It gives a chance to tamper the file with the right tools.
So, instead of File.WriteAllBytes, I went with opening a FileStream FileShare.Read and keeping it open until the execution has finished.
using(var fileStream = new FileStream(filePath, FileMode.CreateNew, FileAccess.Write, FileShare.Read))
{
Process.Start(filePath)
}
But this doesn't work. Process.Start fails with:
System.ComponentModel.Win32Exception (32): The process cannot access the file because it is being used by another process.
This question and its answer explains why I think. In a nutshell, Process.Start will attempt to open the file with FileShare.Read and fail because the open FileStream already has Write Access, hence failing the FileShare.Read attempt of the process.
Is there a way to do this cleanly?
A workaround I can think of is to save the file, close it, open a new FileStream with FileShare.Read and FileAccess.Read, make sure the content is still the same before executing it. But that's not pretty.
What you are describing is a classic case of Time of check to time of use vulnerability.
Any solution that involves checking something and then executing it, and where those two operations are not atomic, will still leave you vulnerable. For example:
make sure the content is still the same before executing it. But that's not pretty
There's still a (smaller) gap (timing window) between the "make sure the content is still the same" and "executing it".
In 2004, an impossibility result was published,showing that there was no portable, deterministic technique for avoiding TOCT-TOU race conditions
- https://web.cecs.pdx.edu/~markem/CS333/handouts/tocttou.pdf
You can do a couple of things to mitigate it:
Don't use files! You say you have some code in memory that you need to execute: can you execute it yourself in the same process?
Reduce the window of time.
Make the file name random and hard to predict for other processes.
Run your program as a separate user where there's less likelyhood an attacker (or malicious program) is running, and restrict the file read/write to the new user only.
I'm using a FileSystemWatcher to watch a directory. I created a _Created() event handler to fire when a file is moved to this folder. My problem is the following:
The files in this directory get created when the user hits a "real life button" (a button in our stock, not in the application). The FileSystemWatcher take this file, do some stuff in the system and then delete it. That wouldn't be a problem when the application runs only once. But it is used by 6 clients. So every application on every client is trying to delete it. If one client is too slow, it will throw an exception because the file is already deleted.
What I'm asking for is: Is there a way to avoid this?
I tried using loops and check if the file still exists, but without any success.
while (File.Exists(file))
{
File.Delete(file);
Thread.Sleep(100);
}
Can someone give me a hint how it could probably work?
Design
If you want a file to be processed by a single instance only (for example, the first instance that reacts gets the job), then you should implement a locking mechanism. Only the instance that is able to obtain a lock on the file is allowed to process and remove it, all other instances should skip the file.
If you're fine with all instances processing the file, and only care that at least one of them succeeds, then you need to figure out which exceptions indicate a genuine failure and which ones indicate a failure caused by the actions of another instance.
Locking
To 'lock' a file, you can open it with share-mode FileShare.None. This prevents other processes from opening it until you close the file. However, you'll then need to close the file before you can delete it, which leaves a small gap during which another instance could open the file.
A better solution is to create a separate lock file for that purpose. Create it with file-mode FileMode.Create and share-mode FileShare.None and keep it open until the whole process is finished, including the removal of the processed file. Then the lock file can be closed and optionally removed.
Exception
As for the UnauthorizedAccessException you got, according to the documentation, that means one of 4 things:
You don't have the required permission
The file is an executable file that is in use
The path is a directory
The file is read-only
1 and 4 seem most likely in this case (if the file was open in another process you'd get an IOException).
If you want to synchronize access between multiple clients on the same computer you should use a Named Mutex.
In my application, I need to delete files and then remove the directory that contains those files. It works great if none of the files are open.
But if any file is open (i.e. index.txt) , it is successfully deletes from directory and at the time of removing the directory it throws an exception like file is used by other application.
Is there any way to close the open file in C# using p/invoke or anything else?
The only way to delete files currently held open by other applications is to have those applications release the lock on the file (usually by closing the file) or by terminating the application itself.
Obviously, forcing an external application to terminate in order to delete a file that the app is currently holding open can often be a recipe for disaster!
If you have noticed, when you open a MS Office file it creates a "shadow" file nearby - that is something like in RAM stored document. So if you delete the real file the "shadow" file remains in directory. So the RAM uses your directory.
In other words I think it is not possible to do it in C#.
Basically, the only way to break this link and be sure about it is to kill other processes from yours. There are various ways to do this, that have been pointed out, but a better question is whether you should.
I recommend looking into a try-catch pattern and making your application report the error to the user, instead of aggressively trying to delete a document which may be open for a very good reason from the perspective of the user or even the system itself.
Also note that killing outside processes is not a garuanteed solution, as there are multiple cases where the "kill outside process" step could fail (targeted process is run as Administrator and your app isn't, targeted process it set up as a Windows Service and restarts itself before you finish deletion, targeted process is a system-critical process which can't be terminated, ect.)
The below code works great with .doc files.
But doesn't work with .txt file. It doesn't throw any exception for .txt
Still searching for solution that works with .txt files.
I am opening files through Window Explorer, not via any code.
public static void Main()
{
String file;
file = "C:\\Temp\\test.doc";
bool b=IsCloseFile(file);
if (b)
MessageBox.Show("File Open");
}
public static bool IsCloseFile(string file)
{
FileStream stream = null;
try
{
stream = File.Open(file, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
catch (IOException e)
{
return true;
}
finally
{
if (stream != null)
stream.Close();
}
return false;
}
}
I have Following Code in a Page_Load called function. When the Page is loaded the first time after starting Visual Studio, everything works out fine.
But any other opening call to the File after that returns IOException: "File is in use by another process", even when directly opening the File in VisualStudio Solution this Error is returned(of course not as Exception)
FileStream mailinglist_FileStream = new FileStream(#"\foobarFile.txt", FileMode.Open);
PeekingStreamReader mailinglist_Reader = new PeekingStreamReader(mailinglist_FileStream);
//Do some stuff with the file
mailinglist_FileStream.Close();
mailinglist_Reader.Close();
mailinglist_Reader.Dispose();
mailinglist_FileStream.Dispose();
Why is the file still locked? and why does fully restarting Visual Studio reset the File?
when checking file-Properties it says:
Build Action: Content
Copy to output directory: do not Copy
I am only reading this File. can i do something similiar to adLockOptimistic, so that multiple processes can access the File?
Why is the file still locked? and why does fully restarting Visual
Studio reset the File? when checking file-Properties it says [...]
I don't know why the file is still locked: probably because your code fails before the stream is closed/disposed.
About "why fully restarting Visual Studio [...]": because you may be using IIS Express or ASP.NET Dev Server whose are closed when you close the IDE, so locks on files are released since the process holding the locks is no longer running.
And about "why is the file still locked?[...]" it could be because the file stream isn't closed because sometimes the thread may not end successfully and the locks aren't released.
As other answer said, check how using block may avoid that IDisposable objects wouldn't be disposed:
// FileShare.ReadWrite will allow other processes
// to read and write the target file even if other processes
// are working with the same file
using var mailinglist_FileStream = new FileStream(#"\foobarFile.txt", FileMode.Open, FileShare.ReadWrite);
using var mailinglist_Reader = new PeekingStreamReader(mailinglist_FileStream);
// Do your stuff. Using blocks will call Dispose() for
// you even if something goes wrong, as it's equal to a try/finally!
I am only reading this File. can i do something similiar to
adLockOptimistic, so that multiple processes can access the File?
Yes, take a look at File.Open method and FileShare enumeration:
File.Open: http://msdn.microsoft.com/en-us/library/y973b725.aspx
FileShare enum: http://msdn.microsoft.com/en-us/library/system.io.fileshare.aspx
Learn to use using:
using (FileStream fileStream = File.Open(#"C:\somefile", FileMode.Open, FileAccess.Read))
{
...
}
The using construct ensures that the file will be closed when you leave the block even if an exception is thrown.
Your problem might not be here, but somewhere else in your code. You'll have to go through all your code and look for places where you have opened files but not put it inside a using statement.
An old question but unfortunately the given answers can be not applicable to the question.
The problem specifically in Windows lies in two aspects of Windows behavior:
a) when the handle to the file, opened for writing, is closed, the Microsoft Antimalware Service opens the file to check the newly written data for malware;
b) the OS itself keeps the file opened for some time after all handles to it are closed. This time can be from seconds to many minutes depending on the nature of the file and other factors.
We saw this problem many times in our products and had to provide special support for this case - our kernel-mode attempts to close the file as soon as the last handle to it is closed.
Try using using blocks, it may not fix your lock problem, but it is better form for disposable objects.
using (FileStream mailinglist_FileStream = new FileStream(#"\foobarFile.txt", FileMode.Open))
{
using (PeekingStreamReader mailinglist_Reader = new PeekingStreamReader(mailinglist_FileStream))
{
...
}
}
Also, try closing mailinglist_Reader before mailinglist_FileStream.
My program creates a log file when it starts. The user has the option through settings to "clear the log" which calls a method to delete the log file.
//calls for a YesNo prompt to delete log or not
result = objectMessageBox.ReturnDeleteLogPrompt();
if (result == DialogResult.Yes)
{
//throw prompt
if (File.Exists(objectLog.GetLogLocation()) == true)
{
try
{
//delete the log file
File.Delete(objectLog.GetLogLocation());
//throw balloon tip saying log was cleared
ShowBalloonTip("LogCleared");
}
catch (Exception ee)
{
MessageBox.Show("Error thrown deleting log: " + ee);
System.Windows.Forms.Clipboard.SetText(ee.ToString());
}
}
}
Because I have deleted the log file entirely I need to then recreate it. So I call a method that has this:
try
{
//we create a new log file so it seems that the log has just been cleared
objectLog.CreateLog();
}
catch (Exception ee)
{
MessageBox.Show("Error occured while clearing log:\n" + ee);
}
But when it attempts to recreate the log file it throws an error that says:
"System.IO.IOException: The process cannot access the file '~~' because it is being used by another process."
So it seems that during my call to delete the file it keeps accessing it? Do I need to dispose of something when I call the file.delete?
I don't know the details, but there are numerous reasons for why a filename isn't immediately available for recreation after deleting an existing file:
The delete operation is still pending by the operating system
An antivirus program or similar security feature opened up the file in response to it being deleted, for pre-deletion analysis
An antivirus program or similar security feature already had the file open while you were using it, and is still in the progress of responding to your deletion request
Mercurial had this problem on Windows as well. If you executed one command that locked the repository (which was done using temporary files), and then immediately executed another command that either needed to lock, or at least ensure no lock was present, it could fail with the same type of error, the file was in use, even though this was two distinct processes and the first had already exited.
In other words, the timeline was as follows:
hg.exe instance #1 starts up, locks the repository by creating the temp file
hg.exe does what it needs to do
hg.exe deletes the file, then exits
hg.exe instance #2 starts up, attempts to lock the repository, fails because file is in use
Their hack to "fix" this was to simply pick a random filename that wasn't used in the directory, rename the file to that name, and then delete it. This did not solve the problem of the file lingering for a short while, but it did free up the filename and make it available for new files right away.
There already is an accepted answer, but perhaps someone finds this useful (or laugh at it if I missed something obvious again and wasted my time completely)
I had the impression that File.Delete would either delete the file and then return, or otherwise throw an exception - until I read this thread.
The windows API mentions that by calling DeleteFile the file is "marked for deletion on close" since it allows calling delete on an open file. After a file is marked for deletion, an attempt to open it will fail as "Access denied". When the last handle for this file is closed, the file is actually deleted.
If windows actually deletes the file before returning from the last CloseHandle call on the file, in theory this code would guarantee that the file is deleted below the using block:
using (File.Open(path, FileMode.Open, FileAccess.Read, FileShare.Delete))
{
File.Delete(path);
}
The File.Open would fail if another process currently has the file open.
Note the difference here that File.Delete even succeeds if a file does not exist (unless the directory does not exist).
Instead of deleting and recreating the same file, can you just clear it out?
Something like this should work for you:
FileStream f = File.Open(#[filename], FileMode.Create);
f.Close();
You could use System.IO.FileInfo.Delete to delete the file, and then System.IO.FileInfo.Refresh() before creating the file again. The Refresh should stop the exception from happening on re-creating the file. Or as nycdan says, use the FileMode.Create enum.