I have some code that saves a string to a file, something like this:
string TheFileJS = HttpRuntime.AppDomainAppPath + "\\SomePath\\" + ClientFileName + ".js";
if (File.Exists(TheFileJS) == true)
{
File.Delete(TheFileJS);
}
File.WriteAllText(TheFileJS, TheJS);
I'm using File.WriteAllText because I thought it would prevent problems with file locking but what's happening is that sometimes I get the error File is being used by another process. The problem is that it rarely happens, but once this exception occurs on a particular file, all client calls to this file then result in a 404.
What do I need to change in my code to make sure this error never happens?
I would imagine that you are running into problems with the lock still being open after the delete causing you to be unable to then rewrite the file.
The good news is that this is easily solvable by not deleting the file first.
From the docs for WriteAllText it says "Creates a new file, writes the specified string to the file, and then closes the file. If the target file already exists, it is overwritten."
What this means is that it effectively deletes the contents of the file anyway so checking if the file exists first is unnecessary. If you are not doing that check then you shouldn't run into any problems.
This is also where exception handling would come in handy. If this is critical code then you could retry or alert an admin immediately of the problem hopefully preventing all your clients then 404ing.
Try this. During file creation, if any stream is opened, it will be closed.
FileStream stream = File.Create(TheFileJS);
stream.Close();
Related
This question already has answers here:
File.Move Does Not Work - File Already Exists
(9 answers)
Closed 2 years ago.
I'm developing a multi threaded application. I have somewhere in my code :
File.Delete(sidetapedata);
File.Move(sidetapedata2, sidetapedata); //sidetapedata and sidetapedata2 are two file paths that correspond to sidetapedata.txt and sidetaptdata2.txt in some directory.
The second line sometimes runs fine and other times, it throws an IOException :
Cannot create a file when that file already exists.
There is one more thread that is accessing the sidetapedata file but that one is only reading this file, no write operations. I am using locks to protect race conditions. Don't know why this is happening.
UPDATE : even when visual c# debugger shows me this exception, looking into the directory that contains these files, I see there is no sidetapedata.txt file but there is a sidetapedata2.txt file!
UPDATE2 : Also, this behavior only happens when sidetapedata.txt and sidetapedata2.txt are both blank
Not sure why this would happen unless there's some event triggered in the file system by the Delete call which means it's not actually deleted until slightly after the call returns. A few options:
You could loop (with some sort of maximum number of loops before erroring) where you check for the file's existence before trying the move, and sleep briefly if it still exists after deletion
You could use File.Copy(sidetapedata, sidetapedata2, true) to copy instead of moving, and then delete the source file. This will be less efficient though, assuming the move would be handled by a simple file system directory entry change (rather than really copying the data)
You could use File.Move on the target file instead of File.Delete to move it to some harmless other filename, then delete that afterwards, hoping that the Move is more atomic than the Delete.
I suspect the threading is irrelevant here - I suggest you write a short but complete program to validate that, so you can rule it out (and easily test the workarounds).
I am unsure if this is the same for .NET, but according to the win32 DeleteFile api reference:
The DeleteFile function marks a file for deletion on close. Therefore, the file deletion does not occur until the last handle to the file is closed.
So there is probably a window of time between the call to Delete returning and Windows closing the last handle to the file. It looks like you are calling Move during this period of time.
In .NET Core 3.0 and later versions, you can call Move(String, String, Boolean) setting the parameter overwrite to true, which will replace the file if it exists.
see https://learn.microsoft.com/en-us/dotnet/api/system.io.file.move?view=netcore-3.0
As per this answer : use FileStream with FileShare
FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
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.
I have the following code:
using (MemoryStream str = new MemoryStream())
{
Program.api.GetDocument(result, str);
using (FileStream fileStream = File.Create(filePath))
{
str.WriteTo(fileStream);
}
}
Whenever a file is written, it is always locked afterwards - attempting to delete it or modify it causes Windows to tell me the file is in use, even after closing my application. Am I missing something?
Your problem is most likely caused by Windows Search Indexing which is a part of Windows Search. If you attempt to access the file immediately (or very shortly) after modifying it, you may run into the sort of issues you are seeing. The best way around this is to add retry logic to the file operation you are performing, which waits some small period of times and re-attempts the file op.
If you would like to confirm that the problem is cause by Windows File Search Indexing, you can disable it for the file type and/or location where you are writing your file to see if that makes the problem go away.
I am trying to create a new file and write XML to it:
FileStream output = File.Create(Path.Combine(PATH_TO_DATA_DIR, fileName));
The argument evaluates to:
C:\path\to\Data\test.xml
The exception is:
The process cannot access the file 'C:\path\to\Data\test.xml' because it is being used by another process.
What am I doing wrong here?
UPDATE: This code throws the same exception:
StreamWriter writer = new StreamWriter(Path.Combine(PATH_TO_DATA_DIR, fileName));
UPDATE 2: The file I am trying to create does not exist in the file system. So how can be it in use?
UPDATE 3: Changed to a new file name, and now it works. I'm not sure why. Perhaps I unknowing created it in the first call, and it failed on subsequent calls?
The message means that another process is using that file. If you have it open it could be using it, or when it was originally created if the stream was not closed properly that could do it also.
First check to make sure you do not have it open. I would try to change the fileName and see if you get the same error. If you do get the same error than some place in your code it is not closing a stream that when it is done with the file.
Your program keeps a handle on your file after it's being created to return to your FileStream object. Because you don't specify the access to it, perhaps it won't let you get a grab on it. Perhaps should you consider closing it, and then reopen it in a proper manner by specifying how you want it open (ReadOnly, ReadWrite, WriteOnly) ?
Not trying to sound insulting, but does the folder exist? Does the file already exist but is hidden by the system? And does the user account that is running the program have write permissions to the folder? Have you tried creating a file using a different method (like with File.WriteAllText(<path>, "Testing") just to see if it's your particular call to File.Create?
I have many processes reading a file stored on a network share. Originally I was only able to have one process read the file, all the others would throw exceptions. I implemented the following code to deal with that:
using (StreamReader fileStreamReader = new StreamReader(File.Open(path, FileMode.Open, FileAccess.Read, FileShare.Read)))
{
content = fileStreamReader.ReadToEnd();
}
This let multiple processes read the same file, however it still seems to have issues, because sometimes multiple processes still can't access the file. Yet I can go back later when the file isn't in use and open it just fine. Right now I have some retry behavior with random delays implemented that so far, seem to help. It seems a little quirky to me to do it this way, so what would be a better method?
This is the weird part, the exception I'm getting is not from file IO at all, it's from a library called CommStudio. In short, I dump the file to a string, i modify it slightly, dump it into a memory stream, and ship it off over ymodem on rs232. The exception is telling me the remote system has canceled. The device getting the data reports that there was a transmission error, which usually means that an incomplete/empty file was received.
Normally I would blame the library on this, but it works flawlessly at desk-testing and when there is only one process accessing the file. The only thing that really seems to be consistent is that it is likely to fail when multiple processes are accessing a file.
had a similar problem but not allot of time to find an ideal solution. I created a webservice and stuck the file local to the webservice app.. then created a simple one liner GET API which was called over the office intranet.. thus ensureing only the calling application edited the log file.. messy but functional.
I have had a similar problem in the past. Try changing how you access the file to something like this.
//Use FileInfo to get around OS locking of the file
FileInfo fileInfo = new FileInfo(path);
//I actually wanted unblocked read write access so change your access and share appropriately
using (FileStream fs = fileInfo.Open(FileMode.Open, FileAccess.Write, FileShare.ReadWrite))
{
//I'm using CopyTo but use whatever method matches your need
fileInfo.CopyTo(Path.Combine(destination, fileName), false);
}