Is there anyway to wait for this file to open before reading it? The file that is being read will be writing to quite a bit and dont want this error to keep happening. Should I do a while loop with a delay before trying to read it? This is a live stats page so reloading of that page will happen quite a bit.
System.IO.IOException: The process cannot access the file because it is being used by another process.
To test if the file is locked, you can use this function:
protected virtual bool IsFileLocked(string filePath)
{
FileInfo file = new FileInfo(filePath);
FileStream stream = null;
try
{
stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
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;
}
finally
{
if (stream != null)
stream.Close();
}
//file is not locked
return false;
}
Usually it is not good to use exceptions in your normal logic, but in this case you may not have a choice. You could call this every X seconds possibly, to check for locks. An alternative may be to use a file system watcher object to monitor a file. It's hard to say without knowing more about your specific use case.
Related
I am been fighting with this problem the last couple of days, it works fine when I am on my dev machine, but on the client it is showing this error.
Now this is the code that I have that seems to be showing the error so any help or guidance would be amazing, thank you in advance.
private void document()
{
StreamWriter sWrite = new StreamWriter("C:\\Demo\\index.html");
//LOTS OF SWRITE LINES HERE
sWrite.Close();
System.Diagnostics.Process.Start("C:\\Demo\\index.html");
}
So I have no idea what it keeps telling me the file is already being used by another process if I run this method twice.
Some of it depends on the exact behavior. This could be for a few reasons: it could be, for example, due to an exception. The following code will produce the exception you've described.
for (int i = 0; i < 10; i++)
{
const string path = #"[path].xml";
try
{
// After the first exception, this call will start throwing
// an exception to the effect that the file is in use
StreamWriter sWrite = new StreamWriter(path, true);
// The first time I run this exception will be raised
throw new Exception();
// Close will never get called and now I'll get an exception saying that the file is still in use
// when I try to open it again. That's because the file lock was never released due to the exception
sWrite.Close();
}
catch (Exception e)
{
}
//LOTS OF SWRITE LINES HERE
Process.Start(path);
}
A "using" block will fix this because it's equivalent to:
try
{
//...
}
finally
{
stream.Dispose();
}
In the context of your code, if you're doing a whole bunch of line writes it actually does make sense to consider if (and when) you want to call Flush at some point. The question is whether the write should be "all or none" - i.e. if an exception occurs, do you want the previous lines to still be written? If not, just use a "using" block - it'll call "Flush" once at the end in the "Dispose." Otherwise, you can call "Flush" earlier. For example:
using (StreamWriter sw = new StreamWriter(...))
{
sw.WriteLine("your content");
// A bunch of writes
// Commit everything we've written so far to disc
// ONLY do this if you could stop writing at this point and have the file be in a valid state.
sw.Flush();
sw.WriteLine("more content");
// More writes
} // Now the using calls Dispose(), which calls Flush() again
A big possible bug is if you're doing this on multiple threads (especially if you're doing a lot of writes). If one thread calls your method and starts writing to the file, and then another thread calls it too and tries to start writing to the file as well, the second thread's call will fail because the first thread's still using the file. If this is the case, you'll need to use some kind of lock to make sure that the threads "take turns" writing to the file.
here is what you can do for example before trying to open the file from the Process.Start
var path = #"C:\Demo\index.html";
using (FileStream fs = new FileStream(path, FileMode.Append, FileAccess.Write))
using (StreamWriter sw = new StreamWriter(fs))
{
sw.WriteLine("Your contents to be written go here");
}
System.Diagnostics.Process.Start(path);
This is a simplified example
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
new Thread(() => Method1()).Start();
new Thread(() => Method2()).Start();
Console.Read();
}
private static void Method1()
{
using (StreamWriter sw = new StreamWriter(#"h:\data.txt"))
{
int i = 100000000;
while (true)
{
Thread.Sleep(100);
sw.WriteLine(i);
i++;
}
}
}
private static void Method2()
{
Thread.Sleep(6000);
throw null;
}
}
}
StreamWriter doesn't write the data into file if exception occurs too early and in another thread. File data.txt is empty at the time when exception occurs.
I played with this situation a little and found a bunch of workarounds:
If I increase the sleep interval for the exception's thread (and decrease interval between writings into file) the file will be filled with data. It is not a choice because I don't know when an exception occurs.
As a consequence of previous workaround I can decrease the buffer size of the stream writer. But it seems not working if I set it too small - for example, this code
FileStream fs = new FileStream(#"h:\data.txt", FileMode.Create);
using (StreamWriter sw = new StreamWriter(fs, Encoding.Default, 10))
doesn't work because the first writing operation occurs only when about 385 integers are waiting in the buffer to be written into file.
File will be filled if I close the writer before exception occurs. But that is not a good choice - I have to write into file from 1 to 10 times per second. It is not a good idea to open and close the writer so frequently, is it?
I can catch the exception like this
private static void Method2()
{
try
{
Thread.Sleep(6000);
throw null;
}
catch
{
Console.WriteLine("Exception!");
}
}
and all will be OK - no application termination and file will be filled pack by pack. But that is not the case also - I can't control when and where exceptions occur. I try to use try-catch everywhere but I can miss something.
So the situation is: StreamWriter's buffer is not full, exception occured in another thread and is not catched, so the application will be terminated. How not to lose this data and to write it into file?
As I understand your situation you are assuming that there is a bug somewhere and the process might be terminated at any time. You want to save as much data as possible.
You should be able to call Flush on the StreamWriter. This will push the data to the OS. If your process terminates the data will eventually be written by the OS.
In case you cannot convince StreamWriter to actually flush for some reason you can use a FileStream and write to that (pseudocode: fileStream.Write(Encoding.GetBytes(myString))). You can then flush the FileStream or use a buffer size of 1.
Of course it's best if you prevent the process from being terminated in the first place. This usually is straight forward with Task as opposed to using raw Threads.
Flushing the stream will ensure that all of it's content is pushed into it's underlying file.
That will take care of ensring all of the data is saved after you completed an operation and that a following exception will not make your application loose data.
I have spent quite some time figuring out how to do this but did not find any usefull solution.
Here is what I want to do. I am generating a huge binary file in my application. The tricky thing is that, the process requires me to occasionally close the FileStream.
The problem now is, that occasionally other applications (i.e. my virus scanner) are using this brief moment where the file is not locked anymore, to lock the file itself. or other applications like dropbox etc...
The result is, that next time I need to open the file stream, it says that it is locked by another process. All this only happens very rarely but it is still annoying when it happens.
And even if i still get the file access, I still don't want i.e. dropbox to upload this file until its done (which can take several minutes).
What I would need is a possibility to manually lock a file so that my application can still open file streams on this file, but no other application can until I manually unlock it again.
I picture something like this in pseudocode:
File.Lock(filepath);
//... do something that opens and closes filestreams on this file
File.Unlock(filepath);
Is there a way to do this? The solution "keep the file stream open" is not valid. I explicitly try to avoid that so please keep that in mind.
As you noticed yourself, the best way to lock a file is to open a handle to it using a FileStream. Your main FileStream gets closed, you say, but you can simulate a lock using one. Here's a sample class, using IDisposable so that the FileLock object itself is the lock, and disposing it releases it:
public class FileLock : IDisposable
{
private FileStream _lock;
public FileLock(string path)
{
if (File.Exists(path))
{
_lock = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.None);
IsLocked = true;
}
}
public bool IsLocked { get; set; }
public void Dispose()
{
if (_lock != null)
{
_lock.Dispose();
}
}
}
And usage:
using (FileLock lock = new FileLock(filePath))
{
// Safe in the knowledge that the file is out of harm's way.
}
I want to create a file ONLY if it doesn't already exists.
A code like:
if (!File.Exists(fileName))
{
fileStream fs = File.Create(fileName);
}
Leave it open for a race-condition in case the file will be created between the "if" to the "create".
How can I avoid it?
EDIT:
locks can't be used here because it's a different processes (multiple instances of the same application).
You can also use
FileStream fs = new FileStream(fileName, FileMode.OpenOrCreate);
However, you should look into thread locking as if more than one thread tries to access the file you'll probably get an exception.
Kristian Fenn answer was almost what I needed, just with a different FileMode. This is what I was looking for:
FileStream fs = new FileStream(fileName, FileMode.CreateNew);
Is this not a better solution. Also notice the using(var stream...) Use it to close the stream to avoid IO Exceptions.
if (!File.Exists(filePath))
{
using (var stream = File.Create(filePath)) { }
}
If the contending attempts to create the file are in the same process, you can use a lock statement around your code to prevent contention.
If not, you may occasionally get an exception when you call File.Create. Just appropriately handle that exception. Checking whether the file exists before creating is probably advisable even if you are handling an exception when the file does exist because a thrown exception is relatively expensive. It would not be advisable only if the probability of the race condition is low.
First you Lock or Monitor.Enter or TryEnter APIs to lock the portion of the code.
Second you can use FileStream API with FileMode.OpenOrCreate API. If the file exists, it just uses it or else it just creates it.
I have two ASP.NET web application. One is responsible for processing some info and writing to a log file, and the other application is reponsible for reading the log file and displays the information based on user request.
Here's my code for the Writer
public static void WriteLog(String PathToLogFile, String Message)
{
Mutex FileLock = new Mutex(false, "LogFileMutex");
try
{
FileLock.WaitOne();
using (StreamWriter sw = File.AppendText(FilePath))
{
sw.WriteLine(Message);
sw.Close();
}
}
catch (Exception ex)
{
LogUtil.WriteToSystemLog(ex);
}
finally
{
FileLock.ReleaseMutex();
}
}
And here's my code for the Reader :
private String ReadLog(String PathToLogFile)
{
FileStream fs = new FileStream(
PathToLogFile, FileMode.Open,
FileAccess.Read, FileShare.ReadWrite);
StreamReader Reader = new StreamReader(fs);
return Reader.ReadToEnd();
}
My question, is the above code enough to prevent locking in a web garden environemnt?
EDIT 1 : Dirty read is okay.
EDIT 2 : Creating Mutex with new Mutex(false, "LogFileMutex"), closing StreamWriter
Sounds like your trying to implement a basic queue. Why not use a queue that gives you guarenteed availability. You could drop the messages into an MSMQ, then implement a windows service which will read from the queue and push the messages to the DB. If the writting to the DB fails you simply leave the message on the queue (Although you will want to handle posion messages so if it fails cause the data is bad you don't end up in an infinite loop)
This will get rid of all locking concerns and give you guarenteed delivery to your reader...
You should also be disposing of your mutex, as it derives from WaitHandle, and WaitHandle implements IDisposable:
using (Mutex FileLock = new Mutex(true, "LogFileMutex"))
{
// ...
}
Also, perhaps consider a more unique name (a GUID perhaps) than "LogFileMutex", since another unrelated process could possibly use the same name inadvertantly.
Doing this in a web based environment, you are going to have a lot of issues with file locks, can you change this up to use a database instead?
Most hosting solutions are allowing up to 250mb SQL databases.
Not only will a database help with the locking issues, it will also allow you to purge older data more easily, after a wile, that log read is going to get really slow.
No it won't. First, you're creating a brand new mutex with every call so multiple threads are going to access the writing critical section. Second, you don't even use the mutex in the reading critical section so one thread could be attempting to read the file while another is attempting to write. Also, you're not closing the stream in the ReadLog method so once the first read request comes through your app won't be able to write any log entries anyway until garbage collection comes along and closes the stream for you... which could take awhile.