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Multi-threaded C# application log file locking issue [duplicate]

This question already has answers here:
How to write in a single file with multiple threads?
(3 answers)
Closed 8 years ago.
I have a bi-threaded c# application where both threads write to the same log and I'm getting an error.
'The process cannot access the file 'logfile.log' because it is being used by another process.'
I'm somewhat new to C# and multi-threading in general so if anyone can point me in the right direction here it would be much appreciated.

The error message says that the file is held open by another process, not thread. Are you sure the file is being closed properly each time you run your app ?
Generally speaking, you should use a lock to control access to a shared resource inside your application. For example, if you have a LogWriter class with a Log() function, a minimal implementation with no queuing might look like this:
public class LogWriter
{
private readonly object _lock = new object;
public void Log(string message)
{
lock(_lock)
{
//write message to log file
string appName = Path.GetFileNameWithoutExtension(Environment.GetCommandLineArgs()[0]);
StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + Path.DirectorySeparatorChar + appName + ".log", true);
sw.WriteLine(string.Format("{0:u} {1}", DateTime.Now, message));
sw.Flush();
sw.Close();
}
}
The lock(_lock) ensures that only one thread accesses the file at a time. The sw.Close() ensures that the file is not left open when the process terminates ;-)

As the error message states, you can't create a writer from multiple different threads. You'll need to find some way of either synchronizing the access to the file between threads, which is to say ensuring that one thread waits to try to access the file until all other threads are done with it, or you could designate a single thread as being the thread responsible for accessing that file, requiring all other threads that want to access that file to request the other thread to do it for them.

You need to create a list(or whatever data structure you like) that is accessible to parallel threads. Then add record to that list in each thread
lock (yourList) {
// here you can add items to the List
}
When finished, dump your list to a file.
Or create a new List per each thread, return them all back, and then join all lists into one.
Or use a Database and add records to a table (the most logical solution)
As alternative solution
Create a global List
pass that list to all threads and lock it(example above) and write to it; unlock it
Create another infinite loop thread with delay
Inside infinite loop, after delay, lock the list, get all the data, write to file, empty the list
release the list

How to check that a file can be read successfully? [duplicate]

This question already has answers here:
Detecting whether a file is locked by another process (or indeed the same process) [duplicate]
(4 answers)
Closed 9 years ago.
In my program I pass the path to a file which may be still being written to by another process (independent external non-c#).
How can I check the file is fully readable and accessible before I actually send the path back to the client? FileIOPermission with Demand for reading did not work for me, so I am wondering if there is some other way of doing this without attempting to read the whole file upfront.
Thank you.

The trouble with checking to see if a file is accessible and fully readable, and then opening the file for reading is that potentially, the state could change between the check and the open where you actually want to be doing something with it.
My suggestion is to go ahead and open the file for reading, being sure to catch the appropriate exception if a problem occurs. You can then wait a little and try again, or do something else.
Remember that just because your program has a sequence of events that are logical to you, many things are going on in an operating system and the state can easily change between two lines of code that seem watertight for you, but have an epoch between them at a multitasking level.

Moo-Juice is right. You can't check whether the file can be read successfully and then open it for reading... things can change and you might get an exception anyway. Best to just read it and catch the exception.
public bool TryReadFile(String path, out String contentsOfFile)
{
try
{
// Try reading file
contentsOfFile = File.ReadAllText(path);
// Success! Yay!
return true;
}
catch (IOException)
{
// Oops! Can't read that file!
// Return some default value and let the caller know we failed
contentsOfFile = String.Empty;
return false;
}

Best way would be to read the file in a normal try catch block, and then apply the logic to continue or not, based on if an exception was thrown or not.
Also, a secondary way is to check if a file is not of zero size, but purely as a secondary check.

Create a loop and try to open the file
If exception occur make your thread sleep for some seconds
and repeat the process

When the external process is complete, does it close? If so, you could modify the solution found here. It would look something like the following:
//While the process is found running
while (System.Diagnostics.Process.GetProcessesByName(process).Length != 0)
{
//Sleep for three seconds
System.Threading.Thread.Sleep(3000);
}
//open file for reading
If the process doesn't close when complete, however, the above won't work. You could try placing the following in a loop that tries to open the file exclusively until it's successful:
System.IO.File.Open(PathToFile, FileMode.Open, FileAccess.Read, FileShare.None);
Both of these methods should also have some kind of count added to ensure that they eventually stop trying and error out, or else you could end up with an infinite loop.

Check If File Is In Use By Other Instances of Executable Run

Before I go into too detail, my program is written in Visual Studio 2010 using C# .Net 4.0.
I wrote a program that will generate separate log files for each run. The log file is named after the time, and accurate up at millisecond (for example, 20130726103042375.log). The program will also generate a master log file for the day if it has not already exist (for example, *20130726_Master.log*)
At the end of each run, I want to append the log file to a master log file. Is there a way to check if I can append successfully? And retry after Sleep for like a second or something?
Basically, I have 1 executable, and multiple users (let's say there are 5 users).
All 5 users will access and run this executable at the same time. Since it's nearly impossible for all user to start at the exact same time (up to millisecond), there will be no problem generate individual log files.
However, the issue comes in when I attempt to merge those log files to the master log file. Though it is unlikely, I think the program will crash if multiple users are appending to the same master log file.
The method I use is
File.AppendAllText(masterLogFile, File.ReadAllText(individualLogFile));
I have check into the lock object, but I think it doesn't work in my case, as there are multiple instances running instead of multiple threads in one instance.
Another way I look into is try/catch, something like this
try
{
stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
catch {}
But I don't think this solve the problem, because the status of the masterLogFile can change in that brief millisecond.
So my overall question is: Is there a way to append to masterLogFile if it's not in use, and retry after a short timeout if it is? Or if there is an alternative way to create the masterLogFile?
Thank you in advance, and sorry for the long message. I want to make sure I get my message across and explain what I've tried or look into so we are not wasting anyone's time.
Please let me know if there's anymore information I can provide to help you help me.

Your try/catch is the way to do things. If the call to File.Open succeeds, then you can write to to the file. The idea is to keep the file open. I would suggest something like:
bool openSuccessful = false;
while (!openSuccessful)
{
try
{
using (var writer = new StreamWriter(masterlog, true)) // append
{
// successfully opened file
openSuccessful = true;
try
{
foreach (var line in File.ReadLines(individualLogFile))
{
writer.WriteLine(line);
}
}
catch (exceptions that occur while writing)
{
// something unexpected happened.
// handle the error and exit the loop.
break;
}
}
}
catch (exceptions that occur when trying to open the file)
{
// couldn't open the file.
// If the exception is because it's opened in another process,
// then delay and retry.
// Otherwise exit.
Sleep(1000);
}
}
if (!openSuccessful)
{
// notify of error
}
So if you fail to open the file, you sleep and try again.
See my blog post, File.Exists is only a snapshot, for a little more detail.

I would do something along the lines of this as I think in incurs the least overhead. Try/catch is going to generate a stack trace(which could take a whole second) if an exception is thrown. There has to be a better way to do this atomically still. If I find one I'll post it.

Streamwriter Lock Not Working

I'm taking over a C# project, and when testing it out I'm getting errors. The error is that the log file cannot be written to because it is in use by another process. Here's the code:
public void WriteToLog(string msg)
{
if (!_LogExists)
{
this.VerifyOrCreateLogFile(); // Creates log file if it does not already exist.
}
// do the actual writing on its own thread so execution control can immediately return to the calling routine.
Thread t = new Thread(new ParameterizedThreadStart(WriteToLog));
t.Start((object)msg);
}
private void WriteToLog(object msg)
{
lock (_LogLock)
{
string message = msg as string;
using (StreamWriter sw = File.AppendText(LogFile))
{
sw.Write(message);
sw.Close();
}
}
}
_LogLock is defined as a class variable:
private object _LogLock = 0;
Based on my research and the fact that this has been working fine in a production system for a few years now, I don't know what the problem could be. The lock should prevent another thread from attempting to write to the log file.
The changes I've made that need to be tested are a lot more log usage. We're basically adding a debug mode to save much more info to the log than used to be saved.
Thanks for any help!
EDIT:
Thanks for the quick answers! The code for VerifyOrCreateLogFile() does use the _LogLock, so that shouldn't be an issue. It does do some writing to the log before it errors out, so it gets past creating the file just fine.
What seems to be the problem is that previously only one class created an instance of the log class, and now I've added instances to other classes. It makes sense that this would create problems. Changing the _LogLock field to be static fixes the issue.
Thanks again!

The lock should prevent another thread from attempting to write to the log file.
This is only true if you're using a single instance of this class.
If each (or even some) of the log requests use a separate instance, then the lock will not protect you.
You can easily "correct" this by making the _LogLock field static:
private static object _LogLock = 0;
This way, all instances will share the same lock.

I see 2 problems with the code:
Lock must be the same among all "users" of ths Log class, easiest solution is to make either _LogLock or the complete class static
VerifyOrCreateLogFile could pose a problem if 2 or more parallel threads call WriteToLog when _LogExists is false...

One possibility is that the OS isn't releasing the file lock quickly enough before you exit the lock in WriteToLog and another thread that was blocked waiting for the lock tried to open it before the OS finished releasing the file lock. Yes, it can happen. You either need to sleep for a little before trying to open the file, centralize the writing to the log to a dedicated object (so that he and only he has access to this file and you don't have to worry about file lock contentions).
Another possibility is that you need to lock around
if (!_LogExists) {
this.VerifyOrCreateLogFile(); // Creates log file if it does not already exist.
}
The third possibility is that you have multiple instances of whatever class is housing these methods. The lock object won't be shared across instances (make it static to solve this).
At the end of the day, unless you're an expert in writing safe multi-threaded code, just let someone else worry about this stuff for you. Use a framework that handles these issues for you (log4net?).

you can do the code executable by simply
removing sw.Close(); from your code ...
do it....
it will work fine.....

Saving XML stream in C# says that it's being used by another process

I have this open-source library that I'm having some trouble fixing a problem... This library allows to easily create an XML file to store application settings. But I'm having an issue saving the changes.
I have another application where I'm using this library and every time that application window is done resizing, I call the Save() method of the library to save the window size/position to the XML file.
Most of times it works fine, everything is saved. Once in a while though, I get an exception saying the file is being used by another process.
I really need to make sure that changes are saved every time the Save() method is called, I need to handle this exception somehow or prevent it from happening.
What are you guys suggestions for best handling this situation?
The code for the Save() method is the following:
public void Save() {
// Create a new XML file if there's no root element
if(xDocument.DocumentElement == null) {
xDocument = new XmlDocument();
xDocument.LoadXml("<?xml version=\"1.0\" encoding=\"utf-8\" ?>\n" +
"<" + XmlRootElement + ">\n</" + XmlRootElement + ">");
}
// OMITTED CODE WAS HERE (NOT IMPORTANT FOR THE PROBLEM)
// Create a new XML writer for the XML file
XmlWriter xWriter = XmlWriter.Create(XmlFilePath, new XmlWriterSettings() {
Indent = true,
IndentChars = "\t"
});
// Sort the XML file using the XSL sylesheet and save it
xslTransform.Transform(xDocument, xWriter);
// Clear the buffer and close the XML writer stream
xWriter.Flush();
xWriter.Close();
}

XmlWriter is IDisposable. You should wrap it in a using() clause.
http://msdn.microsoft.com/en-us/library/system.xml.xmlwriter.aspx

I have to go with a combination of the answers already given here.
Your XmlWriter should be in a using block for several reasons. You should dispose it so that your resources are freed as soon as possible. Also, what if you throw an exception while interacting with it. The file wouldn't be closed properly, at least until the finalizer kicks in and frees your resources.
Even with the using statement, you "might" have contention on the file and need to place the Save code in a lock statement. The method is non-reentrant by nature because the file is a shared resource. Putting a lock around it might be over kill if you don't have multiple threads, but you would ensure that you properly controlled access to the file.
The other thing to consider is that you might want to move the saving operation to a background thread to write the file out. If you get a large settings file you might cause strange UI interactions because you are waiting on the file to write every time the user resizes and this happens on the UI thread. If you did this you would definitely need to lock access to the file resource.

It could be the case that the window-resizing-completed events are firing so quickly, that the save function is being called, the called again before it finishes running the first time. This would result in the error you're describing (the other process using the file is... YOU!). Try surrounding your code with a lock, thusly:
lock(some_shared_object)
{
//Your code here
}

Also you might try using a lock statement. It could be that the methods are overrunning one another.