Develop Reference

How to start the c# FileSystemWatcher function indefinitely when executing the program?

I am developing windows program using .Net Framework.
I want to create a program that executes a function when a file is created in a specific folder using FileSystemWatcher.
Below is my code.
public async Task<int> CollectFunc() {
string path = #"C:\test";
try
{
FileSystemWatcher watcher = new FileSystemWatcher
{
Path=path
Filter="test.log"
};
watcher.Created += new FileSystemEventHandler(WatcherFunc);
watcher.IncludeSubdrectories=true;
watcher.EnableRaisingEvents=true;
}
catch
{
Console.WriteLine("Error");
}
while(true)
{
await Task.Delay(100000);
}
}
public async void WatcherFunc(object source, FileSystemEventArgs e) {
Console.WriteLine("File Created: " + e.FullPath);
}
When I start the program, file creation is monitored until I close the program.
An example is shown below.
On September 1st, the following file is created.
C:\test\20200901\test.log
The program then prints "File Created: C:\test\20200901\test.log".
And on September 2nd
C:\test\20200902\test.log file is created,
The program will then output "File Created: C:\test\20200902\test.log".
...
But sometimes the Watcher doesn't work and I have to reboot the program.
Please let me know if there is any better or more stable logic than my source code.
I look forward to your kind reply.

Try these changes:
// Introduce a class field, to prevent the watcher reference from going out of scope.
private FileSystemWatcher watcher = null;
public void CollectFunc() { // no need for async any more ...
string path = #"C:\test";
try
{
// Init class field
watcher = new FileSystemWatcher
{
Path=path
Filter="test.log"
};
watcher.Created += new FileSystemEventHandler(WatcherFunc);
watcher.IncludeSubdrectories=true;
watcher.EnableRaisingEvents=true;
}
catch (Exception ex)
{
// Better know what the problem actually was.
Console.WriteLine($"Error: {ex.Message}");
}
// It's a winforms app - we don't need to block this => away with while(true)
}
public async void WatcherFunc(object source, FileSystemEventArgs e)
{
// Just in case, catch and log exceptions
try{
Console.WriteLine("File Created: " + e.FullPath);
} catch( Exception ex ) {
// TODO: Log Exception or handle it.
}
}
On top of that: It is a known issue, that a high number and frequency of changes can lead to some buffer to overflow in the watcher (if that still applies, but I remember running into this some years ago).
The problem with buffer overflow is mentioned here : https://learn.microsoft.com/en-us/dotnet/api/system.io.filesystemwatcher.internalbuffersize?view=netcore-3.1#remarks
It may also be worthwhile to register a handler to the Error event: https://learn.microsoft.com/en-us/dotnet/api/system.io.filesystemwatcher.error?view=netcore-3.1
I guess that your Console.WriteLine in the event handler is just an example code and you are actually doing more than that. In the past, I found that it releaves stress from the FileSystemWatcher's buffer if I keep code very small here and handle the event as quickly as possible.
So, what I did was enqueue the file path in a qeue and have that queue handled on a different thread. This ensures that event are handled as quickly as possible while not losing any. Peeks can be caught by the queue getting bigger and be dealt with independently by another thread. In other words: Things pile up outside the watcher's buffers.

Tell if a file is written to a networkshare completely [duplicate]

When a file is created (FileSystemWatcher_Created) in one directory I copy it to another. But When I create a big (>10MB) file it fails to copy the file, because it starts copying already, when the file is not yet finished creating...
This causes Cannot copy the file, because it's used by another process to be raised. ;(
Any help?
class Program
{
static void Main(string[] args)
{
string path = #"D:\levan\FolderListenerTest\ListenedFolder";
FileSystemWatcher listener;
listener = new FileSystemWatcher(path);
listener.Created += new FileSystemEventHandler(listener_Created);
listener.EnableRaisingEvents = true;
while (Console.ReadLine() != "exit") ;
}
public static void listener_Created(object sender, FileSystemEventArgs e)
{
Console.WriteLine
(
"File Created:\n"
+ "ChangeType: " + e.ChangeType
+ "\nName: " + e.Name
+ "\nFullPath: " + e.FullPath
);
File.Copy(e.FullPath, #"D:\levan\FolderListenerTest\CopiedFilesFolder\" + e.Name);
Console.Read();
}
}

There is only workaround for the issue you are facing.
Check whether file id in process before starting the process of copy. You can call the following function until you get the False value.
1st Method, copied directly from this answer:
private bool IsFileLocked(FileInfo file)
{
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;
}
2nd Method:
const int ERROR_SHARING_VIOLATION = 32;
const int ERROR_LOCK_VIOLATION = 33;
private bool IsFileLocked(string file)
{
//check that problem is not in destination file
if (File.Exists(file) == true)
{
FileStream stream = null;
try
{
stream = File.Open(file, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
}
catch (Exception ex2)
{
//_log.WriteLog(ex2, "Error in checking whether file is locked " + file);
int errorCode = Marshal.GetHRForException(ex2) & ((1 << 16) - 1);
if ((ex2 is IOException) && (errorCode == ERROR_SHARING_VIOLATION || errorCode == ERROR_LOCK_VIOLATION))
{
return true;
}
}
finally
{
if (stream != null)
stream.Close();
}
}
return false;
}

From the documentation for FileSystemWatcher:
The OnCreated event is raised as soon as a file is created. If a file
is being copied or transferred into a watched directory, the
OnCreated event will be raised immediately, followed by one or more
OnChanged events.
So, if the copy fails, (catch the exception), add it to a list of files that still need to be moved, and attempt the copy during the OnChanged event. Eventually, it should work.
Something like (incomplete; catch specific exceptions, initialize variables, etc):
public static void listener_Created(object sender, FileSystemEventArgs e)
{
Console.WriteLine
(
"File Created:\n"
+ "ChangeType: " + e.ChangeType
+ "\nName: " + e.Name
+ "\nFullPath: " + e.FullPath
);
try {
File.Copy(e.FullPath, #"D:\levani\FolderListenerTest\CopiedFilesFolder\" + e.Name);
}
catch {
_waitingForClose.Add(e.FullPath);
}
Console.Read();
}
public static void listener_Changed(object sender, FileSystemEventArgs e)
{
if (_waitingForClose.Contains(e.FullPath))
{
try {
File.Copy(...);
_waitingForClose.Remove(e.FullPath);
}
catch {}
}
}

It's an old thread, but I'll add some info for other people.
I experienced a similar issue with a program that writes PDF files, sometimes they take 30 seconds to render.. which is the same period that my watcher_FileCreated class waits before copying the file.
The files were not locked.
In this case I checked the size of the PDF and then waited 2 seconds before comparing the new size, if they were unequal the thread would sleep for 30 seconds and try again.

You're actually in luck - the program writing the file locks it, so you can't open it. If it hadn't locked it, you would have copied a partial file, without having any idea there's a problem.
When you can't access a file, you can assume it's still in use (better yet - try to open it in exclusive mode, and see if someone else is currently opening it, instead of guessing from the failure of File.Copy). If the file is locked, you'll have to copy it at some other time. If it's not locked, you can copy it (there's slight potential for a race condition here).
When is that 'other time'? I don't rememeber when FileSystemWatcher sends multiple events per file - check it out, it might be enough for you to simply ignore the event and wait for another one. If not, you can always set up a time and recheck the file in 5 seconds.

Well you already give the answer yourself; you have to wait for the creation of the file to finish. One way to do this is via checking if the file is still in use. An example of this can be found here: Is there a way to check if a file is in use?
Note that you will have to modify this code for it to work in your situation. You might want to have something like (pseudocode):
public static void listener_Created()
{
while CheckFileInUse()
wait 1000 milliseconds
CopyFile()
}
Obviously you should protect yourself from an infinite while just in case the owner application never releases the lock. Also, it might be worth checking out the other events from FileSystemWatcher you can subscribe to. There might be an event which you can use to circumvent this whole problem.

When the file is writing in binary(byte by byte),create FileStream and above solutions Not working,because file is ready and wrotted in every bytes,so in this Situation you need other workaround like this:
Do this when file created or you want to start processing on file
long fileSize = 0;
currentFile = new FileInfo(path);
while (fileSize < currentFile.Length)//check size is stable or increased
{
fileSize = currentFile.Length;//get current size
System.Threading.Thread.Sleep(500);//wait a moment for processing copy
currentFile.Refresh();//refresh length value
}
//Now file is ready for any process!

So, having glanced quickly through some of these and other similar questions I went on a merry goose chase this afternoon trying to solve a problem with two separate programs using a file as a synchronization (and also file save) method. A bit of an unusual situation, but it definitely highlighted for me the problems with the 'check if the file is locked, then open it if it's not' approach.
The problem is this: the file can become locked between the time that you check it and the time you actually open the file. Its really hard to track down the sporadic Cannot copy the file, because it's used by another process error if you aren't looking for it too.
The basic resolution is to just try to open the file inside a catch block so that if its locked, you can try again. That way there is no elapsed time between the check and the opening, the OS does them at the same time.
The code here uses File.Copy, but it works just as well with any of the static methods of the File class: File.Open, File.ReadAllText, File.WriteAllText, etc.
/// <param name="timeout">how long to keep trying in milliseconds</param>
static void safeCopy(string src, string dst, int timeout)
{
while (timeout > 0)
{
try
{
File.Copy(src, dst);
//don't forget to either return from the function or break out fo the while loop
break;
}
catch (IOException)
{
//you could do the sleep in here, but its probably a good idea to exit the error handler as soon as possible
}
Thread.Sleep(100);
//if its a very long wait this will acumulate very small errors.
//For most things it's probably fine, but if you need precision over a long time span, consider
// using some sort of timer or DateTime.Now as a better alternative
timeout -= 100;
}
}
Another small note on parellelism:
This is a synchronous method, which will block its thread both while waiting and while working on the thread. This is the simplest approach, but if the file remains locked for a long time your program may become unresponsive. Parellelism is too big a topic to go into in depth here, (and the number of ways you could set up asynchronous read/write is kind of preposterous) but here is one way it could be parellelized.
public class FileEx
{
public static async void CopyWaitAsync(string src, string dst, int timeout, Action doWhenDone)
{
while (timeout > 0)
{
try
{
File.Copy(src, dst);
doWhenDone();
break;
}
catch (IOException) { }
await Task.Delay(100);
timeout -= 100;
}
}
public static async Task<string> ReadAllTextWaitAsync(string filePath, int timeout)
{
while (timeout > 0)
{
try {
return File.ReadAllText(filePath);
}
catch (IOException) { }
await Task.Delay(100);
timeout -= 100;
}
return "";
}
public static async void WriteAllTextWaitAsync(string filePath, string contents, int timeout)
{
while (timeout > 0)
{
try
{
File.WriteAllText(filePath, contents);
return;
}
catch (IOException) { }
await Task.Delay(100);
timeout -= 100;
}
}
}
And here is how it could be used:
public static void Main()
{
test_FileEx();
Console.WriteLine("Me First!");
}
public static async void test_FileEx()
{
await Task.Delay(1);
//you can do this, but it gives a compiler warning because it can potentially return immediately without finishing the copy
//As a side note, if the file is not locked this will not return until the copy operation completes. Async functions run synchronously
//until the first 'await'. See the documentation for async: https://msdn.microsoft.com/en-us/library/hh156513.aspx
CopyWaitAsync("file1.txt", "file1.bat", 1000);
//this is the normal way of using this kind of async function. Execution of the following lines will always occur AFTER the copy finishes
await CopyWaitAsync("file1.txt", "file1.readme", 1000);
Console.WriteLine("file1.txt copied to file1.readme");
//The following line doesn't cause a compiler error, but it doesn't make any sense either.
ReadAllTextWaitAsync("file1.readme", 1000);
//To get the return value of the function, you have to use this function with the await keyword
string text = await ReadAllTextWaitAsync("file1.readme", 1000);
Console.WriteLine("file1.readme says: " + text);
}
//Output:
//Me First!
//file1.txt copied to file1.readme
//file1.readme says: Text to be duplicated!

You can use the following code to check if the file can be opened with exclusive access (that is, it is not opened by another application). If the file isn't closed, you could wait a few moments and check again until the file is closed and you can safely copy it.
You should still check if File.Copy fails, because another application may open the file between the moment you check the file and the moment you copy it.
public static bool IsFileClosed(string filename)
{
try
{
using (var inputStream = File.Open(filename, FileMode.Open, FileAccess.Read, FileShare.None))
{
return true;
}
}
catch (IOException)
{
return false;
}
}

I would like to add an answer here, because this worked for me. I used time delays, while loops, everything I could think of.
I had the Windows Explorer window of the output folder open. I closed it, and everything worked like a charm.
I hope this helps someone.

FileSystemWatcher - is File ready to use

When a file is being copied to the file watcher folder, how can I identify whether the file is completely copied and ready to use? Because I am getting multiple events during file copy. (The file is copied via another program using File.Copy.)

When I ran into this problem, the best solution I came up with was to continually try to get an exclusive lock on the file; while the file is being written, the locking attempt will fail, essentially the method in this answer. Once the file isn't being written to any more, the lock will succeed.
Unfortunately, the only way to do that is to wrap a try/catch around opening the file, which makes me cringe - having to use try/catch is always painful. There just doesn't seem to be any way around that, though, so it's what I ended up using.
Modifying the code in that answer does the trick, so I ended up using something like this:
private void WaitForFile(FileInfo file)
{
FileStream stream = null;
bool FileReady = false;
while(!FileReady)
{
try
{
using(stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
FileReady = true;
}
}
catch (IOException)
{
//File isn't ready yet, so we need to keep on waiting until it is.
}
//We'll want to wait a bit between polls, if the file isn't ready.
if(!FileReady) Thread.Sleep(1000);
}
}

Here is a method that will retry file access up to X number of times, with a Sleep between tries. If it never gets access, the application moves on:
private static bool GetIdleFile(string path)
{
var fileIdle = false;
const int MaximumAttemptsAllowed = 30;
var attemptsMade = 0;
while (!fileIdle && attemptsMade <= MaximumAttemptsAllowed)
{
try
{
using (File.Open(path, FileMode.Open, FileAccess.ReadWrite))
{
fileIdle = true;
}
}
catch
{
attemptsMade++;
Thread.Sleep(100);
}
}
return fileIdle;
}
It can be used like this:
private void WatcherOnCreated(object sender, FileSystemEventArgs e)
{
if (GetIdleFile(e.FullPath))
{
// Do something like...
foreach (var line in File.ReadAllLines(e.FullPath))
{
// Do more...
}
}
}

I had this problem when writing a file. I got events before the file was fully written and closed.
The solution is to use a temporary filename and rename the file once finished. Then watch for the file rename event instead of file creation or change event.

Note: this problem is not solvable in generic case. Without prior knowledge about file usage you can't know if other program(s) finished operation with the file.
In your particular case you should be able to figure out what operations File.Copy consist of.
Most likely destination file is locked during whole operation. In this case you should be able to simply try to open file and handle "sharing mode violation" exception.
You can also wait for some time... - very unreliable option, but if you know size range of files you may be able to have reasonable delay to let Copy to finish.
You can also "invent" some sort of transaction system - i.e. create another file like "destination_file_name.COPYLOCK" which program that copies file would create before copying "destination_file_name" and delete afterward.

private Stream ReadWhenAvailable(FileInfo finfo, TimeSpan? ts = null) => Task.Run(() =>
{
ts = ts == null ? new TimeSpan(long.MaxValue) : ts;
var start = DateTime.Now;
while (DateTime.Now - start < ts)
{
Thread.Sleep(200);
try
{
return new FileStream(finfo.FullName, FileMode.Open);
}
catch { }
}
return null;
})
.Result;
...of course, you can modify aspects of this to suit your needs.

One possible solution (It worked in my case) is to use the Change event. You can log in the create event the name of the file just created and then catch the change event and verify if the file was just created. When I manipulated the file in the change event it didn't throw me the error "File is in use"

If you are doing some sort of inter-process communication, as I do, you may want to consider this solution:
App A writes the file you are interested in, eg "Data.csv"
When done, app A writes a 2nd file, eg. "Data.confirmed"
In your C# app B make the FileWatcher listen to "*.confirmed" files. When you get this event you can safely read "Data.csv", as it is already completed by app A.
(Edit: inspired by commets) Delete the *.confirmed filed with app B when done processing the "Data.csv" file.

I have solved this issue with two features:
Implement the MemoryCache pattern seen in this question: A robust solution for FileSystemWatcher firing events multiple times
Implement a try\catch loop with a timeout for access
You need to collect average copy times in your environment and set the memory cache timeout to be at least as long as the shortest lock time on a new file. This eliminates duplicates in your processing directive and allows some time for the copy to finish. You will have much better success on first try, which means less time spent in the try\catch loop.
Here is an example of the try\catch loop:
public static IEnumerable<string> GetFileLines(string theFile)
{
DateTime startTime = DateTime.Now;
TimeSpan timeOut = TimeSpan.FromSeconds(TimeoutSeconds);
TimeSpan timePassed;
do
{
try
{
return File.ReadLines(theFile);
}
catch (FileNotFoundException ex)
{
EventLog.WriteEntry(ProgramName, "File not found: " + theFile, EventLogEntryType.Warning, ex.HResult);
return null;
}
catch (PathTooLongException ex)
{
EventLog.WriteEntry(ProgramName, "Path too long: " + theFile, EventLogEntryType.Warning, ex.HResult);
return null;
}
catch (DirectoryNotFoundException ex)
{
EventLog.WriteEntry(ProgramName, "Directory not found: " + theFile, EventLogEntryType.Warning, ex.HResult);
return null;
}
catch (Exception ex)
{
// We swallow all other exceptions here so we can try again
EventLog.WriteEntry(ProgramName, ex.Message, EventLogEntryType.Warning, ex.HResult);
}
Task.Delay(777).Wait();
timePassed = DateTime.Now.Subtract(startTime);
}
while (timePassed < timeOut);
EventLog.WriteEntry(ProgramName, "Timeout after waiting " + timePassed.ToString() + " seconds to read " + theFile, EventLogEntryType.Warning, 258);
return null;
}
Where TimeoutSeconds is a setting that you can put wherever you hold your settings. This can be tuned for your environment.

File Access in Windows Service

I have a Windows Service which has 8 timers running parallel (elapsed time = 10 sec) and each timer is performing some activity and logs the write_time when it enters the timer and end_time when it exits the timer and this happens for all the timers.
I have a ASP.net application which reads the logs for the write_time and end_time for each timer and displays it on a grid.
Commonly I get an error for file operations which causes my timer to stop. The code block is below.
Write_time
FileInfo file = null;
StreamWriter write = null;
try
{
file = new FileInfo(ConfigurationManager.AppSettings["SupportFilePath"].ToString() + processName + "_Log.txt");
write = new StreamWriter(file.FullName);
write.Write(string.Empty);
write.Write(processName + "_" + time + " at: _" + System.DateTime.Now.ToString());
write.Close();
write.Dispose();
}
catch (System.Exception ex)
{
_errorMonitoringEngine.ErrorInfo(" ", ex.StackTrace.ToString(), ex.Message, "Email Notification Engine", "WriteTimeProcess2");
}
I get maximun times the exception The process cannot access the file. Please advise how to get rid of it.

Most likely two or more threads are trying to write to the same file at the same time.
Create an instance of an object somewhere in your class and lock it whenever you need to write to the file.
public class Example
{
// ...
// Depending on whether there are one or many instances of
// this class determines whether this needs to be static
// or not. If it needs to be static, use a static constructor.
private object syncObject = new object();
// ...
void WriteToFile()
{
lock (syncObject)
{
// Do file IO
// Now no two threads will attempt to access the file at the same time
}
}
// ...
}
It would also be wise to wrap the StreamWriter in a using statement.

Azure Queue throwing Exception on DeleteMessage

I'm working on an Azure based project for some research and have been running into some issues when deleting messages from a CloudQueue instance. The code is fairly straightforward, so I'm a bit baffled as to why an exception is being thrown when I try to delete a message from the queue.
Here is the code that produces data for the queue:
foreach (var cell in scheme(cells))
{
string id = Guid.NewGuid().ToString();
var blob = sweepItemContainer.GetBlobReference(id);
using (BlobStream stream = blob.OpenWrite())
{
BinaryFormatter bf = new BinaryFormatter();
bf.Serialize(stream, cell);
}
sweepItemQueue.AddMessage(new CloudQueueMessage(id), new TimeSpan(1, 0, 0));
}
Here is the code that consumes the data from the queue:
var msgs = sweepItemsQueue.GetMessages(MsgAmt);
foreach (var msg in msgs)
{
_handleMessage(msg, sweepItemsContainer);
sweepItemsQueue.DeleteMessage(msg);
mergeItemsQueue.AddMessage(new CloudQueueMessage(msg.AsString), new TimeSpan(1, 0, 0));
}
I don't see how the message cannot exist in the queue. Nothing else is mutating the queue besides other consumers. But I am assured that they cannot get the same message (so long as the timespan doesn't run out), so how is this happening?

There are two timeouts that you need to worry about, how long the message lives in the queue (which you've specified in the your .AddMessage() call and the visibility timeout that is set when you call .GetMessages() (by default this is 30 seconds, there is an overload that allows you to specify the timeout). When you call .GetMessages() all of the messages returned are invisible to other consumers for the period 'visibilityTimeout'. Once this period finishes all of the messages you haven't already deleted become visible to all other consumers.
To check if this is the problem I would try using the overload of .GetMessages() with it's maximum visibility timeout of 2 hours. If this is the problem you can fine tune this value down to a more sensible number. Another option would be to just retrieve one message at a time.

Another answer from Steve Marx, basically look at the storage exception and move on. I have seen this in other frameworks too.:
Steve Marx blog post
try
{
q.DeleteMessage(msg);
}
catch (StorageClientException ex)
{
if (ex.ExtendedErrorInformation.ErrorCode == "MessageNotFound")
{
// pop receipt must be invalid
// ignore or log (so we can tune the visibility timeout)
}
else
{
// not the error we were expecting
throw;
}
}