FileSystemWatcher and reading from file

Marius

So, my situation is something like this:

What I want to do is to detect when the contents of a file change, and then read from that file (and use the data for whatever). The trouble is, when I try to do that, I get a "File is in use" exception.

Some code:

FileSystemWatcher watcher = new FileSystemWatcher();
watcher.NotifyFilter = NotifyFilters.LastWrite;
watcher.Path = pathToFile;
watcher.Filter = fileName;

//then the event:

watcher.Changed = new FileSystemEventHandler(watcher_Changed);
watcher.EnableRaisingEvents = true;

//and the handler

void watcher_Changed(object sender, FileSystemEventArgs e)
{
FileStream fs = new FileStream(pathToFile + fileName, FileMode.Open, FileAccess.Read);

//read from file, etc etc
}

So, this basically says that the file cannot be accessed because it's open by another process. I assume the FileStream locks the file or something like that?
It works when not trying to read from the file...

Anyway I can fix this?

Thanks in advance.

15 Replies

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Vulpes15y ago
If Sam's code is working and you're using the same version of VS as him (probably VS 2010), then evidently the file is not being opened with exclusive read/write access and so the answer must lie elsewhere.

I can't find any option to open the file with different file share settings, in any case.
Sam Hobbs15y ago
In your watcher_Changed, the file that is opened is "pathToFile + fileName". What is that? More specifically, what is fileName? Are you sure that "pathToFile + fileName" is valid? Have you verified (using the debugger or some other diagnostic technique during execution) that it is what you think it is?

The following is for a simple console application. I can execute the program and while it is executing I can have a file open in VS and edit the file and save the changes and this program then reads it.

static void Main(string[] args)
{
FileSystemWatcher watcher = new FileSystemWatcher();
watcher.NotifyFilter = NotifyFilters.LastWrite;
watcher.Path = pathToFile;
watcher.Filter = "";
watcher.EnableRaisingEvents = true;
watcher.Changed += new FileSystemEventHandler(watcher_Changed);
Console.ReadKey();
}

static void watcher_Changed(object sender, FileSystemEventArgs e)
{
Console.WriteLine("File has changed: {0} {1}", e.Name, e.FullPath);
using (StreamReader sr = new StreamReader(e.FullPath))
{
while (!sr.EndOfStream)
Console.WriteLine(sr.ReadLine());
}
Console.WriteLine();
}
Marius15y ago
Isn't there some way I can do this directly, without having to have other applications open?

For instance, is there some setting in VS that allows me to access the file from within the application, while it's also open in the VS editor?
Vulpes15y ago
It would appear that, when a file is opened in Visual Studio, it's opened with exclusive read/write access which means, of course, that the file is totally locked and nothing else can even read from it whilst it's still open.

What you could do instead is to create a simple console application, open the file with the Read/Write share and then write to it from there. If you then run your main program simultaneously, again opening the file with the Read/Write share, then hopefully you won't be told that the file is in use when you try to read from it.
Marius15y ago
It's really strange that I can't read from the file while I have it open. I'm modifying it manually in the VS editor, yes, but for reading from it, it shouldn't matter that I have it open elsewhere. And as I said, FileShare.ReadWrite makes no difference.

To clarify further, this is a file that contains GLSL code (shading language, runs on the GPU, etc). What I want is to have it automatically commit the changes that I make to the file in the resulting rendered image. I don't want to have to close the application and relaunch it, as there is a lot of pre-processing being done and it takes quite a while. Right now I have a button that I click on after I've made modifications to the shader file, but it would still be a lot faster if the changes would take place when I modify the file and hit Ctrl+S.
Sam Hobbs15y ago
I often delete projects and usually I must exit from Visual Studio to delete all the files, even if I close the solution. I have even tried opening a different solution yet I cannot delete all files for projects. The point I am getting at is that the problem might be that the file has been opened by VS. I am able to write to a file that is also open in VS so if your application is only trying to read from it then it does not make sense that the reason the application can't read is that VS has it open.

It is easy enough to just exit from VS and try the application.

Earlier when I said to Dispose the FileStream object I should say to Dispose of anything in your application that used the file.

Try the Handle utility. I am not sure whether it will tell you what you need to know. Note that it is a command-line tool and if you don't specify any options it will produce many lines of output. I think I would begin using it by redirecting the output to a file.
Vulpes15y ago
Yes, it could be the problem if you're not closing the file after writing to it or you're not using FileShare.ReadWrite when opening the FileStream.
Marius15y ago
I have it open in Visual Studio and I write to it. Hmm... could that be the problem?
Vulpes15y ago
So, what is actually writing to this file?

Is it another process running on the same machine?
Marius15y ago
Tried. Still no change...
Sam Hobbs15y ago
Dispose the FileStream object when you are through with it.
Marius15y ago
No, there's no networking involved.
Vulpes15y ago
Is it possible that the file is being opened with exclusive access by other network users and they're not closing it immediately after they've written to it?
Marius15y ago
Yeah, I'm closing the stream right after I've read the data from the file.

The FileShare thing doesn't change anything :(
Vulpes15y ago
Firstly, I assume you're closing the FileStream after you've finished with it?

I'd try using one of the overloads of the FileStream constructor which takes a FileShare argument:

FileStream fs = new FileStream(pathToFile + fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);