Our application needs to lock down a file to read-only when it is opened the first time. I have the code below, which doesn't seem to work anymore.
Now, for some reason, it never triggers the exception when I open a second file and just passes through the first statement every time.
From what I understand, the logic behind this code is this: The first file opens the file with Read/Write (FileAccess.ReadWrite) and sets the file to Read for subsequent users (FileShare.Read).
When it is opened the second time, it will try and open it with Read/Write but is restricted to Read-Only upon which it triggers the exception and goes to the second statement. There it will open the file as Read-Only(FileAccess.Read) and set it back to read-write(FileAccess.ReadWrite) for subsequent users, this to ensure that the first document does not get locked out of its already permitted Write rights, which caused an exception.
I tested this already, it used to work with below code. I can still verify that the read-lock is set. When I open the file in another PDF editor, it cannot save, just read. When I do it in my own application, I can save whenever I want.
Am I missing something, or was there a fluke so that it temporarily worked ?
try
{
m_FileStream = File.Open(fileName, FileMode.Open, FileAccess.ReadWrite, FileShare.Read);
m_bCanWrite = true;
}
catch (Exception)
{
m_FileStream = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
m_bCanWrite = false;
}
I changed the code so it uses using blocks, with the same params, but the result is also the same.
try
{
FileInfo info = new FileInfo(fileName);
using (FileStream readWriteStream = info.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.Read))
{
m_bcanWrite = true;
m_PdfDocument = new PDFDoc(readWriteStream);
}
}
catch (Exception)
{
FileInfo info = new FileInfo(fileName);
using (FileStream readStream = info.Open(FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
m_bcanWrite = false;
m_PdfDocument = new PDFDoc(readStream);
}
}
As we figured out in comments - the reason was premature closing of the file stream. Of course when you close filestream - all locks on it are released. It's not a good idea to hold the file open for the duration of the whole application run time as you suggested in comment - just close the file when you are done with it, not sooner but not later.
Related
I'm attempting to use StreamReader and StreamWriter to grab a temporary output log (.txt format) from another application.
The output log is always open and constantly written to.
Unhelpfully if the application closes or crashes, the log file ends up deleted - hence the need for a tool that can grab the information from this log and save it.
What my program currently does is:
Create a new .txt file, and stores the path of that file as the
string "destinationFile".
Finds the .txt log file to read, and stores the path of that file as
the string "sourceFile"
It then passes those two strings to the method below.
Essentially I'm trying to read the sourceFile one line at a time.
Each time one line is read, it is appended to destinationFile.
This keeps looping until the sourceFile no longer exists (i.e. the application has closed or crashed and deleted its log).
In addition, the sourceFile can get quite big (sometimes 100Mb+), and this program may be handling more than one log at a time.
Reading the whole log rather than line by line will most likely start consuming a fair bit of memory.
private void logCopier(string sourceFile, string destinationFile)
{
while (File.Exists(sourceFile))
{
string textLine;
using (var readerStream = File.Open(sourceFile,
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite))
using (var reader = new StreamReader(readerStream))
{
while ((textLine = reader.ReadLine()) != null)
{
using (FileStream writerStream = new FileStream(destinationFile,
FileMode.Append,
FileAccess.Write))
using (StreamWriter writer = new StreamWriter(writerStream))
{
writer.WriteLine(textLine);
}
}
}
}
}
The problem is that my WPF application locks up and ceases to respond when it reaches this code.
To track down where, I put a MessageBox just before the writerStream line of the code to output what the reader was picking up.
It was certainly reading the log file just fine, but there appears to be a problem with writing it to the file.
As soon as it reaches the using (FileStream writerStream = new FileStream part of the code, it stops responding.
Is using the StreamWriter in this manner not valid, or have I just gone and dome something silly in the code?
Am also open to a better solution than what I'm trying to do here.
Simply what I understand is you need to copy a file from source to destination which may be deleted at any time.
I'll suggest you to use FileSystemWatcher to watch for source file changed event, then just simply copy the whole file from source to destination using File.Copy.
I've just solved the problem, and the issue was indeed something silly!
When creating the text file for the StreamWriter, I had forgotten to use .Dispose();. I had File.Create(filename); instead of File.Create(filename).Dispose(); This meant the text file was already open, and the StreamWriter was attempting to write to a file that was locked / in use.
The UI still locks up (as expected), as I've yet to implement this on a new thread as SteenT mentioned. However the program no longer crashes and the code correctly reads the log and outputs to a text file.
Also after a bit of refinement, my log reader/writer code now looks like this:
private void logCopier(string sourceFile, string destinationFile)
{
int num = 1;
string textLine = String.Empty;
long offset = 0L;
while (num == 1)
{
if (File.Exists(sourceFile))
{
FileStream stream = new FileStream(sourceFile, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (new StreamReader(stream))
{
stream.Seek(offset, SeekOrigin.Begin);
TextReader reader2 = new StreamReader(stream);
while ((textLine = reader2.ReadLine()) != null)
{
Thread.Sleep(1);
StreamWriter writer = new StreamWriter(destinationFile, true);
writer.WriteLine(textLine);
writer.Flush();
writer.Close();
offset = stream.Position;
}
continue;
}
}
else
{
num = 0;
}
}
}
Just putting this code up here in case anyone else is looking for something like this. :)
I am getting this error: The process cannot access the file (...) because it is being used by another process.
I have tried to use
File.WriteAllText;
StreamWriter sw = new StreamWriter(myfilepath);
sw.Write(mystring);
sw.Close();
sw.Dispose();
;
using (FileStream fstr = File.Create(myfilepath))
{
StreamWriter sw = new StreamWriter(myfilepath);
sw.Write(mystring);
sw.Close();
sw.Dispose();
fstr.Close();
}
All I am trying to do is to access a file, write on it, then close it. I might be making a silly mistake but I would like to understand what I am doing wrong and why. How to make sure that the file is closed and not to cause this error again.
Helped by the answers so far I did this:
using (FileStream fstr = File.Open(myfilepath,FileMode.OpenOrCreate,FileAccess.ReadWrite))
{
StreamWriter sw = new StreamWriter(fstr);
sw.Write(mystring);
sw.Close();
}
It seems to be better because it seems to close/stop the process of my file if I try to access another file on the second time I access the page. But if I try to access the same file on a second time, it gives me the error again.
Why not just use:
System.IO.File.WriteAllText(myfilepath, mystring");
That should not lock your file.
Internally WriteAllText uses FileShare.Read and releases that lock as soon as it is done writing.
"because it is being used by another process" that's the clue. Do you by chance have the file open in Notepad or something?
You may need to set the sharing mode when you open the file to allow readers, and ask only for the permission you need (write access).
http://msdn.microsoft.com/en-us/library/5h0z48dh.aspx
I would like to thank everyone for the help.
In fact, apart from this code I found out that I had a stremReader still opened somewhere else after the code above. At the end I changed the code I had before for this:
using (FileStream fstr = File.Open(myfile, FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
StreamWriter sw = new StreamWriter(fstr);
sw.Write(mystring);
sw.Flush();
sw.Dispose();
}
and on my StreamReader I did this:
StreamReader sr = new StreamReader(myfile);
string sometext = sr.ReadToEnd();
sr.Dispose();
I could also use this:
File.ReadAllText(myfile);
If there is something that I could have done in a better way please tell me.
Thank you very much.
Try this
FileStream fs = new FileStream(myfilepath, FileMode.Create, FileAccess.Write);
byte[] bt = System.Text.Encoding.ASCII.GetBytes(mystring);
fs.Write(bt, 0, bt.Length);
fs.Close();
for an application that uses a File as some sort of global storage for device reservations in a firm I need a way to read and write to a file (or lock a file, read from it, write to it, and unlock it). A little code snippet will shot what I mean:
FileStream in = new FileStream("storage.bin", FileMode.Open);
//read the file
in.Close();
//!!!!!
//here is the critical section since between reading and writing, there shouldnt
//be a way for another process to access and lock the file, but there is the chance
//because the in stream is closed
//!!!!!
FileStream out = new FileStream("storage.bin", FileMode.Create);
//write data to file
out.Close();
this should get something like this
LockFile("storage.bin");
//read from it...
//OVERwrite it....
UnlockFile("storage.bin");
the method should be absolute safe, since the program should run on 2000 devices at the same time
Simply holding a FileStream open with exclusive (not shared) access will prevent other processes from accessing the file. This is the default when opening a file for read/write access.
You can 'overwrite' a file that you currently hold open by truncating it.
So:
using (var file = File.Open("storage.bin", FileMode.Open))
{
// read from the file
file.SetLength(0); // truncate the file
// write to the file
}
the method should be absolute safe, since the program should run on 2000 devices at the same time
Depending on how often you're writing to the file, this could become a chokepoint. You probably want to test this to see how scalable it is.
In addition, if one of the processes tries to operate on the file at the same time as another one, an IOException will be thrown. There isn't really a way to 'wait' on a file, so you probably want to coordinate file access in a more orderly fashion.
You need a single stream, opened for both reading and writing.
FileStream fileStream = new FileStream(
#"c:\words.txt", FileMode.OpenOrCreate,
FileAccess.ReadWrite, FileShare.None);
Alternatively you can also try
static void Main(string[] args)
{
var text = File.ReadAllText(#"C:\words.txt");
File.WriteAllText(#"C:\words.txt", text + "DERP");
}
As per http://msdn.microsoft.com/en-us/library/system.io.fileshare(v=vs.71).aspx
FileStream s2 = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.None);
You need to pass in a FileShare enumeration value of None to open on the FileStream constructor overloads:
fs = new FileStream(#"C:\Users\Juan Luis\Desktop\corte.txt", FileMode.Open,
FileAccess.ReadWrite, FileShare.None);
I ended up writing this helper class to do this:
public static class FileHelper
{
public static void ReplaceFileContents(string fileName, Func<String, string> replacementFunction)
{
using (FileStream fileStream = new FileStream(
fileName, FileMode.OpenOrCreate,
FileAccess.ReadWrite, FileShare.None))
{
StreamReader streamReader = new StreamReader(fileStream);
string currentContents = streamReader.ReadToEnd();
var newContents = replacementFunction(currentContents);
fileStream.SetLength(0);
StreamWriter writer = new StreamWriter(fileStream);
writer.Write(newContents);
writer.Close();
}
}
}
which allows you to pass a function that will take the existing contents and generate new contents and ensure the file is not read or modified by anything else whilst this change is happening
You are likely looking for FileStream.Lock and FileStream.Unlock
I think you just need to use the FileShare.None flag in the overloaded Open method.
file = File.Open("storage.bin", FileMode.Open, FileShare.None);
I need to read a Windows file that may be locked, but I don't want to create any kind lock that will prevent other processes from writing to the file.
In addition, even if the file is locked for exclusive use, I'd like to see what's inside.
Although this isn't my exact use case, consider how to read a SQL/Exchange log or database file while it's in use and mounted. I don't want to cause corruption but I still want to see the insides of the file and read it.
You can do it without copying the file, see this article:
The trick is to use FileShare.ReadWrite (from the article):
private void LoadFile()
{
try
{
using(FileStream fileStream = new FileStream(
"logs/myapp.log",
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite))
{
using(StreamReader streamReader = new StreamReader(fileStream))
{
this.textBoxLogs.Text = streamReader.ReadToEnd();
}
}
}
catch(Exception ex)
{
MessageBox.Show("Error loading log file: " + ex.Message);
}
}
The accepted answer is not correct. If the file is really locked, you cannot just change the file share. This would work if the lock has been set with this fileshare option too but it does not mean that it is the case. In fact, you can test #CaffGeek solution pretty easily by opening the file without the FileShare.ReadWrite and than trying to open it with this flag to ReadWrite. You will get that the file is using by another process.
Code:
string content;
var filePath = "e:\\test.txt";
//Lock Exclusively the file
var r = File.Open(filePath, FileMode.Open, FileAccess.Write, FileShare.Write);
//CaffGeek solution
using (FileStream fileStream = new FileStream(
filePath,
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite))
{
using (StreamReader streamReader = new StreamReader(fileStream))
{
content = streamReader.ReadToEnd();
}
}
As you can see, it crashes. This result is the same with any FileStream method like the File.Open. It will crash what ever you put for FileShare during the open stage.
//OPEN FOR WRITE with exclusive
var r = File.Open(filePath, FileMode.Open, FileAccess.Write, FileShare.Write);
//OPEN FOR READ with file share that allow read and write
var x = File.Open(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite); //Crash
Copying the file is not also an option. You can try it your self by opening the file exclusively and try to copy the file on Windows Explorer or by code:
var filePath = "e:\\test.txt";
var filePathCopy = "e:\\test.txt.bck";
//Lock the file
var r = File.Open(filePath, FileMode.Open, FileAccess.Write, FileShare.Write);
File.Copy(filePath, filePathCopy);
var x = File.Open(filePathCopy, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (var reader = new StreamReader(x))
{
content = reader.ReadToEnd();
}
r.Close();
File.Delete(filePathCopy);
This code crash when you hit the File.Copy line. The exception is the same as before : file is being using by another process.
You need to kill the process that has the lock of the file if you want to read it OR if you have the source code of the file that is locking the file to change this one to use FileShare.ReadWrite instead of just FileShare.Write.
You can probably create a copy and read that, even if the file is locked.
Or maybe a StreamReader on a FileStream depending on how SQL opened the file?
new FileStream("c:\myfile.ext", FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
The StreamReader locks a text file whilst it is reading it.
Can I force the StreamReader to work in a "read-only" or "non locking" mode?
My workaround would be to copy the file to a temp location and read it from there but I would prefer to use the StreamReader directly if possible.
Any alternative suggetions?
Background:
I've written a small app to get some stats out of a log file. This file is constantly being updating (several times a second) by an outside program lets call AAXXYY.
Reviewing the output suggests that my app may be locking the file and preventing AAXXYY from writing.
This is what I'm doing
private void btnGetStats_Click(object sender, EventArgs e)
{
int countStarts = 0;
int countEnds = 0;
IList<string> sessions = new List<string>();
using(StreamReader stRead = new StreamReader(openFileDialog1.FileName,Encoding.Unicode))
{
while(!stRead.EndOfStream)
{
string line = stRead.ReadLine();
if(line.Contains("Session start"))
{
countStarts++;
sessions.Add(line.Substring(line.IndexOf("["), line.LastIndexOf("]") - line.IndexOf("[")));
}
if (line.Contains("Session end"))
{
countEnds++;
sessions.Remove(line.Substring(line.IndexOf("["), line.LastIndexOf("]") - line.IndexOf("[")));
}
}
}
txtStarts.Text = countStarts.ToString();
txtEnds.Text = countEnds.ToString();
txtDifference.Text = (countStarts - countEnds).ToString();
listBox1.DataSource = sessions;
}
You can pass a FileStream to the StreamReader, and create the FileStream with the proper FileShare value. For instance:
using (var file = new FileStream (openFileDialog1.FileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var reader = new StreamReader (file, Encoding.Unicode)) {
}
Thought I'd add some context, StreamReader does not lock a file for reading only for writing whist it is being read. Take a look at the code below from the StreamReader class.
new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, FileOptions.SequentialScan);
Notice the default FileAccess.Read parameter taken for MSDN http://msdn.microsoft.com/en-us/library/system.io.fileshare.aspx
Allows subsequent opening of the file for reading. If this flag is not specified, any request to open the file for reading (by this process or another process) will fail until the file is closed. However, even if this flag is specified, additional permissions might still be needed to access the file.
Again taken from MSDN to allow reading and writing use FileAccess.ReadWrite instead (as suggested by Jb Evain).
Allows subsequent opening of the file for reading or writing. If this
flag is not specified, any request to open the file for reading or
writing (by this process or another process) will fail until the file
is closed. However, even if this flag is specified, additional
permissions might still be needed to access the file.