How can I read a text file without locking it?

Question

I have a windows service writes its log in a text file in a simple format.

Now, I'm going to create a small application to read the service's log and shows both the existing log and the added one as live view.

The problem is that the service locks the text file for adding the new lines and at the same time the viewer application locks the file for reading.

The Service Code:

void WriteInLog(string logFilePath, data)
{
    File.AppendAllText(logFilePath, 
                       string.Format("{0} : {1}\r\n", DateTime.Now, data));
}

The viewer Code:

int index = 0;
private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                using (StreamReader sr = new StreamReader(logFilePath))
                {
                    while (sr.Peek() >= 0)  // reading the old data
                    {
                        AddLineToGrid(sr.ReadLine());
                        index++;
                    }
                    sr.Close();
                }

                timer1.Start();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }


private void timer1_Tick(object sender, EventArgs e)
        {
            using (StreamReader sr = new StreamReader(logFilePath))
            {
                // skipping the old data, it has read in the Form1_Load event handler
                for (int i = 0; i < index ; i++) 
                    sr.ReadLine();

                while (sr.Peek() >= 0) // reading the live data if exists
                {
                    string str = sr.ReadLine();
                    if (str != null)
                    {
                        AddLineToGrid(str);
                        index++;
                    }
                }
                sr.Close();
            }
        }

Is there any problem in my code in reading and writing way?

How to solve the problem?

Possible duplicate of Can I prevent a StreamReader from locking a text file whilst it is in use? — Gerardo Lima
– Gerardo Lima, Commented Sep 15, 2017 at 9:49

Manfred · Accepted Answer · 2020-04-10 21:32:11Z

145

You need to make sure that both the service and the reader open the log file non-exclusively. Try this:

For the service - the writer in your example - use a FileStream instance created as follows:

var outStream = new FileStream(logfileName, FileMode.Open, 
                               FileAccess.Write, FileShare.ReadWrite);

For the reader use the same but change the file access:

var inStream = new FileStream(logfileName, FileMode.Open, 
                              FileAccess.Read, FileShare.ReadWrite);

Also, since FileStream implements IDisposable make sure that in both cases you consider using a using statement, for example for the writer:

using(var outStream = ...)
{
   // using outStream here
   ...
}

Good luck!

edited Apr 10, 2020 at 21:32

answered Aug 10, 2010 at 11:14

Manfred

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4 Comments

Colin Over a year ago

There's also the ReadLines() version at stackoverflow.com/questions/5338450/…

neminem Over a year ago

Perfect - I had thought File.Open() with FileAccess.Read was enough, but it is not. This is, though. :)

crokusek Over a year ago

On the writer, wouldn't FileShare.Read be enough since there is only the one writer?

weeksdev Over a year ago

Also, worth noting that FileStream implements IDisposable

Indy9000 · Accepted Answer · 2013-03-27 00:38:35Z

34

Explicit set up the sharing mode while reading the text file.

using (FileStream fs = new FileStream(logFilePath, 
                                      FileMode.Open, 
                                      FileAccess.Read,    
                                      FileShare.ReadWrite))
{
    using (StreamReader sr = new StreamReader(fs))
    {
        while (sr.Peek() >= 0) // reading the old data
        {
           AddLineToGrid(sr.ReadLine());
           index++;
        }
    }
}

edited Mar 27, 2013 at 0:38

Indy9000

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answered Aug 10, 2010 at 11:13

heads5150

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3 Comments

nothrow Over a year ago

it is unnecessary to explicitly close the stream, as StreamReader.Dispose closes it automatically.

L.E.O Over a year ago

I believe the file share should be set on BOTH read and write streams, and not only read.

Eregrith Over a year ago

StreamReader.Dispose also disposes of the FileStream if I'm not mistaken. Here it's disposed twice.

Indy9000 · Accepted Answer · 2013-03-27 00:39:54Z

16

new StreamReader(File.Open(logFilePath, 
                           FileMode.Open, 
                           FileAccess.Read, 
                           FileShare.ReadWrite))

-> this doesn't lock the file.

edited Mar 27, 2013 at 0:39

Indy9000

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answered Aug 10, 2010 at 11:15

nothrow

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4 Comments

webzy Over a year ago

Seems to work when reading data. When writing the locked file error comes up.

Fandango68 Over a year ago

@webzy of course. You cannot write to a file without locking it

nothrow Over a year ago

@Fandango68 The accepted answer elaborates on the solution better than mine, but your statement is completely not true. It is possible to write to files without holding any lock.

Fandango68 Over a year ago

!! surprised. I should try it. So how does Windows update a log file in real time, whilst running the above?

Gary · Accepted Answer · 2024-06-07 03:01:05Z

10

The problem is when you are writing to the log you are exclusively locking the file down so your StreamReader won't be allowed to open it at all.

You need to try open the file in readonly mode.

using (FileStream fs = new FileStream("myLogFile.txt", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
    using (StreamReader sr = new StreamReader(fs))
    {
        while (!sr.EndOfStream)
        {
            string line = sr.ReadLine();
            // Your code here
        }
    }
}

edited Jun 7, 2024 at 3:01

Gary

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answered Aug 10, 2010 at 11:07

James

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2 Comments

bohdan_trotsenko Over a year ago

As now I know, File.ReadAllText() fails with read-only mode.

James Over a year ago

@modosansreves yeah I removed that portion of the answer as the docs state that it will throw an UnauthorizedAccessException if the file is read only - must have missed that at the time of answering!

dotmartin · Accepted Answer · 2010-08-10 11:16:22Z

5

I remember doing the same thing a couple of years ago. After some google queries i found this:

    FileStream fs = new FileStream(@”c:\test.txt”, 
                                   FileMode.Open, 
                                   FileAccess.Read,        
                                   FileShare.ReadWrite);

i.e. use the FileShare.ReadWrite attribute on FileStream().

(found on Balaji Ramesh's blog)

answered Aug 10, 2010 at 11:16

dotmartin

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Comments

Tom Gullen · Accepted Answer · 2010-08-10 11:05:42Z

1

Have you tried copying the file, then reading it?

Just update the copy whenever big changes are made.

answered Aug 10, 2010 at 11:05

Tom Gullen

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1 Comment

Seichi Over a year ago

I've tried this and it's not the best solution. Copying the file takes too long, for a 4mb file takes like 20 seconds.

Amit Kumawat · Accepted Answer · 2016-06-23 07:53:56Z

-4

This method will help you to fastest read a text file and without locking it.

private string ReadFileAndFetchStringInSingleLine(string file)
    {
        StringBuilder sb;
        try
        {
            sb = new StringBuilder();
            using (FileStream fs = File.Open(file, FileMode.Open))
            {
                using (BufferedStream bs = new BufferedStream(fs))
                {
                    using (StreamReader sr = new StreamReader(bs))
                    {
                        string str;
                        while ((str = sr.ReadLine()) != null)
                        {
                            sb.Append(str);
                        }
                    }
                }
            }
            return sb.ToString();
        }
        catch (Exception ex)
        {
            return "";
        }
    }

Hope this method will help you.

answered Jun 23, 2016 at 7:53

Amit Kumawat

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1 Comment

default locale Over a year ago

As far as I understand, this method reads a whole file into a string and swallows exceptions along the way. I can't see how is that supposed to help with file locking.

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