c# continuously read file

Question

I want to read file continuously like GNU tail with "-f" param. I need it to live-read log file. What is the right way to do it?

Answer 1

You want to open a FileStream in binary mode. Periodically, seek to the end of the file minus 1024 bytes (or whatever), then read to the end and output. That's how tail -f works.

Answers to your questions:

Binary because it's difficult to randomly access the file if you're reading it as text. You have to do the binary-to-text conversion yourself, but it's not difficult. (See below)

1024 bytes because it's a nice convenient number, and should handle 10 or 15 lines of text. Usually.

Here's an example of opening the file, reading the last 1024 bytes, and converting it to text:

static void ReadTail(string filename)
{
    using (FileStream fs = File.Open(filename, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
    {
        // Seek 1024 bytes from the end of the file
        fs.Seek(-1024, SeekOrigin.End);
        // read 1024 bytes
        byte[] bytes = new byte[1024];
        fs.Read(bytes, 0, 1024);
        // Convert bytes to string
        string s = Encoding.Default.GetString(bytes);
        // or string s = Encoding.UTF8.GetString(bytes);
        // and output to console
        Console.WriteLine(s);
    }
}

Note that you must open with FileShare.ReadWrite, since you're trying to read a file that's currently open for writing by another process.

Also note that I used Encoding.Default, which in US/English and for most Western European languages will be an 8-bit character encoding. If the file is written in some other encoding (like UTF-8 or other Unicode encoding), It's possible that the bytes won't convert correctly to characters. You'll have to handle that by determining the encoding if you think this will be a problem. Search Stack overflow for info about determining a file's text encoding.

If you want to do this periodically (every 15 seconds, for example), you can set up a timer that calls the ReadTail method as often as you want. You could optimize things a bit by opening the file only once at the start of the program. That's up to you.

Answer 2

More natural approach of using FileSystemWatcher:

    var wh = new AutoResetEvent(false);
    var fsw = new FileSystemWatcher(".");
    fsw.Filter = "file-to-read";
    fsw.EnableRaisingEvents = true;
    fsw.Changed += (s,e) => wh.Set();

    var fs = new FileStream("file-to-read", FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    using (var sr = new StreamReader(fs))
    {
        var s = "";
        while (true)
        {
            s = sr.ReadLine();
            if (s != null)
                Console.WriteLine(s);
            else
                wh.WaitOne(1000);
        }
    }

    wh.Close();

Here the main reading cycle stops to wait for incoming data and FileSystemWatcher is used just to awake the main reading cycle.