How can I do an atomic write/append in C#, or how do I get files opened with the FILE_APPEND_DATA flag?

Question

Under most Unixes and Posix conforming operating systems performing an open() operating system call with the O_APPEND indicates to the OS that writes are to be atomic append

Answer 1

Use one of the overloads of the FileStream constructor:

new FileStream(FileName, FileMode.Open, FileSystemRights.AppendData,
            FileShare.Write, 4096, FileOptions.None)

FileSystemRights.AppendData corresponds with FILE_APPEND_DATA

FileStream seems to insist on buffering, so make sure the buffer is large enough for each write and call Flush() after each write.

Tiny example:

    private void button1_Click(object sender, EventArgs e) {
        Thread t1 = new Thread(DoIt);
        Thread t2 = new Thread(DoIt);
        t1.Start("a");
        t2.Start("b");
        Thread.Sleep(2000);
        Environment.Exit(0);
    }

    private void DoIt(object p) {
        using (FileStream fs = new FileStream(FileName, FileMode.Open, FileSystemRights.AppendData,
            FileShare.Write, 4096, FileOptions.None)) {
            using (StreamWriter writer = new StreamWriter(fs)) {
                writer.AutoFlush = true;
                for (int i = 0; i < 20; ++i)
                    writer.WriteLine("{0}: {1:D3} {2:o} hello", p, i, DateTime.Now);
            }
        }
    }

Answer 2

Why can't you use

System.IO.File.AppendAllText("C:\\somefile.txt", "some content");

? That's a thread-safe/"atomic" call as well.

Answer 3

You can call CreateFile using PInvoke with the required parameters and pass the resulting handle to one of the FileStream Constructors which takes SafeFileHandle as a parameter.