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

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Problem :

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 and write operations. With this behavior,for local filesystems when you do a write, you know it get appended to the end of the file.

The Windows operating systems support the same functionality by passing FILE_APPEND_DATA in the appropriate parameter to the Win32 CreateFile() system call.


My problem is this, I cannot determine how to get this behavior under C# using the Net Framework
Libraries, is there a way to get such behavior using the Net Framework?
I do not believe using FileMode.Append gives such behavior, by the way.

Solution :

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);

    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);

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.

Why can’t you use

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

? That’s a thread-safe/”atomic” call as well.

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