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 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.

references:

http://www.google.com/search?q=msdn+createfile or: http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx  http://www.google.com/search?q=msdn+IoCreateFileSpecifyDeviceObjectHint or: http://www.google.com/search?q=msdn+IoCreateFileSpecifyDeviceObjectHint 

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.

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

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.