How do I write to a hidden file?

It seems that the problem is that kind of a File.Exists() check is done internally, which fails if the file is hidden (e.g. tries to do a FileMode.Create on a file which already exists).

Therefore, use FileMode.OpenOrCreate to make sure that the file is opened or created even if it is hidden, or just FileMode.Open if you do not want to create it if it doesn't exist.

When FileMode.OpenOrCreate is used though, the file will not be truncated, so you should set its length at the end to make sure that there is no leftover after the end of the text.

using (FileStream fs = new FileStream(filename, FileMode.Open)) {
  using (TextWriter tw = new StreamWriter(fs)) {
    // Write your data here...
    tw.WriteLine("foo");
    // Flush the writer in order to get a correct stream position for truncating
    tw.Flush();
    // Set the stream length to the current position in order to truncate leftover text
    fs.SetLength(fs.Position);
  }
}

If you use .NET 4.5 or later, there is a new overload which prevents the disposal of the StreamWriter to also dispose the underlying stream. The code could then be written slighly more intuitively like this:

using (FileStream fs = new FileStream(filename, FileMode.Open)) {
  using (TextWriter tw = new StreamWriter(fs, Encoding.UTF8, 1024, true)) {
    // Write your data here...
    tw.WriteLine("foo");
  }
  // Set the stream length to the current position in order to truncate leftover text
  fs.SetLength(fs.Position);
}

EDIT 2: This answer solve the problem, but is not the correct way to deal with the problem. You should look for Lucero's answer.

Took this answer from: http://www.dotnetspark.com/Forum/314-accessing-hidden-files-and-write-it.aspx

1- Set File as Visible so it can be overwritten

// Get file info
FileInfo myFile= new FileInfo(Environment.CurrentDirectory + @"\hiddenFile.txt");

// Remove the hidden attribute of the file
myFile.Attributes &= ~FileAttributes.Hidden;

2- Make changes to the file

// Do foo...

3- Set back file as hidden

// Put it back as hidden
myFile.Attributes |= FileAttributes.Hidden;

EDIT: I fixed some problem on my answer as mentionned by briler

If that's an option for you you could try to use File.SetAttributes to remove the hidden attribute temporarily, do your work and then set it back to the previous state.

You can unhide the file before writing into it and after complete writing hide it again.

Once you've opened a file, you can change its attributes (including to readonly) and continue writing to it. This is one way to prevent a file from being overwritten by other processes.

So it would seem to be possible to unhide the file, open it, then reset it to hidden, even while you've got it open.

For example, the following code works:

public void WriteToHiddenFile(string fname)
{
    TextWriter    outf;
    FileInfo      info;  

    info = new FileInfo(fname);
    info.Attributes = FileAttributes.Normal;    // Set file to unhidden
    outf = new StreamWriter(fname);             // Open file for writing
    info.Attributes = FileAttributes.Hidden;    // Set back to hidden
    outf.WriteLine("test output.");             // Write to file
    outf.Close();                               // Close file
}

Note that the file remains hidden while the process writes to it.