Overwrite file but still read content

Question

I'm trying to find the most reasonable way to open a file, modify its content and then write it back to file.

If I have the following "MyFile.xml"

<?xml version="1.0" encoding="utf-8"?>
<node>
  <data>this is my data which is long</data>
</node>

And then want to modify it according to this:

private static void Main(string[] args)
{
    using (FileStream stream = new FileStream("Myfile.xml", FileMode.Open))
    {
        XDocument doc = XDocument.Load(stream);
        doc.Descendants("data").First().Value = "less data";
        stream.Position = 0;
        doc.Save(stream);
    }
}

I get the following result. Note that, since the total file length is less than before I get incorrect data at the ending.

<?xml version="1.0" encoding="utf-8"?>
<node>
  <data>less data</data>
</node>/node>

I guess I could use File.ReadAll* and File.WriteAll* but that would mean two File openings. Isn't there some way to say "I want to open this file, read its data and when I save delete the old content" without closing and reopening the file? Other solutions that I have found include FileMode.Truncate, but that would imply that I cannot read the content.

Does this method help: http://msdn.microsoft.com/en-us/library/system.io.filestream.setlength.aspx ? — Vitalii, Mar 25 '13 at 10:22
duplicate of http://stackoverflow.com/questions/8464261/filestream-and-streamwriter-how-to-truncate-the-remainder-of-the-file-after-wr ? — Lanorkin, Mar 25 '13 at 10:23

score 3 · Accepted Answer · answered Mar 25 '13 at 10:22

3

You'll have to use FileStream.SetLength like this:

stream.SetLength(stream.Position);

After you have finished writing.
Of course, assuming that the position is at the end of the written data.

answered Mar 25 '13 at 10:22

Vercas

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That would be it. This is the method I'm looking for. Thanks! (will accept after time limit) – default Mar 25 '13 at 10:24
I think you should do what Tomtom suggested below. – Matthew Watson Mar 25 '13 at 10:42
@MatthewWatson That would work for his very specific case. My solution covers a larger spectrum of cases. – Vercas Mar 25 '13 at 11:49
@Vercas: So for his *specific* case, I would use Tomtom's solution. :) – Matthew Watson Mar 25 '13 at 13:30

score 2 · Answer 2 · answered Mar 25 '13 at 10:23

Why do you read the file into a filestream first?

You can do the following:

private static void Main(string[] args]
{
   string path = "MyFile.xml";
   XDocument doc = XDocument.Load(path);
   // Check if the root-Node is not null and other validation-stuff
   doc.Descendants("data").First().Value = "less data";
   doc.Save(path);
}

The problem with the stream is, that you can either read or write.

I've read, that with the .net-Framework 4.5 it's also possible to read and write on a stream, but haven't tried it yet.

Overwrite file but still read content

2 Answers2