Open, Lock, Truncate and Write to a file

Question

I'm trying to replicate Excel's behaviour when opening and editing files, that is:

File is opened and locked for exclusive write access
Other users are allowed to read the file

I'm using the following code to open the file, read and prepare for later writing:

_fileStream = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.Read);
_fileReader = new StreamReader(_fileStream);
_fileWriter = new StreamWriter(_fileStream);
var contents = _fileReader.ReadToEnd();

(In a class that implements IDisposable and disposes the streams)

It apparently works fine aside from one issue when I later save using the following code:

_fileWriter.BaseStream.Seek(0, SeekOrigin.Begin);
_fileWriter.Write(content);
_fileWriter.Flush();

The problem is that I need to truncate the file before saving, and the above only overwrites. If the new content is shorter than what was loaded, I end up with a bad file due to partial old content.

How do I truncate a filestream before writing (but not before reading)?

Is there perhaps a better way to gain exclusive access to the file than what I am doing?

@GSerg: Thanks. I found another way, but SetLength is an even simpler option, so I'll replace my code with that. — Troels Larsen, Oct 02 '19 at 20:23

score 0 · Answer 1 · answered Oct 02 '19 at 20:22

0

I don't know why I didn't think of this until now, but I found my answer in the source code for System.IO.File:

[SecurityCritical]
private static void InternalWriteAllText(
  string path,
  string contents,
  Encoding encoding,
  bool checkHost)
{
  using (StreamWriter streamWriter = new StreamWriter(path, false, encoding, 1024, checkHost))
    streamWriter.Write(contents);
}

I didn't realize that I could safely open the file for writing, not from the existing FileStream, but by specifying the path. This allowed me to set append to false.

EDIT:

A simpler solution is found in this answer: How to truncate a file in c#?

Just call SetLength(0) before writing.

answered Oct 02 '19 at 20:22

Troels Larsen

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Please note that you can't share a FileStream between multiple StreamReader/StreamWriter instances. The FileShare.xxx options control how multiple handles/FileStreams can be used against the same file. – David Browne - Microsoft Oct 02 '19 at 20:31
@DavidBrowne-Microsoft: Hm.. I had actually expected that to be the case, but I've yet to encounter adverse effects of doing so. What sort of problems should I be seeing as it appears to work fine? And how would I then lock a file for Write elsewhere yet allow my own application to write to it? – Troels Larsen Oct 03 '19 at 21:05
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The Readers and Writers have internal buffers, and expect the FileStream location to _not change_ without them knowing about it. The FileStream has only _one_ Location at which the next Read or Write will occur. – David Browne - Microsoft Oct 03 '19 at 21:09
@DavidBrowne-Microsoft: Ah thank you very much. The reason I'm not seeing it is because I only use the Reader once. I'll refactor my code just to guard against future changes. – Troels Larsen Oct 04 '19 at 06:28

Open, Lock, Truncate and Write to a file

1 Answers1