Question

We perform updates of large text files by writing new records to a temp file, then replacing the old file with the temp file. A heavily abbreviated version:

var tpath = Path.GetTempFileName(); try {     using (var sf = new StreamReader(sourcepath))     using (var tf = new StreamWriter(tpath))     {         string line;         while ((line = sf.ReadLine()) != null)             tf.WriteLine(UpdateLine(line));     }      File.Delete(sourcepath);     File.Move(tpath, sourcepath); } catch {     File.Delete(tpath);     throw; } 

If anything throws an exception (file not found, no permission), the original file is left untouched, which is what we want.

However, the code has the following problems:

1. Is there a real-world situation where the Delete works but the Move fails? This would delete the original and updated data. This would be bad.

2. The most common failure is the source file being open from another application, and the Delete fails. This means all the Update work is discarded. Is there a way to see if the source file is deletable at the start, and abandon the update if not?

3. We have users putting Windows Explorer Summary properties, like Title or Comments, on files. These are discarded when we delete the file. Is there a way to copy the old file’s Summary properties to a new file? Should we do this?

Answer 1

Lots of good suggestions. I was able to solve the problems with:

var sInfo = new FileInfo(sourcePath); if (sInfo.IsReadOnly)     throw new IOException('File '' + sInfo.FullName + '' is read-only.');  var tPath = Path.GetTempFileName(); try {     // This throws if sourcePath does not exist, is opened, or is not readable.     using (var sf = sInfo.OpenText())     using (var tf = new StreamWriter(tPath))     {         string line;         while ((line = sf.ReadLine()) != null)             tf.WriteLine(UpdateLine(line));     }      string backupPath = sInfo.FullName + '.bak';     if (File.Exists(backupPath))         File.Delete(backupPath);      File.Move(tPath, backupPath);     tPath = backupPath;     File.Replace(tPath, sInfo.FullName, null); } catch (Exception ex) {     File.Delete(tPath);     throw new IOException('File '' + sInfo.FullName + '' could not be overwritten.', ex); } 

OpenText throws if the source file is open or not readable, and the update is not done. If anything throws, the original file is left unchanged. Replace copies the old files’ Summary properties to the new file. This works even if the source file is on a different volume than the temp folder.