Question

A Winforms program needs to save some run time information to an XML file. The file can sometimes be a couple of hundred kilobytes in size. During beta testing we found some users would not hesitate to terminate processes seemingly at random and occasionally causing the file to be half written and therefore corrupted.

As such, we changed the algorithm to save to a temp file and then to delete the real file and do a move.

Our code currently looks like this..

private void Save()
{
    XmlTextWriter streamWriter = null;
    try
    {
        streamWriter = new XmlTextWriter(xmlTempFilePath, System.Text.Encoding.UTF8);

        XmlSerializer xmlSerializer = new XmlSerializer(typeof(MyCollection));

        xmlSerializer.Serialize(streamWriter, myCollection);

        if (streamWriter != null)
            streamWriter.Close();

        // Delete the original file
        System.IO.File.Delete(xmlFilePath);

        // Do a move over the top of the original file 
        System.IO.File.Move(xmlTempFilePath, xmlFilePath);
    }
    catch (System.Exception ex)
    {
        throw new InvalidOperationException("Could not save the xml file.", ex);
    }
    finally
    {
        if (streamWriter != null)
            streamWriter.Close();
    }
}

This works in the lab and in production almost all of the time. The program is running on 12 computers and this code is called on average once every 5 min. About once or twice a day we get this exception:

System.InvalidOperationException: 
Could not save the xml file. 
---> System.IO.IOException: Cannot create a file when that file already exists.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.__Error.WinIOError()
at System.IO.File.Move(String sourceFileName, String destFileName)
at MyApp.MyNamespace.InternalSave()

It is as if the Delete is not actually issued to the hard drive before the Move is issued.

This is happening on Win7 machines.

A couple of questions: Is there some concept of a Flush() one can do for the entire disk operating system? Is this a bug with my code, .net, the OS or something else? Should I be putting in some Thread.Sleep(x)? Maybe I should do a File.Copy(src, dest, true)? Should I write the following code? (But it looks pretty silly.)

while (System.IO.File.Exists(xmlFilePath))
{
    System.IO.File.Delete(xmlFilePath);
}

// Do a move over the top of the main file 
bool done = false;
while (!done)
{
    try
    {
        System.IO.File.Move(xmlTempFilePath, xmlFilePath);
        done = true;
    }
    catch (System.IO.IOException)
    {
        // let it loop
    }
}

Has anyone seen this before?

Answer 1

how about using Move.Copy with overwrite set to true so that it overwrites your app state and then you can delete your temp state ?

You can also attach to App_Exit event and try to perform clean shut down?