Cet article est disponible en francais.

Sometimes, the most simple examples are the best.


Let’s say you have a configuration file, but you want to make a copy of it before you modify it. Easy, you copy that file to “filename.bak”. But what happens there’s already that file ? Well, either you replace it, or you create an autoincremented file.


If you want to do the latter, you could do it using a for loop. But since you’re a happy functional programming guy, you want to make it using LINQ.


You then can do it like this :

    public static string CreateNewFileName(string filePath)
        if (!File.Exists(filePath))
            return filePath;

        // Don't do that for each file.
        var name = Path.GetFileNameWithoutExtension(filePath);
        var extension = Path.GetExtension(filePath);

        // Now find the next available file
        var fileQuery = from index in Enumerable.Range(2, 10000)

                        // Build the file name
                        let fileName = string.Format("{0} ({1}){2}", name, index, extension)

                        // Does it exist ?
                        where !File.Exists(fileName)

                        // No ? Select it.
                        select fileName;

        // Return the first one.
        return fileQuery.First();

Note the use of the let operator, which allows the reuse of what is called a “range variable”. In this case, it avoids using string.Format multiple times.


The case of Infinity

There’s actually one problem with this implementation, which is the arbitrary “10000”. This might be fine if you don’t intend to make more than 10000 backups of your configuration file. But if you do, to lift that limit, we could write this iterator method :

    public static IEnumerable<int> InfiniteRange(int start)
             yield return start++;

Which basically will return an new value each time you ask for one. To use that method you have to make sure that you have an exit condition (the file does not exist, in the previous example), or you may well be enumerating until the end of times... Actually up to int.MaxValue, for the nit-pickers, but .NET 4.0 adds System.Numerics.BigInteger to be sure to get to the end of times. You never know.


To use this iterator, just replace :

        var fileQuery = from index in Enumerable.Range(2, 10000)


        var fileQuery = from index in InfiniteRange()

And you’re done.