C#: How would you make a unique filename by adding a number?

Question

I would like to create a method which takes either a filename as a string or a FileInfo and adds an incremented number to the filename if the file exists. But can't quite wrap my head around how to do this in a good way.

For example, if I have this FileInfo

var file = new FileInfo(@"C:\file.ext");

I would like the method to give me a new FileInfo with C:\file 1.ext if C:\file.ext existed, and C:\file 2.ext if C:\file 1.ext existed and so on. Something like this:

public FileInfo MakeUnique(FileInfo fileInfo)
{
    if(fileInfo == null)
        throw new ArgumentNullException("fileInfo");
    if(!fileInfo.Exists)
        return fileInfo;

    // Somehow construct new filename from the one we have, test it, 
    // then do it again if necessary.
}

Answer 1

This is just a string operation; find the location in the filename string where you want to insert the number, and re-construct a new string with the number inserted. To make it re-usable, you might want to look for a number in that location, and parse it out into an integer, so you can increment it.

Please note that this in general this way of generating a unique filename is insecure; there are obvious race condition hazards.

There might be ready-made solutions for this in the platform, I'm not up to speed with C# so I can't help there.

Answer 2

Take a look at the methods in the Path class, specifically Path.GetFileNameWithoutExtension(), and Path.GetExtension().

You may even find Path.GetRandomFileName() useful!

Edit:

In the past, I've used the technique of attempting to write the file (with my desired name), and then using the above functions to create a new name if an appropriate IOException is thrown, repeating until successful.

Answer 3

public FileInfo MakeUnique(string path)
{            
    string dir = Path.GetDirectoryName(path);
    string fileName = Path.GetFileNameWithoutExtension(path);
    string fileExt = Path.GetExtension(path);

    for (int i = 1; ;++i) {
        if (!File.Exists(path))
            return new FileInfo(path);

        path = Path.Combine(dir, fileName + " " + i + fileExt);
    }
}

Obviously, this is vulnerable to race conditions as noted in other answers.

Answer 4

If checking if the file exists is too hard you can always just add a date and time to the file name to make it unique:

FileName.MMDDYY.HHMMSS

Maybe even add milliseconds if necessary.

Answer 5

If the format doesn't bother you then you can call:

try{
    string tempFile=System.IO.Path.GetTempFileName();
    string file=System.IO.Path.GetFileName(tempFile);
    //use file
    System.IO.File.Delete(tempFile);
}catch(IOException ioe){
  //handle 
}catch(FileIOPermission fp){
  //handle
}

PS:- Please read more about this at msdn before using.

Answer 6

Insert a new GUID into the file name.

Answer 7

This answer by Marc should do it pretty efficiently.

Answer 8

The idea is to get a list of the existing files, parse out the numbers, then make the next highest one.

Note: This is vulnerable to race conditions, so if you have more than one thread creating these files, be careful.

Note 2: This is untested.

public static FileInfo GetNextUniqueFile(string path)
{
    //if the given file doesn't exist, we're done
    if(!File.Exists(path))
        return new FileInfo(path);

    //split the path into parts
    string dirName = Path.GetDirectoryName(path);
    string fileName = Path.GetFileNameWithoutExtension(path);
    string fileExt = Path.GetExtension(path);

    //get the directory
    DirectoryInfo dir = new DirectoryInfo(dir);

    //get the list of existing files for this name and extension
    var existingFiles = dir.GetFiles(Path.ChangeExtension(fileName + " *", fileExt);

    //get the number strings from the existing files
    var NumberStrings = from file in existingFiles
                        select Path.GetFileNameWithoutExtension(file.Name)
                            .Remove(0, fileName.Length /*we remove the space too*/);

    //find the highest existing number
    int highestNumber = 0;

    foreach(var numberString in NumberStrings)
    {
        int tempNum;
        if(Int32.TryParse(numberString, out tempnum) && tempNum > highestNumber)
            highestNumber = tempNum;
    }

    //make the new FileInfo object
    string newFileName = fileName + " " + (highestNumber + 1).ToString();
    newFileName = Path.ChangeExtension(fileName, fileExt);

    return new FileInfo(Path.Combine(dirName, newFileName));
}

Answer 9

Lots of good advice here. I ended up using a method written by Marc in an answer to a different question. Reformatted it a tiny bit and added another method to make it a bit easier to use "from the outside". Here is the result:

    private static string numberPattern = " ({0})";

    public static string NextAvailableFilename(string path)
    {
        // Short-cut if already available
        if (!File.Exists(path))
            return path;

        // If path has extension then insert the number pattern just before the extension and return next filename
        if (Path.HasExtension(path))
            return GetNextFilename(path.Insert(path.LastIndexOf(Path.GetExtension(path)), numberPattern));

        // Otherwise just append the pattern to the path and return next filename
        return GetNextFilename(path + numberPattern);
    }

    private static string GetNextFilename(string pattern)
    {
        string tmp = string.Format(pattern, 1);
        if (tmp == pattern)
            throw new ArgumentException("The pattern must include an index place-holder", "pattern");

        if (!File.Exists(tmp))
            return tmp; // short-circuit if no matches

        int min = 1, max = 2; // min is inclusive, max is exclusive/untested

        while (File.Exists(string.Format(pattern, max)))
        {
            min = max;
            max *= 2;
        }

        while (max != min + 1)
        {
            int pivot = (max + min) / 2;
            if (File.Exists(string.Format(pattern, pivot)))
                min = pivot;
            else
                max = pivot;
        }

        return string.Format(pattern, max);
    }

Only partially tested it so far, but will update if I find any bugs with it. (Marcs code works nicely!) If you find any problems with it, please comment or edit or something :)

Answer 10

/// <summary>
/// Created a Unique filename for the given filename
/// </summary>
/// <param name="filename">A full filename, e.g., c:\temp\myfile.tmp</param>
/// <returns>A filename like c:\temp\myfile633822247336197902.tmp</returns>
public string GetUniqueFilename(string filename)
{
    string basename = Path.Combine(Path.GetDirectoryName(filename), Path.GetFileNameWithoutExtension(filename));
    string uniquefilename = string.Format("{0}{1}{2}", basename, DateTime.Now.Ticks, Path.GetExtension(filename));
    // Thread.Sleep(1); // To really prevent collisions, but usually not needed
    return uniquefilename;
}

As DateTime.Ticks has a resolution of 100 nanoseconds, collisions are extremely unlikely. However, a Thread.Sleep(1) will ensure that, but I doubt that it's needed

Answer 11

Instead of poking the disk a number of times to find out if it has a particular variant of the desired file name, you could ask for the list of files that already exist and find the first gap according to your algorithm.

public static class FileInfoExtensions
{
    public static FileInfo MakeUnique(this FileInfo fileInfo)
    {
     if (fileInfo == null)
     {
      throw new ArgumentNullException("fileInfo");
     }

     string newfileName = new FileUtilities().GetNextFileName(fileInfo.FullName);
     return new FileInfo(newfileName);
    }
}

public class FileUtilities
{
    public string GetNextFileName(string fullFileName)
    {
     if (fullFileName == null)
     {
      throw new ArgumentNullException("fullFileName");
     }

     if (!File.Exists(fullFileName))
     {
      return fullFileName;
     }
     string baseFileName = Path.GetFileNameWithoutExtension(fullFileName);
     string ext = Path.GetExtension(fullFileName);

     string filePath = Path.GetDirectoryName(fullFileName);
     var numbersUsed = Directory.GetFiles(filePath, baseFileName + "*" + ext)
      .Select(x => Path.GetFileNameWithoutExtension(x).Substring(baseFileName.Length))
      .Select(x =>
        {
         int result;
         return Int32.TryParse(x, out result) ? result : 0;
        })
      .Distinct()
      .OrderBy(x => x)
      .ToList();

     var firstGap = numbersUsed
      .Select((x, i) => new { Index = i, Item = x })
      .FirstOrDefault(x => x.Index != x.Item);
     int numberToUse = firstGap != null ? firstGap.Item : numbersUsed.Count;
     return Path.Combine(filePath, baseFileName) + numberToUse + ext;
    }
}