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

Answer 1

Here's one that decouples the numbered naming question from the check of the filesystem:

/// 

/// Finds the next unused unique (numbered) filename.
/// 
/// Name of the file.
/// Function that will determine if the name is already in use
/// The original filename if it wasn't already used, or the filename with " (n)"
/// added to the name if the original filename is already in use.
private static string NextUniqueFilename(string fileName, Func inUse)
{
    if (!inUse(fileName))
    {
        // this filename has not been seen before, return it unmodified
        return fileName;
    }
    // this filename is already in use, add " (n)" to the end
    var name = Path.GetFileNameWithoutExtension(fileName);
    var extension = Path.GetExtension(fileName);
    if (name == null)
    {
        throw new Exception("File name without extension returned null.");
    }
    const int max = 9999;
    for (var i = 1; i < max; i++)
    {
        var nextUniqueFilename = string.Format("{0} ({1}){2}", name, i, extension);
        if (!inUse(nextUniqueFilename))
        {
            return nextUniqueFilename;
        }
    }
    throw new Exception(string.Format("Too many files by this name. Limit: {0}", max));
}

And here's how you might call it if you are using the filesystem

var safeName = NextUniqueFilename(filename, f => File.Exists(Path.Combine(folder, f)));