EnsureUniqueFilename helps to generate a unique filename

EnsureUniqueFilename

public class EnsureUniqueFilename
{
    private readonly string _path;
    private readonly Dictionary<string, bool> _used = new Dictionary<string, bool>(StringComparer.OrdinalIgnoreCase);
    private string _format = "{0}_{1}{2}";
    private int _startingSeq = 2;

    public EnsureUniqueFilename(string path)
    {
        _path = path;
    }

    public string Seperator
    {
        set { Format = "{0}" + value + "{1}{2}"; }
    }

    public int StartingSeq
    {
        get { return _startingSeq; }
        set { _startingSeq = value; }
    }

    public string Format
    {
        get { return _format; }
        set { _format = value; }
    }

    public void MarkUsed(string fname)
    {
        _used[fname] = true;
    }

    public string GetFilename(string suggested)
    {
        var seq = GetStartingSeq(suggested);
        RemoveSuffix(ref suggested, seq);
        string fname = suggested;
        while (AlreadyUsed(fname))
            fname = string.Format(Format, Path.GetFileNameWithoutExtension(suggested), seq++, Path.GetExtension(suggested));
        _used[fname] = true;
        return fname;
    }

    private int GetStartingSeq(string suggested)
    {
        var matchCollection = Regex.Matches(suggested, "[0-9]+").Cast<Match>().LastOrDefault();
        if (matchCollection == null) return StartingSeq;

        int seq;
        return !int.TryParse(matchCollection.Value, out seq) ? StartingSeq : seq;
    }

    private void RemoveSuffix(ref string fname, int seq)
    {
        var suffix = string.Format(Format, "", seq, "");
        var name = Path.GetFileNameWithoutExtension(fname);
        var ext = Path.GetExtension(fname);
        if (name != null && name.EndsWith(suffix))
            fname = name.Substring(0, name.Length - suffix.Length) + ext;
    }

    private bool AlreadyUsed(string suggested)
    {
        if (_path != null) return File.Exists(Path.Combine(_path, suggested));
        bool alreadyUsed;
        return _used.TryGetValue(suggested, out alreadyUsed) && alreadyUsed;
    }
}