C# Sanitize File Name

I recently have been moving a bunch of MP3s from various locations into a repository. I had been constructing the new file names using the ID3 tags (thanks, TagLib-Sharp!), and I noticed that I was getting a System.NotSupportedException:

“The given path’s format is not supported.”

This was generated by either File.Copy() or Directory.CreateDirectory().

It didn’t take long to realize that my file names needed to be sanitized. So I did the obvious thing:

public static string SanitizePath_(string path, char replaceChar)
{
    string dir = Path.GetDirectoryName(path);
    foreach (char c in Path.GetInvalidPathChars())
        dir = dir.Replace(c, replaceChar);

    string name = Path.GetFileName(path);
    foreach (char c in Path.GetInvalidFileNameChars())
        name = name.Replace(c, replaceChar);

    return dir + name;
}

To my surprise, I continued to get exceptions. It turned out that ‘:’ is not in the set of Path.GetInvalidPathChars(), because it is valid in a path root. I suppose that makes sense – but this has to be a pretty common problem. Does anyone have some short code that sanitizes a path? The most thorough I’ve come up with this, but it feels like it is probably overkill.

    // replaces invalid characters with replaceChar
    public static string SanitizePath(string path, char replaceChar)
    {
        // construct a list of characters that can't show up in filenames.
        // need to do this because ":" is not in InvalidPathChars
        if (_BadChars == null)
        {
            _BadChars = new List<char>(Path.GetInvalidFileNameChars());
            _BadChars.AddRange(Path.GetInvalidPathChars());
            _BadChars = Utility.GetUnique<char>(_BadChars);
        }

        // remove root
        string root = Path.GetPathRoot(path);
        path = path.Remove(0, root.Length);

        // split on the directory separator character. Need to do this
        // because the separator is not valid in a filename.
        List<string> parts = new List<string>(path.Split(new char[]{Path.DirectorySeparatorChar}));

        // check each part to make sure it is valid.
        for (int i = 0; i < parts.Count; i++)
        {
            string part = parts[i];
            foreach (char c in _BadChars)
            {
                part = part.Replace(c, replaceChar);
            }
            parts[i] = part;
        }

        return root + Utility.Join(parts, Path.DirectorySeparatorChar.ToString());
    }

Any improvements to make this function faster and less baroque would be much appreciated.