ListAllFiles.cs

public static IEnumerable<string> ListAllFiles(string path)
        {
            var directories = Directory.GetDirectories(path);
            foreach (var directory in directories)
            {
                foreach (string x in ListAllFiles(directory))
                {
                    yield return x;
                }
            }

            var files = Directory.GetFiles(path);
            foreach (var file in files)
            {
                yield return  file;
            }
        }