brute force timing

    static void Main()
    {
        var sw = Stopwatch.StartNew();
        int attempt = 0;
        foreach (var str in Strings(8)) {
            ++attempt;
            if (attempt > 1000*1000)
                break;
            PasswordCheck.IsCorrect(str);
        }
        sw.Stop();

        // attempts per second
        Console.WriteLine(1000.0*1000 / sw.Elapsed.TotalSeconds);

    }

    static IEnumerable<string> Strings(int length, StringBuilder sb = null)
    {
        if (length == 0) {
            if (sb == null)
                yield return "";
            else
                yield return sb.ToString();
            yield break;
        }
        if (sb == null) {
            sb = new StringBuilder();
        }
        // password is not necessarily a-z!
        for (char ch = 'a'; ch <= 'z'; ++ch)
        {
            sb.Append(ch);
            foreach (var str in Strings(length - 1, sb)) {
                yield return str;
            }
            sb.Remove(sb.Length - 1, 1);
        }
    }