例文： 
This section is written by M. Jordan.

希望得到如下集合：
Dictionary<int, string> words = new Dictionary<int, string>();
words[0] = "This";
words[1] = "section";
words[2] = "is";
words[3] = "written";
words[4] = "by";
words[5] = "M.";  // 如果只有一个字母后面跟"." 则保留"."
words[6] = "Jordan"; // 如果多个字母后面跟"." 则忽略"."

我目前有可以实现这个功能的函数，但是效率不高，希望高手能够指点高效的方法。
        public static Dictionary<int, string> BreakToWords3(string sentence)
        {
            Dictionary<int, string> dicWords = new Dictionary<int, string>();
            if (sentence == null || sentence.Length == 0) return dicWords;
            sentence = sentence.ToLower();

            bool start = false;
            const int MAXWORDLEN = 30;
            int index = 0, cindex = 0;
            char[] cs = null;

            foreach (char c in sentence)
            {
                if (char.IsLetter(c))
                {
                    if (!start)
                    {
                        cs = new char[MAXWORDLEN];
                        //tmpStr = string.Empty;
                        start = true;
                    }
                    if (cindex < MAXWORDLEN)
                    {
                        cs.SetValue(c, cindex);
                        cindex++;
                    }
                    else
                    {
                        cs = null;
                        cindex = 0;
                        start = false;
                    }
                    //tmpStr = string.Concat(tmpStr, c);
                    continue;
                }
                if (start && cindex == 1 && c == '.')
                {
                    cs.SetValue(c, cindex);
                    cindex++;
                    //tmpStr = string.Concat(tmpStr, c);
                    continue;
                }
                if (cindex > 0)
                {
                    Array.Resize<char>(ref cs, cindex);
                    dicWords.Add(index, new string(cs));
                    cindex = 0;
                    index++;
                }
                start = false;
            }
            if (start)
            {
                Array.Resize<char>(ref cs, cindex);
                dicWords.Add(index, new string(cs));
                start = false;
                cs = null;
            }

            return dicWords;
        }