c#实现随机字符串(不包含数字)

class RandomString
    {      
        Random rnd = new Random();
        public char getRandomChar()
        {
            int ret = rnd.Next(122);
            while(ret<65||(ret>90&&ret<97)||ret>122)
            {
                ret = rnd.Next(122);
            }
            return (char)ret;
        }
        public string getRandomString(int length)
        {
            StringBuilder sb = new StringBuilder(length);
            for (int i = 0; i < length; i++)
            {
                sb.Append(getRandomChar());
            }
            return sb.ToString();
        }
    }