Eric 的随机文件名 生成方法

利用 当前时间与2000-1-1 0:0:0 相差的毫秒数转成36进制字符串 加上4位随机字符串 生成一个随机文件名. 算是长度比较短而唯一性比较高的随机文件名生成方法了

        private static string charDictionary = "0123456789abcdefghijklmnopqrstuvwxyz";

        public static string GetRandomString(int length)
        ...{
            return GetRandomString(new Random(), length);
        }

        public static string GetRandomString(string dictionary, int length)
        ...{
            return GetRandomString(new Random(), dictionary, length);
        }

        public static string GetRandomString(Random r, int length)
        ...{
            return GetRandomString(r, charDictionary, length);
        }

        public static string GetRandomString(Random r, string dictionary, int length)
        ...{
            if(r == null)
            ...{
                r = new Random();
            }

            StringBuilder sb = new StringBuilder();
            char[] chars = dictionary.ToCharArray();

            for(int i = 0; i < length; i++)
            ...{
                sb.Append(chars[r.Next(0, chars.Length)]);
            }

            return sb.ToString();
        }

        public static string GetRandomFileName()
        ...{
            return GetRandomFileName(new Random());
        }

        public static string GetRandomFileName(Random r)
        ...{
            string fileName = ConvertToRadix(GetTimeSpan("f"), 36);
            fileName += GetRandomString(r, 4);
            return fileName.ToLower();
        }

        internal static long GetTimeSpan(string dateTimePart)
        ...{
            long now = DateTime.Now.Ticks;
            long begin = DateTime.Parse("2000-1-1 0:0:0").Ticks;
            long timeSpan = now - begin;

            switch(dateTimePart.ToLower())
            ...{
                case "f":
                    timeSpan = timeSpan / TimeSpan.TicksPerMillisecond;
                    break;
                case "s":
                    timeSpan = timeSpan / TimeSpan.TicksPerSecond;
                    break;
                case "m":
                    timeSpan = timeSpan / TimeSpan.TicksPerMinute;
                    break;
                case "h":
                    timeSpan = timeSpan / TimeSpan.TicksPerHour;
                    break;
                case "d":
                    timeSpan = timeSpan / TimeSpan.TicksPerDay;
                    break;
                default:
                    timeSpan = 0;
                    break;
            }

            return timeSpan;
        }

        public static string ConvertToRadix(long number, byte scale)
        ...{
            if(scale < 2 || scale > 36)
            ...{
                throw new Exception("Scale number must not be less than 2 and bigger then 36.");
            }

            char[] dictionary = charDictionary.ToCharArray()
            List<char> charList = new List<char>();
            long positive = Math.Abs(number);

            for(int i = 0; i < 64; i++)
            ...{
                if(positive == 0)
                ...{
                    break;
                }

                charList.Add(dictionary[positive % scale]);
                positive /= scale;
            }

            charList.Reverse();
            return new string(charList.ToArray());
        }