C# 产生真随机数（RNGCryptoServiceProvider）
参考：http://www.cftea.com/c/2017/02/6808.asppublic static string GetRandomString(int length, bool useNum = false, bool useLow = false, bool useUpp = false, bool useSpe = false) { RNGCryptoServiceProvider rng = new RNGCryptoServiceProv
