生成随机字符串

        /// <summary>
        /// 生成随机字符串
        /// </summary>
        /// <param name="length">目标字符串的长度</param>
        /// <param name="useNum">是否包含数字，true=包含</param>
        /// <param name="useLow">是否包含小写字母，true=包含</param>
        /// <param name="useUpp">是否包含大写字母，true=包含</param>
        /// <param name="useSpe">是否包含特殊字符，true=包含</param>
        /// <param name="custom">要包含的自定义字符，直接输入要包含的字符列表</param>
        /// <returns>指定长度的随机字符串</returns>
        internal static string GetRnd(int length, bool useNum, bool useLow, bool useUpp, bool useSpe, string custom)
        {
            byte[] b = new byte[4];
            new RNGCryptoServiceProvider().GetBytes(b);
            Random r = new Random(BitConverter.ToInt32(b, 0));
            string s = null, str = custom;

            if (useNum == true) { str += "0123456789"; }
            if (useLow == true) { str += "abcdefghijklmnopqrstuvwxyz"; }
            if (useUpp == true) { str += "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; }
            if (useSpe == true) { str += "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"; }

            for (int i = 0; i < length; i++) { s += str.Substring(r.Next(0, str.Length - 1), 1); }

            return s;
        }