C# 生成随机字符串

#region 5.0 生成随机字符串 + static string GetRandomString(int length, bool useNum, bool useLow, bool useUpp, bool useSpe, string custom)
        ///<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，1=包含，默认为包含</param>
        ///<param name="useLow">是否包含小写字母，1=包含，默认为包含</param>
        ///<param name="useUpp">是否包含大写字母，1=包含，默认为包含</param>
        ///<param name="useSpe">是否包含特殊字符，1=包含，默认为不包含</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>
        public static string GetRandomString(int length, bool useNum, bool useLow, bool useUpp, bool useSpe, string custom)
        {
            byte[] b = new byte[4];
            new System.Security.Cryptography.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;
        } 
        #endregion