C#生成隨機字符串

///<summary>
    ///生成隨機字符串 
    ///</summary>
    ///<param name="length">目標字符串的長度</param>
    ///<param name="useNum">是否包含數字，1=包含，默認為包含</param>
    ///<param name="useLow">是否包含小寫字母，1=包含，默認為包含</param>
    ///<param name="useUpp">是否包含大寫字母，1=包含，默認為包含</param>
    ///<param name="useSpe">是否包含特殊字符，1=包含，默認為不包含</param>
    ///<param name="custom">要包含的自定義字符，直接輸入要包含的字符列表</param>
    ///<returns>指定長度的隨機字符串</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;
    }