using System;

using Newtonsoft.Json;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class GetCreditCardListResponseCache

    {

        public List<CreditCardListResponseCache> Families { get; set; }

    }

​

    public class CreditCardListResponseCache

    {

        public List<CreditCardListResponse2Cache> CreditCards { get; set; }

    }

​

    public class CreditCardListResponse2Cache

    {

        public string AccountIndex { get; set; }

        public string CardNumber { get; set; }

    }

    public static void Main()

    {

        var cacheCreditCardList = JsonConvert.DeserializeObject<GetCreditCardListResponseCache>("{\"Families\":[{\"CreditCards\":[{\"AccountIndex\":\"5\",\"CardNumber\":\"4730148900110111\"},{\"AccountIndex\":\"6\",\"CardNumber\":\"4730148900110210\"}]},{\"CreditCards\":[{\"AccountIndex\":\"7\",\"CardNumber\":\"5444828900066116\"},{\"AccountIndex\":\"8\",\"CardNumber\":\"5444828900066215\"}]}]}");

        var creditCard = cacheCreditCardList.Families.SelectMany(a => a.CreditCards).Where(b => b.AccountIndex == "8").FirstOrDefault();

        var logdata = JsonConvert.SerializeObject(creditCard);

        Console.WriteLine(logdata);

    }

}