Snippet for - https://stackoverflow.com/questions/47407574/

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ListListTest
{
    class Program
    {
        static void Main(string[] args)
        {
            List<MainCat> AllMainCat = new List<MainCat>();
            List<subcat> Allsubcat = new List<subcat>();

            subsubcat ss11 = new subsubcat { id = "ss11", name = "ss11" };
            subsubcat ss21 = new subsubcat { id = "ss21", name = "ss21" };
            subsubcat ss31 = new subsubcat { id = "ss31", name = "ss31" };
            List<subsubcat> ss1List = new List<subsubcat>();
            ss1List.Add(ss11);
            ss1List.Add(ss21);
            ss1List.Add(ss31);

            subsubcat ss12 = new subsubcat { id = "ss12", name = "ss12" };
            subsubcat ss22 = new subsubcat { id = "ss22", name = "ss22" };
            subsubcat ss32 = new subsubcat { id = "ss32", name = "ss32" };
            List<subsubcat> ss2List = new List<subsubcat>();
            ss2List.Add(ss12);
            ss2List.Add(ss22);
            ss2List.Add(ss32);

            subsubcat ss13 = new subsubcat { id = "ss13", name = "ss13" };
            subsubcat ss23 = new subsubcat { id = "ss23", name = "ss23" };
            subsubcat ss33 = new subsubcat { id = "ss33", name = "ss33" };
            List<subsubcat> ss3List = new List<subsubcat>();
            ss3List.Add(ss13);
            ss3List.Add(ss23);
            ss3List.Add(ss33);

            subcat s1 = new subcat { id = "s1", name = "s1", subsubcat = ss1List };
            subcat s2 = new subcat { id = "s2", name = "s2", subsubcat = ss2List };
            subcat s3 = new subcat { id = "s3", name = "s3", subsubcat = ss3List };
            List<subcat> s1List = new List<subcat>();
            s1List.Add(s1);
            s1List.Add(s2);
            List<subcat> s2List = new List<subcat>();
            s1List.Add(s2);
            s1List.Add(s3);


            MainCat m1 = new MainCat { id = "m1", name = "m1", subcat = s1List };
            MainCat m2 = new MainCat { id = "m2", name = "m2", subcat = s2List };


            Console.WriteLine(string.Format("m1 can be found in {0}", Find("m1", m1)));
            Console.WriteLine(string.Format("s2 can be found in {0}", Find("s2", m1)));
            Console.WriteLine(string.Format("ss33 can be found in {0}",Find("ss33", m1)));
            Console.WriteLine(string.Format("ss22 can be found in {0}", Find("ss22", m1)));
            Console.WriteLine(string.Format("dummy can be found in {0}", Find("dummy", m1)));

            Console.ReadKey();
        }

        public static System.Type Find(string id, MainCat m)
        {
            if (m.id.Equals(id))
            {
                return m.GetType();
            }
            else if (m.subcat.Any(a => a.id.Equals(id)))
            {
                return typeof(subcat);
            }
            else if (m.subcat.Any(a => a.subsubcat.Any(b => b.id.Equals(id))))
            {
                return typeof(subsubcat);
            }
            return null;
        }
    }


    public class MainCat
    {
        public string id { get; set; }
        public string name { get; set; }
        public List<subcat> subcat { get; set; }
    }

    public class subcat
    {
        public string id { get; set; }
        public string name { get; set; }
        public List<subsubcat> subsubcat { get; set; }
    }

    public class subsubcat
    {
        public string id { get; set; }
        public string name { get; set; }
    }
}