using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

namespace ConsoleApp4

{

    class Program

    {

        static void Main(string[] args)

        {

            int fleetId = 2;

​

            var a = FindFleetId(TestA(), fleetId);

            foreach (var x in a)

                Console.WriteLine(x.Item1.ToString() + " | " + x.Item2.ToString());

​

            var b = FindFleetId(TestB(), fleetId);

            foreach (var x in b)

                Console.WriteLine(x.Item1.ToString() + " | " + x.Item2.ToString());

​

            var c = FindFleetId(TestC(), fleetId);

            foreach (var x in c)

                Console.WriteLine(x.Item1.ToString() + " | " + x.Item2.ToString());

​

            var d = FindFleetId(TestD(), fleetId);

            foreach (var x in d)

                Console.WriteLine(x.Item1.ToString() + " | " + x.Item2.ToString());

        }

​

        private static IEnumerable<(object, object)> FindFleetId(object rootObj, int fleetId)

        {

            if (rootObj == null)

                Enumerable.Empty<(object, object)>();

​

            return Parse(rootObj, rootObj, fleetId);

        }

        private static IEnumerable<(object, object)> Parse(object rootObj, object currentObj, int fleetId)

        {

            if (currentObj == null)

                yield break;

​

            var type = currentObj.GetType();

            var enumerable = currentObj as IEnumerable;

            if (enumerable != null && type != typeof(string))

            {

                foreach (var item in enumerable)

                {

                    var newRootObj = object.ReferenceEquals(rootObj, currentObj) ? item : rootObj;

                    foreach (var nestedObj in Parse(newRootObj, item, fleetId))

                        yield return nestedObj;

                }

​

                yield break;

            }

​

            var properties = type.GetProperties();

            foreach (var propertyInfo in properties)

            {

                var propValue = propertyInfo.GetValue(currentObj);

​

                if (propertyInfo.Name == "FleetId")

                {

                    if (propValue is int fId && fId == fleetId)

                        yield return (rootObj, currentObj);

​

                    break;

                }

​

                foreach (var nestedObj in Parse(rootObj, propValue, fleetId))

                    yield return nestedObj;    

            }

        }

​

        private static A1[] TestA()

        {

            return new[]

            {

                new A1() {A2 = new A2() {A3 = new A3() {FleetId = 1}}},

                new A1() {A2 = new A2() {A3 = new A3() {FleetId = 2}}},

            };

        }

        private static B1[] TestB()

        {

            return new[]

            {

                new B1() {FleetId = 1},

                new B1() {FleetId = 2},

            };

        }

        private static C1[] TestC()

        {

            return new[]

            {

                new C1() { C2 = new List<C2>() { new C2() { C3 = new C3(){FleetId = 1}}}},

                new C1() { C2 = new List<C2>() { new C2() { C3 = new C3(){FleetId = 2}}}},

            };

        }

        private static D1[] TestD()

        {

            return new[]

            {

                new D1() { D2 = new List<D2>() {new D2(){FleetId = 1}}},

                new D1() { D2 = new List<D2>() {new D2(){FleetId = 2}}},

            };

        }

    }

​

    public class A1

    {

        public A2 A2 { get; set; }

        public override string ToString()

        {

            return "A1." + A2;

        }

    }

    public class A2

    {

        public A3 A3 { get; set; }

        public override string ToString()

        {

            return "A2." + A3;

        }

    }

    public class A3

    {

        public int FleetId { get; set; }

        public override string ToString()

        {

            return "A3." + FleetId;

        }

    }

​

    public class B1

    {

        public int FleetId { get; set; }

        public override string ToString()

        {

            return "B1." + FleetId;

        }

    }

​

    public class C1

    {

        public List<C2> C2 { get; set; }

        public override string ToString()

        {

            return "C1." + string.Join("|", C2);

        }

    }

    public class C2

    {

        public C3 C3 { get; set; }

        public override string ToString()

        {

            return "C2." + C3;

        }

    }

    public class C3

    {

        public int FleetId { get; set; }

        public override string ToString()

        {

            return "C3." + FleetId;

        }

    }

​

    public class D1

    {

        public List<D2> D2 { get; set; }

        public override string ToString()

        {

            return "D1." + string.Join("|", D2);

        }

    }

    public class D2

    {

        public int FleetId { get; set; }

        public override string ToString()

        {

            return "D2." + FleetId;

        }

    }

}

​