using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class Country

    {

        public string Name { get; set; }

        public List<City> Cities { get; set; }

    }

    public class City

    {

        public string Name { get; set; }

    }

    public static void Main()

    {

        var items = new List<Country>()

        {

            new Country() { Name = "Austria", Cities = new List<City>() { new City() { Name = "Vienna" }, new City() { Name = "Linz" } } },

            new Country() { Name = "Australia", Cities = new List<City>() { new City() { Name = "Sydney" }, new City() { Name = "Melbourne" } } },

            new Country() { Name = "USA", Cities = new List<City>() { new City() { Name = "Austin" }, new City() { Name = "New York" } } }

        };

        var searchTerm = "aus";

        foreach (var country in items)

        {

            Console.WriteLine(country.Name);

            foreach (var city in country.Cities)

            {

                Console.WriteLine("    " + city.Name);

            }

        }

        Console.WriteLine("\r\n****FILTERED:\r\n");

        var filteredItems = items.Where(i => i.Name.Contains(searchTerm, StringComparison.OrdinalIgnoreCase) ||

                                             i.Cities.Any(c => c.Name.Contains(searchTerm, StringComparison.OrdinalIgnoreCase)))

                                .Select(i => new Country()

                                        {

                                            Name = i.Name,

                                            Cities = i.Cities.Where(c => c.Name.Contains(searchTerm, StringComparison.OrdinalIgnoreCase) ||

                                                                         i.Name.Contains(searchTerm, StringComparison.OrdinalIgnoreCase)).ToList()

                                        })

                                .ToList();

        foreach (var country in filteredItems)

        {

            Console.WriteLine(country.Name);

            foreach (var city in country.Cities)

            {

                Console.WriteLine("    " + city.Name);

            }

        }

    }

}