using System;

using System.Collections.Generic;

​

public class Program

{

    private class Customer

    {

        public int Id;

        public string Name;

        public int Age;

        public string City;

    }

    private class Policy

    {

        public int Id;

        public int Customer;

        public double Premium;

        public string City;

    }

    public static void Main()

    {

        var customers = new List<Customer>()

        {

            new Customer() { Id = 1, Name = "Abel", Age = 41, City = "Madrid" },

            new Customer() { Id = 2, Name = "Andres", Age = 22, City = "London" },

            new Customer() { Id = 3, Name = "Alice", Age = 38, City = "Lisbon" },

            new Customer() { Id = 4, Name = "Ana", Age = 61, City = "Berlin" },

            new Customer() { Id = 1, Name = "Billy", Age = 35, City = "Rome" },

            new Customer() { Id = 1, Name = "Lock", Age = 25, City = "Rome" },

        };

        var policies = new List<Policy>()

        {

            new Policy() { Id = 50, Customer = 1, Premium = 120.0, City = "Madrid" },

            new Policy() { Id = 500, Customer = 2, Premium = 20.0, City = "London" },

            new Policy() { Id = 5000, Customer = 3, Premium = 100.0, City = "Lisbon" },

            new Policy() { Id = 50000, Customer = 1, Premium = 10.0, City = "Madrid" },

            new Policy() { Id = 40000, Customer = 5, Premium = 1200.0, City = "Athenas" },

            new Policy() { Id = 80000, Customer = 1, Premium = 1000.0, City = "Madrid" },

        };

        var dictionary = new Dictionary<string, List<int>>();

        int countCustomers = customers.Count;

        for(int i = 0; i < countCustomers; i++)

        {

            List<int> records = null;

            var item = customers[i];

            string key = string.Concat(item.Id.ToString(), item.City);

            if(!dictionary.TryGetValue(key, out records))

            {

                records = new List<int>();

                dictionary[key] = records;

            }

            records.Add(i);

        }

        foreach (var item in dictionary)

        {

            Console.WriteLine(item.Key + "\t\t" + string.Join(", ", item.Value));

        }

    }

}