using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var people = new[]

        {

            new

            {

                Name = "Vernon",

                Gender = "Male",

                City = "London",

            },

                new

            {

                Name = "Carrie",

                Gender = "Female",

                City = "London"

            },

                new

            {

                Name = "Joanna",

                Gender = "Female",

                City = "London"

            },

                new

            {

                Name = "Thomas",

                Gender = "Male",

                City = "Paris"

            }

        };

        ILookup<string, string> lookup = people.ToLookup(arg => arg.City, arg => arg.Name);

        IEnumerable<string> peopleInLondon = lookup["London"];

        IEnumerable<string> peopleInParis = lookup["Paris"];

        Console.WriteLine("People in London");

        foreach (var person in peopleInLondon)

        {

            Console.WriteLine(person);

        }

        Console.WriteLine();

        Console.WriteLine("People in Paris");

        foreach (var person in peopleInParis)

        {

            Console.WriteLine(person);

        }

    }

}