using System;

using System.Linq;

using System.Collections.Generic;

class Person

{ 

    public int PersonId; 

    public string car  ; 

}

​

class Result

{ 

   public int PersonId;

   public List<string> Cars; 

}

​

public class Program

{

    public static void Main()

    {

        List<Person> persons = new List<Person>()

        {

            new Person { PersonId = 1, car = "Ferrari" },

            new Person { PersonId = 1, car = "BMW" },

            new Person { PersonId = 2, car = "Audi"}

        };

​

        //With Query Syntax

        List<Result> results1 = (

            from p in persons

            group p by p.PersonId into g

            select new Result()

                {

                    PersonId = g.Key, 

                    Cars = g.Select(c => c.car).ToList()

                }

            ).ToList();

        foreach (Result item in results1)

        {

            Console.WriteLine(item.PersonId);

            foreach(string car in item.Cars)

            {

                Console.WriteLine(car);

            }

        }

        Console.WriteLine("-----------");

        //Method Syntax

        List<Result> results2 = persons

            .GroupBy(p => p.PersonId, 

                     (k, c) => new Result()

                             {

                                 PersonId = k,

                                 Cars = c.Select(cs => cs.car).ToList()

                             }

                    ).ToList();

        foreach (Result item in results2)

        {

            Console.WriteLine(item.PersonId);

            foreach(string car in item.Cars)

            {

                Console.WriteLine(car);

            }

        }

    }

}