using System;

using System.IO;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

​

public class Program

{

    public static void Main()

    {

        var data = new List<UserData>{

            new UserData{ UserId = 1, RoleId = 1, FirstName = "Ted"},

            new UserData{ UserId = 1, RoleId = 2, FirstName = "Ted"},

            new UserData{ UserId = 1, RoleId = 3, FirstName = "Ted"},

            new UserData{ UserId = 2, RoleId = 1, FirstName = "Ted"},

            new UserData{ UserId = 2, RoleId = 3, FirstName = "Ted"},

        };

        //How to do this better?

        var formattedData = data.GroupBy(x=>x.UserId)

            .Select(x=> new {

                UserId = x.FirstOrDefault().UserId,

                FirstName = x.FirstOrDefault().FirstName,

                Roles = x.Select(k=>k.RoleId)

            });

        //Ouptut

        var json = JsonConvert.SerializeObject(formattedData, Formatting.Indented);

        Console.WriteLine(json);

    }

    public class UserData{

        public int UserId{get;set;}

        public int RoleId{get;set;}

        public string FirstName{get;set;}

    }

​

}