using System;

using System.Collections.Generic;

using System.Linq;

using AutoMapper;

using Newtonsoft.Json;

​

​

public class Program

{

    public static void Main()

    {

        var flattenData = new List<CountyAndCity>

        {

            new CountyAndCity { CountyId = 1, CountyName = "Albany", CityId = 10, CityName = "Cohoes", CityPopulation = 1000 },

            new CountyAndCity { CountyId = 1, CountyName = "Albany", CityId = 20, CityName = "Watervliet", CityPopulation = 1000 },

            new CountyAndCity { CountyId = 2, CountyName = "Bronx", CityId = 30, CityName = "Wakefield", CityPopulation = 2000 },

            new CountyAndCity { CountyId = 2, CountyName = "Bronx", CityId = 40, CityName = "Eastchester", CityPopulation = 2000 },

        };

​

        var unflattenData = new List<County>

        {

            new County { Id = 1, Name = "Albany", Cities = new List<City>

            {

                new City { Id = 10, Name = "Cohoes", Population = 1000 },

                new City { Id = 20, Name = "Watervliet", Population = 1000 },

            }},

            new County { Id = 2, Name = "Bronx", Cities = new List<City>

            {

                new City { Id = 30, Name = "Wakefield", Population = 2000 },

                new City { Id = 40, Name = "Eastchester", Population = 2000 },

            }},

        };

​

        var mapperConfiguration = new MapperConfiguration(configuration => configuration.AddProfile(new AutoMapperProfile()));

        var mapper = mapperConfiguration.CreateMapper();

        State mappedData = mapper.Map<State>(flattenData);

        //Should be TRUE after some kind of custom comparison but it is goof enough for the example.

        bool result = Enumerable.SequenceEqual(mappedData.Counties, unflattenData);

        Console.WriteLine(result);

        Console.WriteLine(JsonConvert.SerializeObject(mappedData, Formatting.Indented));

    }

}

​

public class CountyAndCity

{

    public int CountyId { get; set; }

    public string CountyName { get; set; }

    public int CityId { get; set; }

    public string CityName { get; set; }

    public int CityPopulation { get; set; }

}

​

public class City

{

    public int Id { get; set; }

    public string Name { get; set; }

    public int Population { get; set; }

    public override bool Equals (Object obj)

    {

        if (obj is not City)

            return false;

        City b = (City)obj;

        return Id == b.Id

            && Name == b.Name

            && Population == b.Population;

    }

    public override int GetHashCode()

    {

        return this.Id.GetHashCode() ^ this.Name.GetHashCode() ^ this.Population.GetHashCode(); 

    }

}

​

public class County

{

    public int Id { get; set; }

    public string Name { get; set; }

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

    public override bool Equals (Object obj)

    {

        if (obj is not County)

            return false;

        County b = (County)obj;

        return Id == b.Id

            && Name == b.Name

            && Enumerable.SequenceEqual(Cities, b.Cities);

    }

    public override int GetHashCode()

    {

        return this.Id.GetHashCode() ^ this.Name.GetHashCode(); 

    }

}

​

public class State

{

    public List<County> Counties { get; set; }

}

​

public class AutoMapperProfile : Profile

{

    public AutoMapperProfile()

    {

        CreateMap<CountyAndCity, City>()

            .ForMember(destination => destination.Id, options => options.MapFrom(source => source.CityId))

            .ForMember(destination => destination.Name, options => options.MapFrom(source => source.CityName))

            .ForMember(destination => destination.Population, options => options.MapFrom(source => source.CityPopulation));

​

        //CreateMap<List<CountyAndCity>, County>()

       //     .ForMember(destination => destination.Id, options => options.MapFrom(source => source.FirstOrDefault().CountyId))

       //     .ForMember(destination => destination.Name, options => options.MapFrom(source => source.FirstOrDefault().CountyName))

       //     .ForMember(destination => destination.Cities, options => options.MapFrom(source => source));

        //CreateMap<CountyAndCity, County>()

        //    .ForMember(destination => destination.Id, options => options.MapFrom(source => source.CountyId))

        //    .ForMember(destination => destination.Name, options => options.MapFrom(source => source.CountyName))

        //    .ForMember(destination => destination.Cities, options => options.MapFrom(source => source));

​

        CreateMap<List<CountyAndCity>, State>()

            .ForMember(destination => destination.Counties, options => options.MapFrom<StateCountiesResolver>());

    }

}

​

public class StateCountiesResolver : IValueResolver<List<CountyAndCity>, State, List<County>>

{

    public List<County> Resolve(List<CountyAndCity> src, State dest, List<County> destMember, ResolutionContext ctx)

    {

        destMember = src.GroupBy(x => x.CountyId)

            .Select(x => new County

            {

                Id = x.Key,

                Name = x.FirstOrDefault()?.CountyName,

                Cities = ctx.Mapper.Map<List<City>>(x.ToList())

            })

            .ToList();

        return destMember;

    }

}