using System;

using System.Linq;

using AutoMapper;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var config = new MapperConfiguration(cfg => {

            cfg.CreateMap<FromClass, ToClass>()

                .ForMember(i=>i.Foreigns, o=>o.MapFrom((s,to, tp, ctx)=>{

                    tp.Add((LocalId:to?.Id??0,ForeignIds:101)); // here, let's suppose we manipulate the target collection to have it synchronized with the source one

                    return tp.ToList(); // This works

                    // The following doesn't work whereas that is what I want: manipulating the collection without reissuing another one

                    // return tp; 

                }));

        });

        var mapper=config.CreateMapper();

        var to=new ToClass{ Foreigns=new []{(LocalId:1, ForeignIds:100)}.ToList() };

        mapper.Map(new FromClass{Foreigns=new int[]{1,2,3}.ToList()}, to);

        Console.WriteLine(to.Foreigns.Count());

    }

}

public class ToClass

{

    public int Id {get;set;}

     public List<(int LocalId, int ForeignId)> Foreigns {get;set;}

}

public class FromClass

{

    public List<int> Foreigns{get;set;}

}