using System;

using System.Collections.Generic;

using System.Linq;

public class ObjectA

{

    public int Id { get; set; }

    public List<ObjectB> ObjectBs { get; set; }

}

​

public class ObjectB

{

    public int Id { get; set; }

    public string Name { get; set; }

    public int ObjectAId { get; set; }

    public ObjectA ObjectA { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        ObjectA a1 = new()

        {

            Id = 1

        };

        a1.ObjectBs =

        [

            new ObjectB()

            {

                Id = 11,

                Name = "11",

                ObjectAId = 1,

                ObjectA = a1

            },

            new ObjectB()

            {

                Id = 12,

                Name = "12",

                ObjectAId = 1,

                ObjectA = a1

            }

        ];

        ObjectA a2 = new()

        {

            Id = 2

        };

        a2.ObjectBs =

        [

            new ObjectB()

            {

                Id = 21,

                Name = "21",

                ObjectAId = 2,

                ObjectA = a2

            },

            new ObjectB()

            {

                Id = 22,

                Name = "22",

                ObjectAId = 2,

                ObjectA = a2

            }

        ];

        List<ObjectA> As = [a1, a2];

        var result = As

                .SelectMany(a => 

                    a.ObjectBs.Select(b =>

                    new { b = b, a = a }))

                .ToList();

        foreach (var x in result)

        {

            Console.WriteLine($"{x.b.Id} {x.a.Id}");

        }

    }

}