EfProjectionSubEntities.cs

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;

namespace EfSelectIssue
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            using (var ctx = new MyContext())
            {
                var anEntity = new A();
                ctx.As.Add(anEntity);

                ctx.Bs.Add(new B() { A = anEntity, Name = "1" });
                ctx.Bs.Add(new B() { A = anEntity, Name = "2" });
                ctx.Bs.Add(new B() { A = anEntity, Name = "3" });
                ctx.SaveChanges();

                foreach (var thing in ctx.As

                .Select(a => new
                {
                    AId = a.Id,
                    BNames = a.Bs.Select(x => x.Name)
                }))
                {
                    Console.WriteLine(thing.AId + ":" + String.Join(", ", thing.BNames));
                }
            }
        }
    }
    public class MyContext : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"Server=.\;Database=EFCoreDemo;Trusted_Connection=True;MultipleActiveResultSets=true");
        }
        public DbSet<A> As { get; set; }
        public DbSet<B> Bs { get; set; }
    }
    public class A
    {
        public int Id { get; set; }
        public ICollection<B> Bs { get; set; }
    }

    public class B
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public A A { get; set; }
    }
}