using System;

using System.Collections.Generic;

using System.Linq;

 public class Program {

        public static void Main(string[] args) {

            var data = new List<A>();

​

            var a1 = new A() {Name = "first", B = new List<B>()};

            var b10 = new B() {a_pk = 10, system_date_time = DateTime.Now.AddHours(3), A = a1};

            var b11 = new B() {a_pk = 11, system_date_time = DateTime.Now.AddHours(3), A = a1};

            var b12 = new B() {a_pk = 12, system_date_time = DateTime.Now.AddHours(3), A = a1};

            a1.B.Add(b10);

            a1.B.Add(b11);

            a1.B.Add(b12);

            data.Add(a1);

​

            var a2 = new A() {Name = "second", B = new List<B>()};

            var b20 = new B() {a_pk = 20, system_date_time = DateTime.Now.AddHours(2), A = a2};

            var b21 = new B() {a_pk = 21, system_date_time = DateTime.Now.AddHours(2), A = a2};

            var b22 = new B() {a_pk = 22, system_date_time = DateTime.Now.AddHours(2), A = a2};

            a2.B.Add(b20);

            a2.B.Add(b21);

            a2.B.Add(b22);

            data.Add(a2);

​

            var a3 = new A() {Name = "third", B = new List<B>()};

            var b30 = new B() {a_pk = 30, system_date_time = DateTime.Now.AddHours(1), A = a3};

            var b31 = new B() {a_pk = 31, system_date_time = DateTime.Now.AddHours(1), A = a3};

            var b32 = new B() {a_pk = 32, system_date_time = DateTime.Now.AddHours(1), A = a3};

            a3.B.Add(b30);

            a3.B.Add(b31);

            a3.B.Add(b32);

            data.Add(a3);

​

            var allBs = data.SelectMany(x => x.B).ToList().OrderByDescending(x => x.system_date_time);

​

            foreach (var b in allBs) {

                Console.WriteLine(b.system_date_time + ":" + b.A.Name + " " + b.a_pk);

            }

​

            Console.ReadLine();

        }

    }

​

    public partial class A {

        public A() {

            this.B = new HashSet<B>();

        }

​

        public virtual ICollection<B> B { get; set; }

​

        public virtual string Name { get; set; }

    }

​

    public partial class B {

        public Nullable<long> a_pk { get; set; }

​

        public Nullable<System.DateTime> system_date_time { get; set; }

​

        public virtual A A { get; set; }

    }