using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Globalization;

using System.Xml.Serialization;

using System.IO;

public class Program

{

    public static void Main()

    {

         var lstA = new List<A>();

​

    var a1 = new A(){

                     listB = {  new B(){

                                 id = 1 

                                },

                                new B(){

                                 id = 2

                                }

                              }

                    };

    lstA.Add(a1);

    var a2 = new A(){

                     listB = {  new B(){

                                 id = 3 

                                },

                                new B(){

                                 id = 4

                                }

                              }

                    };

    lstA.Add(a2);

        lstA.SelectMany(x=>x.listB.Select(y=>y.id)).Dump();

    }

}

​

class A{

    public A()

    {

        listB = new List<B>();

    }

public List<B> listB {get;set;}

​

}

​

class B {

public int id {get;set;}

}

​