using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var customers = new[] {

            new Customer {

                Invoices = new[] {

                    new Invoice {Id=1},

                    new Invoice {Id=2},

                }

            },

            new Customer {

                Invoices = new[] {

                    new Invoice {Id=3},

                    new Invoice {Id=4},

                }

            },

            new Customer {

                Invoices = new[] {

                    new Invoice {Id=5},

                    new Invoice {Id=6},

                }

            }

        };

        var allInvoicesFromAllCustomers = customers.SelectMany(c => c.Invoices);

        Console.WriteLine(string.Join(",", allInvoicesFromAllCustomers.Select(i => i.Id).ToArray()));

    }

}

​

class Invoice

{

    public int Id { get; set; }

}

​

class Customer

{

    public Invoice[] Invoices {get;set;}

}

​

​

​

​