using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

     public class Customer

     {

         public int customerId;

         public string customerName;

         public List<Order> orders;

     }

​

    public class Order

    {

        public int customerId;

        public int orderId;

        public string OrderName;

    }

    public static void Main()

    {

        List<Customer> _customers = new List<Customer>

        {

            new Customer{customerId = 1, customerName="1"},

            new Customer{customerId = 2, customerName="2"},

            new Customer{customerId = 3, customerName="3"},

        };

        List<Order> _orders = new List<Order>

        {

            new Order{customerId=1,orderId=1},

            new Order{customerId=1,orderId=2},

            new Order{customerId=2,orderId=1},

            new Order{customerId=2,orderId=2},

        };

​

        List<Customer> result = _customers.GroupJoin(_orders,

                                                     c => c.customerId,

                                                     o => o.customerId,

                                                     (c, o) => new Customer

                                                     {

                                                         customerId = c.customerId,

                                                         customerName = c.customerName,

                                                         orders = o.ToList()

                                                     }).ToList();

        result.Dump();

    }

}