我知道使用 using

orders Order.OrderId Customer.CustomerName

using System;
using System.Collections.Generic;
using System.Linq;
using TestExtn2343.Models;

namespace TestExtn2343
{
    class Program
    {
        public static void Main(string[] args)
        {

            var orders = GetOrders(10, 10);

            orders.ForEach(x =>
            {
                Customer customer = x.Customer;
                if (customer != null)
                {
                    //SUCCEEDS:
                    Console.WriteLine("{0}, {1}", x.OrderID);

                    //FAILS: "
                    Console.WriteLine("{0}, {1}", x.OrderID, x.Customer.ContactName.ToString());
                }
            });

            Console.ReadLine();
        }

        public static List<Order> GetOrders(int skip, int take)
        {
            using (MainDataContext db = new MainDataContext())
            {
                List<Order> orders = (from order in db.Orders
                                      select order).Skip(skip).Take(take).ToList();
                return orders;
            }
        }

    }
}

谢谢Adam,根据你的建议,我的代码是这样工作的:

public static void Main(string[] args)
{
    using (MainDataContext db = new MainDataContext())
    {
        GetOrders(db, 10, 10).ForEach(x => Console.WriteLine("{0}, {1}", x.OrderID, x.Customer.ContactName.ToString()));
    }
    Console.ReadLine();
}

public static List<Order> GetOrders(MainDataContext db, int skip, int take)
{
    List<Order> orders = (from order in db.Orders
                          select order).Skip(skip).Take(take).ToList();
    return orders;
}