Linq to SQL设计器创建的类的属性缺少Add()方法

求助：Linq to SQL设计器创建的类的属性缺少Add()方法
我用Linq to SQL设计器创建的类的属性缺少Add()方法，请高手帮忙看看代码，红色是错误的地方，是按书上打的。

dc.Customers.Add(douglas);

C# code

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Data.Linq;using System.Data.Linq.Mapping;namespace Modifying_Data_Using_Linq_To_SQL{    //主程序    public class Tester    {        static void Main()        {            AddCustomer();            UpdateCustomer();            Console.ReadKey();        }                private static void AddCustomer()        {            Console.WriteLine("Adding a new customer...");            MyNewItemDataContext dc = new MyNewItemDataContext();            //加入带有地址的新客户            Customer douglas = new Customer();            douglas.FirstName = "Douglas";            douglas.LastName = "Adams";            douglas.EmailAddress = "douglas0@adventureworks.com";            douglas.PasswordHash = "fake";            douglas.PasswordSalt = "fake";            douglas.ModifiedDate = DateTime.Today;            douglas.rowguid = Guid.NewGuid();            Address addr = new Address();            addr.AddressLine1 = "1c sharp Way";            addr.City = "Seattle";            addr.PostalCode = "98011";            addr.StateProvince = "Washington";            addr.CountryRegion = "United States";            addr.ModifiedDate = DateTime.Today;            addr.rowguid = Guid.NewGuid();            CustomerAddress ca = new CustomerAddress();            ca.AddressType = "Main Office";            ca.Address = addr;            ca.Customer = douglas;            ca.ModifiedDate = DateTime.Today;            ca.rowguid = Guid.NewGuid();                        dc.Customers.Add(douglas);            dc.SubmitChanges();            ShowCustomersByFirstName("Douglas");        }                //更新客户记录        private static void UpdateCustomer()        {            Console.WriteLine("Updating a customer...");            MyNewItemDataContext dc = new MyNewItemDataContext();            Customer dAdams = dc.Customers.Single(                c => (c.FirstName == "Douglas" && c.LastName == "Adams"));            Console.WriteLine("Before:\n{0}", dAdams);            dAdams.Title = "Mr.";            //加一个新的发货地址            Address addr = new Address();            addr.AddressLine1 = "1 Warehouse Place";            addr.City = "Los Angeles";            addr.PostalCode = "30210";            addr.StateProvince = "California";            addr.CountryRegion = "United States";            addr.ModifiedDate = DateTime.Today;            addr.rowguid = Guid.NewGuid();            CustomerAddress ca = new CustomerAddress();            ca.AddressType = "Shipping";            ca.Address = addr;            ca.Customer = dAdams;            ca.ModifiedDate = DateTime.Today;            ca.rowguid = Guid.NewGuid();            dc.SubmitChanges();            Customer dAdams1 = dc.Customers.Single(                c => (c.FirstName == "Douglas" && c.LastName == "Adams"));            Console.WriteLine("After:\n{0}", dAdams);        }        //查找带有制定名字的客户记录列表        private static void ShowCustomersByFirstName(string firstname)        {            MyNewItemDataContext dc = new MyNewItemDataContext();            var customers =                from customer in dc.Customers                where customer.FirstName == "Douglas"                orderby customer.FirstName, customer.LastName                select customer;            Console.WriteLine("Customers whose first name is {0}:", firstname);            foreach (Customer customer in customers)                Console.WriteLine(customer);        }    }    //对创建的客户类加入一个方法来格式化显示客户属性    public partial class Customer    {        public override string ToString()        {            StringBuilder sb = new StringBuilder();            sb.AppendFormat("{0} {1} {2} {3}", Title, FirstName, LastName, EmailAddress);            foreach (CustomerAddress ca in CustomerAddresses)            {                sb.AppendFormat("\n\t{0}: {1}, {2}",                    ca.AddressType, ca.Address.AddressLine1, ca.Address.City);            }            sb.AppendLine();            return sb.ToString();        }    }}

错误提示：“System.Data.Linq.Table<Modifying_Data_Using_Linq_To_SQL.Customer>”不包含“Add”的定义，并且找不到可接受类型为“System.Data.Linq.Table<Modifying_Data_Using_Linq_To_SQL.Customer>”的第一个参数的扩展方法“Add”(是否缺少 using 指令或程序集引用?)

[解决办法]
dc.Customers.Add(douglas);
这句里面的Customers表，
和douglas（Customer ）是一致的吗？？？？
[解决办法]
用 dc.Customers.InsertOnSubmit（douglas）方法

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