如何用LINQ to SQL制作数据库对象

MSDN解释如下

// Northwnd inherits from System.Data.Linq.DataContext.
Northwnd nw = new Northwnd(@"northwnd.mdf");
// or, if you are not using SQL Server Express
// Northwnd nw = new Northwnd("Database=Northwind;Server=server_name;Integrated Security=SSPI");
var companyNameQuery =
    from cust in nw.Customers
    where cust.City == "London"
    select cust.CompanyName;
foreach (var customer in companyNameQuery)
{
    Console.WriteLine(customer);
}

这里我的数据库名称是Madu，表是Student，包含姓名，课程和地址。我的代码在

下面

Madu md = new Madu("Database=Madu;Server=Madu-DV6080;Integrated Security=SSPI");
var courseQuery =from stu in md.Student where stu.Address == "London" select stu.Name;
foreach (var stu in courseQuery)
{
    Console.WriteLine(stu);
}

我添加了"System.Data."Linq"引用也是如此。但是它给了我一个错误"类型或命名空间名称'Madu'找不到(你是否缺少using指令或程序集引用?)"

请帮帮我。提前谢谢…!

// Northwnd inherits from System.Data.Linq.DataContext.

using System.Data.Linq;
namespace Foo
{
    public class Mabu : DataContext
    {
        // Stuff
    }
}