光脚丫学LINQ(032)：探究AssociationAttribute.Storage

视频演示：http://u.115.com/file/f220e439a4 

演示重点
此演示用来专门探究AssociationAttribute.Storage属性，包含了如下的内容：
如果不设置此属性值将会引发【未将对象引用设置到对象实例】的异常信息。不过，这只是针对实体类的属性而言，如果是使用字段来建立映射关系的话，那么可以不设置此属性值。
介绍此属性的作用。
此属性值是区分大小写的，即使是像VB这样不区分大小写的编程语言也是如此。
设置的属性值必须是一个私有字段，公有字段会发生异常。
并且所设置的必须是一个存在的私有字段，否则就算编译通过了，获取数据的时候也会出错。

演示代码下面是建立映射的两个实体类的代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq.Mapping;
using System.Data.Linq;

namespace Demo06Ex03
{
    [Table(Name = "Customers")]
    public class Customer
    {
        [Column(Name = "CustomerID", IsPrimaryKey = true)]
        public string CustomerID;

        [Column(Name = "ContactName")]
        public string ContactName;

        [Column(Name = "Phone")]
        public string Phone;

        private EntitySet<Order> _Orders;
        //private EntitySet<Order> _orders;
        [Association(Storage = "_Orders", ThisKey = "CustomerID", OtherKey = "CustomerID")]
        public EntitySet<Order> Orders
        {
            get { return this._Orders; }
            set { this._Orders.Assign(value); }
        }
    }
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq.Mapping;
using System.Data.Linq;

namespace Demo06Ex03
{
    [Table(Name = "Orders")]
    public class Order
    {
        [Column(Name = "OrderID", IsPrimaryKey = true)]
        public int OrderID;

        [Column(Name = "CustomerID")]
        public string CustomerID;

        [Column(Name = "OrderDate")]
        public DateTime OrderDate;

        [Column(Name = "Freight")]
        public decimal Freight;

        private EntityRef<Customer> _Customer;
        [Association(Storage = "_Customer", ThisKey = "CustomerID", OtherKey = "CustomerID")]
        public Customer Customer
        {
            get { return this._Customer.Entity; }
            set { this._Customer.Entity = value; }
        }
    }
}

下面是创建的自定义数据上下文的代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;

namespace Demo06Ex03
{
    public class NorthwindDataContext : DataContext
    {
        public Table<Customer> Customers
        {
            get
            {
                return this.GetTable<Customer>();
            }
        }

        public Table<Order> Orders
        {
            get
            {
                return this.GetTable<Order>();
            }
        }

        public NorthwindDataContext(string ConnectionString)
            : base(ConnectionString)
        {
        }
    }
}

下面是通过客户对象获取此客户的所有订单对象的测试代码。

// *************************************************
// 通过客户对象获取客户的所有订单记录。
// *************************************************
string DatabaseFileName = @"C:/LINQ/Northwind.mdf";
NorthwindDataContext db = new NorthwindDataContext(DatabaseFileName);
db.Log = Console.Out;

var AllCustomers = from CustomerObject in db.Customers
                   select CustomerObject;

foreach (var CustomerObject in AllCustomers)
{
    Console.WriteLine("---------------------");
    Console.WriteLine("Customer ID : {0}", CustomerObject.CustomerID);
    Console.WriteLine("Customer Name : {0}", CustomerObject.ContactName);
    Console.WriteLine("Phone : {0}", CustomerObject.Phone);

    Thread.Sleep(1000);

    var CustomerOrders = CustomerObject.Orders;

    foreach (var OrderObject in CustomerOrders)
    {
        Console.WriteLine("OrderID={0}, OrderDate={1}, Freight={2}",
            OrderObject.OrderID,
            OrderObject.OrderDate,
            OrderObject.Freight);
    }

    Thread.Sleep(2000);
}

下面的代码则是通过订单对象获取下此订单的客户对象。

// *************************************************
// 通过订单对象获取下订单的客户对象。
// *************************************************
NorthwindDataContext db = new NorthwindDataContext(@"C:/LINQ/Northwind.mdf");
var AllOrders = from OrderObject in db.Orders
                select OrderObject;
foreach (var OrderObject in AllOrders)
{
    Console.WriteLine("---------------------");
    Console.WriteLine("OrderID={0}, OrderDate={1}, Freight={2}",
        OrderObject.OrderID,
        OrderObject.OrderDate,
        OrderObject.Freight);

    Thread.Sleep(1000);

    var OrderCustomer = OrderObject.Customer;
    Console.WriteLine("Customer ID : {0}", OrderCustomer.CustomerID);
    Console.WriteLine("Customer Name : {0}", OrderCustomer.ContactName);
    Console.WriteLine("Phone : {0}", OrderCustomer.Phone);

    Thread.Sleep(2000);
}

以上的代码已经和视频演示文件打包在一起了。