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

时间：2010-11-23 来源：光脚丫思考

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。
设置的属性值应该是一个私有字段，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。