Web.config配置代码如下：

 <!—当前调用的是SqlServer 的实现类 -->
<add key="WebDAL" value="PetShop.SQLServerDAL" />
    <add key="OrdersDAL" value="PetShop.SQLServerDAL" />

　　BLL层的Item类实现业务逻辑，代码如下：

Item item = new Item();        //调用BLL层的Item类
                itemsByProduct =  item.GetItemsByProduct(productId); //实现查询方法

　　BLL层的Item类实现业务逻辑，代码如下：

using System.Collections;
//References to PetShop specific libraries
//PetShop busines entity library
using PetShop.Model;
//PetShop DAL interfaces
using PetShop.IDAL;
namespace PetShop.BLL ...{
    /**//// <summary>
    /// A business component to manage product items
    /// </summary>
    public class Item ...{
        /**//// <summary>
        /// A method to list items by productId
        /// Every item is associated with a parent product
        /// </summary>
        /// <param name="productId">The productId to search by</param>
        /// <param name="productName">Name of the product associated with the product</param>
        /// <returns>An interface to an arraylist</returns>
        public IList GetItemsByProduct(string productId) ...{
            // Validate input
            if (productId.Trim() == string.Empty)
                return null;
            // 通过数据访问工厂类创建访问实例
            IItem dal = PetShop.DALFactory.Item.Create();
            // 实例实现了IDAL接口，调用实例实现查询
            return dal.GetItemsByProduct(productId);
        }
    }
}

　　数据工厂类根据web.config中的配置创建实例，代码如下：

using System;
using System.Reflection;
using System.Configuration;
namespace PetShop.DALFactory ...{
    /**//// <summary>
    /// Factory implementaion for the Item DAL object
    /// </summary>
    public class Item ...{
        public static PetShop.IDAL.IItem Create() ...{
            /**////查找系统设置
            string path = System.Configuration.ConfigurationSettings.AppSettings["WebDAL"];
            string className = path + ".Item";
            // 通过config文件中给定的前缀创建实例
            return (PetShop.IDAL.IItem)Assembly.Load(path).CreateInstance(className);
        }
    }
}

　　IDAL层IItem 定义了数据访问接口，代码如下：

using System.Collections;
//References to PetShop specific libraries
//PetShop busines entity library
using PetShop.Model;
namespace PetShop.IDAL...{
    /**//// <summary>
    /// Interface to the Item DAL
    /// </summary>
    public interface IItem...{
        IList GetItemsByProduct(string productId);
        ItemInfo GetItem(string itemId);
    }
}

　　SQLServerDAL层实现了IDAL层接口，完成了具体的数据库访问，代码如下：

using System;
using System.Collections;
using System.Data;
using System.Diagnostics;
using System.Data.SqlClient;
using PetShop.Model;
using PetShop.IDAL;

namespace PetShop.SQLServerDAL ...{

    public class Item : IItem...{

        // Static constants
        private const string SQL_SELECT_ITEM = "SELECT Item.ItemId, Item.Attr1, Inventory.Qty, Item.ListPrice, Product.Name, Product.Descn FROM Item INNER JOIN Inventory ON Item.ItemId = Inventory.ItemId INNER JOIN Product ON Item.ProductId = Product.ProductId WHERE Item.ItemId = @ItemId";
        private const string SQL_SELECT_ITEMS_BY_PRODUCT = "SELECT ItemId, Attr1, ListPrice, Name FROM Item INNER JOIN Product ON Item.ProductId = Product.ProductId WHERE Item.ProductId = @ProductId";

        private const string PARM_ITEM_ID = "@ItemId";
        private const string PARM_PRODUCT_ID = "@ProductId";

        /**//// <summary>
        /// Function to get a list of items within a product group
        /// </summary>
        /// <param name="productId"></param>
        /// <returns></returns>
        public IList GetItemsByProduct(string productId) ...{

            IList itemsByProduct = new ArrayList();

            SqlParameter parm = new SqlParameter(PARM_PRODUCT_ID, SqlDbType.Char, 10);
            parm.Value = productId;

            //Execute the query against the database
            using (SqlDataReader rdr = SQLHelper.ExecuteReader(SQLHelper.CONN_STRING_NON_DTC, CommandType.Text, SQL_SELECT_ITEMS_BY_PRODUCT, parm)) ...{
                // Scroll through the results
                while (rdr.Read())...{
                    ItemInfo item = new ItemInfo(rdr.GetString(0).Trim(), rdr.GetString(1), rdr.GetDecimal(2), rdr.GetString(3), null);
                    //Add each item to the arraylist
                    itemsByProduct.Add(item);
                }
            }

            return itemsByProduct;
        }


        /**//// <summary>
        /// Get an individual item based on a the unique key
        /// </summary>
        /// <param name="itemId">unique key</param>
        /// <returns></returns>
        public ItemInfo GetItem(string itemId) ...{

            //Set up a return value
            ItemInfo item = null;

            //Create a parameter
            SqlParameter parm = new SqlParameter(PARM_ITEM_ID, SqlDbType.Char, 10);
            //Bind the parameter
            parm.Value = itemId;

            //Execute the query
            using (SqlDataReader rdr = SQLHelper.ExecuteReader(SQLHelper.CONN_STRING_NON_DTC, CommandType.Text, SQL_SELECT_ITEM, parm)) ...{
                rdr.Read();
                item = new ItemInfo(rdr.GetString(0).Trim(), rdr.GetString(1), rdr.GetInt32(2), rdr.GetDecimal(3), rdr.GetString(4), rdr.GetString(5));

            }
            return item;
        }
    }
}

　　同样，OracleDAL层也实现了IDAL层接口，完成了Oracle的数据库访问。

图3：.NET Pet Shop 3.0 中的数据工厂的实现

　　这意味着，如果要创建应用程序的 DB2 版本，我们不需要改动业务逻辑层（或者 UI 层）。 创建 DB2 兼容版本的步骤如下：

　　1.创建 DB2 的数据库访问类，它应该实现 IDAL 接口。
 
　　2.将 DB2 访问类编译成一个程序集。

　　3.测试和部署新的数据程序集到一台运行中的服务器上。

　　4.更改配置文件，指向新的数据库访问类。

　　无需更改或重新编译业务逻辑组件。

　　联系作者：tyrwjz2003@vip.sina.com