调用可移植数据访问层（一）

此类型的接口可以类似于如下所示：

using System;
using System.Data; 

namespace Common
{
    public interface IDbCustomers
    {
        DataTable GetCustomers();
        DataTable GetCustomerOrders(string CustomerID);
        DataTable GetCustomersByCountry(string CountryCode);
        bool InsertCustomer();
    }
}

如果希望将 Microsoft® SQL Server™ 和 Microsoft® Access 作为数据源为其提供支持，则应该在 Microsoft® Visual Studio® .NET 中创建两个不同项目，每个数据源分别创建一个。

为 SQL Server 创建的项目将类似于如下所示：

using System;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;  
using System.Configuration;    
using Common;

namespace DAL
{
   public class CustomersData : IDbCustomers
   {
      public DataTable GetCustomers()
      {
         string ConnectionString = 
            ConfigurationSettings.AppSettings
            ["ConnectionString"];

         using (SqlConnection cnn = new SqlConnection
                  (ConnectionString))
         {
            string cmdString = "SELECT CustomerID," +
               "CompanyName,ContactName " +
               "FROM Customers";
            SqlCommand cmd = 
               new SqlCommand (cmdString, cnn);

            SqlDataAdapter da = new SqlDataAdapter(cmd); 

            DataTable dt = new DataTable("Customers");

            da.Fill(dt); 

            return dt;
         }
      }
      public DataTable GetCustomerOrders(string CustomerID)
      {
         // 待定
         return null;
      }
      public DataTable GetCustomersByCountry
         (string CountryCode)
      {
         // 待定
         return null;
      }
      public bool InsertCustomer()
      {
         // 待定
         return false;
      }
   }
}

从 Microsoft® Access 进行数据检索的代码类似于如下所示：

using System;
using System.Data;
using System.Data.Common;
using System.Data.OleDb;  
using System.Configuration;    
using Common;

namespace DAL
{
   public class CustomersData : IDbCustomers
   {
      public DataTable GetCustomers()
      {
         string ConnectionString = 
            ConfigurationSettings.AppSettings
            ["ConnectionString"];

         using (OleDbConnection cnn = new OleDbConnection
                  (ConnectionString))
         {
            string cmdString = "SELECT CustomerID," +
               "CompanyName,ContactName " +
               "FROM Customers";

            OleDbCommand cmd = 
               new OleDbCommand (cmdString, cnn);

            OleDbDataAdapter da = new 
               OleDbDataAdapter(cmd); 

            DataTable dt = new DataTable("Customers");

            da.Fill(dt); 

            return dt;
         }
      }
      public DataTable GetCustomerOrders(string CustomerID)
      {
         // 待定
         return null;
      }
      public DataTable GetCustomersByCountry
         (string CountryCode)
      {
         // 待定
         return null;
      }
      public bool InsertCustomer()
      {
         // 待定
         return false;
      }
   }
}

CustomersData 类实现 IdbCustomers 接口。需要支持新数据源时，只能创建一个实现该接口的新类。