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。