C#:System.Data.Common命名空间（数据库抽象工厂的使用）

///在使用本例之前要先手动引用system.configration组件，然后建立config文件，config文件名为当前程序文件名的后面加.config比如：a.exe.config
using System;
using System.Collections.Generic;
using System.Text;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Data.Common;

namespace ConsoleApplication313
{
    class Program
    {
        static void Main(string[] args)
        {
            ConnectionStringSettings cfg = ConfigurationManager.ConnectionStrings["My DB"];
            DbProviderFactory factory = DbProviderFactories.GetFactory(cfg.ProviderName);
            DbConnection cnx = factory.CreateConnection();
            DbCommand cmd = factory.CreateCommand();
            cnx.ConnectionString = cfg.ConnectionString;
            cmd.Connection = cnx;
            cnx.Open();
            cmd.CommandText = string.Format("select * from student");
            DbDataReader dbdatareader=
            cmd.ExecuteReader();
            while (dbdatareader.Read())
            {
                Console.Write(string.Format("{0} {1} {2} {3} {4} {5}",dbdatareader[0],dbdatareader[1],
                    dbdatareader[2],dbdatareader[3],dbdatareader[4],dbdatareader[5]));
                Console.WriteLine();
            }
            System.Console.ReadLine();
        }
    }
}

下面是XML文件的内容：

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<connectionStrings>
   <add name="My DB" providerName="System.Data.SqlClient" connectionString="server=.;uid=sa;pwd=sqlserver;database=master"/>
</connectionStrings>
</configuration>