在项目中通常可能会使用不同的数据源,可能是SQL Server也可能是ACCESS或者是Oracle,那么如何保证在使用不同数据源的时候,使项目代码更改的代价最小呢?
对,使用工厂模式.在Net1.1的时候,这需要项目实施者自己来完成.在Net2.0中,MS已经新增了几个用于实施工厂模式的类库.
首先我现在应用程序当前目录下新建Databases目录,再新建一个Access数据库与Sqlserver数据库
其中这2个数据库的结构都是一样的,都包含一个SampleData表,有ID,与IntegerValue字段

然后回到VS中新建一个WinForm项目,然后编辑App.Config文件如下:

xml version="1.0" encoding="utf-8" ?> configuration>     connectionStrings>             <add name="SQL Server" providerName="System.Data.SqlClient"                  connectionString="Data Source=./SQLEXPRESS;AttachDbFilename=|DataDirectory|/Databases/MyData.mdf;Integrated Security=True;User Instance=True" />         <add name="MS Access" providerName="System.Data.OleDb"           connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|/Databases/MyData.mdb;Persist Security Info=True" />        connectionStrings> configuration>

然后在主窗体中的Button事件中编写如下代码

private void getDataButton_Click(object sender, EventArgs e)         {             try             {                 //创建一个新的StopWatch来监视连接性能[Net2.0新增]                 Stopwatch myWatch = new Stopwatch();                 // 开始计算connection and retrieval of data所花费的时间                 myWatch.Start();                 // 选择要使用的数据源                 string strDataSource="MS Access";                 // 根据选者的数据源获得连接字符串的配置对象                                ConnectionStringSettings objConnectionSettings = ConfigurationManager.ConnectionStrings[strDataSource];                 // 通过配置文件创建数据库驱动工厂的实例                 DbProviderFactory objProviderFactory = DbProviderFactories.GetFactory(objConnectionSettings.ProviderName);                 // 通过数据库驱动工厂创建DBConnection实例                 using (DbConnection objConnection = objProviderFactory.CreateConnection())                 {                                 // 从 objConnectionSettings 中获取连接字符串                 objConnection.ConnectionString = objConnectionSettings.ConnectionString;                 // 打开 connection                 objConnection.Open();                 // 通过数据驱动工厂创建　数据适配器和 Command                 DbDataAdapter myAdapter = objProviderFactory.CreateDataAdapter();                 DbCommand myCommand = objProviderFactory.CreateCommand();                                             string myQuery = "SELECT * FROM SampleData";                 DataSet myDataSet = new DataSet();                 myCommand.Connection = objConnection;                 myCommand.CommandText = myQuery;                 myAdapter.SelectCommand = myCommand;                 myAdapter.Fill(myDataSet);                                 displayDataGridView.DataSource = myDataSet.Tables[0];                 // 停止StopWatch来查看连接和返回数据所花费的时间                 myWatch.Stop();                 elapsedTimeTextLabel.Text = "消耗时间: " +  myWatch.ElapsedMilliseconds.ToString() + " ms";                 providerLabel.Text = "数据驱动: " + objConnectionSettings.ProviderName.ToString();                 connectionStringLabel.Text = objConnectionSettings.ConnectionString.ToString();             }             }             catch             {                 MessageBox.Show("出现错误.", "Alert");             }                     }     }

这样,只需要更改strDataSource就可以使用不同的数据源,而且整个项目都不需要为不同的数据库而烦恼