基于基类的提供者模型 
      提供者程序
     1.得到machine.config文件中所有注册的DbProvider 
      this.dataGridView1.DataSource = DbProviderFactories.GetFactoryClasses();
     2.指定一个连接信息，知道某种数据库程序，来得到连接字符串

 DbConnection conn = null;
               string strConnectionString = ADODOTNET2.Properties.Settings.Default.NorthwindConnectionString;
            DbProviderFactory f = DbProviderFactories.GetFactory("System.Data.SqlClient");
            conn = f.CreateConnection();
            conn.ConnectionString = strConnectionString;
               this.label1.Text = conn.ConnectionString.ToString();

 Cursor.Current = Cursors.WaitCursor;
            try
            {
                // see this method in the example above
                DbProviderFactory f = DbProviderFactories.GetFactory("System.Data.SqlClient");
                // if our factory supports creating a DbConnection, return it.
                if (f.CanCreateDataSourceEnumerator)
                {
                    DbDataSourceEnumerator enu = f.CreateDataSourceEnumerator();
                    DataTable t = enu.GetDataSources();
                    // uses chooses a Data Row r
                    //DataRow r = t.Rows[0];
                    //string dataSource = (string)r["ServerName"];
                    //if (r[InstanceName] != null)
                    //    dataSource += ("\\" + r["InstanceName"]);
                    //// this method is defined below
                    //RewriteConnectionStringAndUpdateConfigFile(f, dataSource);

                    this.dataGridView1.DataSource = t;
                }
                else
                    Console.WriteLine("Source must be changed manually");
            }
            finally
            {
                Cursor.Current = Cursors.Default;
            }