3. 工厂方法模式（Factory Method）---------- 创建型模式

1. 工厂方法和抽象工厂不同 ,抽象工厂解决的问题是一个系列的对象的创建，而工厂方法解决的是某个对象的创建。抽象工厂生成的实例是在抽象工厂类中实现, 工厂方法则是使一个类的实例化延迟到其子类

2. 举例

还是数据库访问.

public abstract class DBAccesss
{
      public abstract void Open();
}

public class SQLDBAccess:DBAccesss
{
      public override void Open()
      {
          Console.WriteLine("Open SQL SERVER DATABASE!");
      }
}

public class ORADBAccess:DBAccesss
{
      public override void Open()
      {
          Console.WriteLine("Open ORACLE!");
      }
}

public abstract class DBFactory
{
      public abstract DBAccesss Create();
}

public class SQLDBFactory:DBFactory
{
      public SQLDBAccess Create()
      {
          return new SQLDBAccess();
      }
}

public class ORADBFactory:DBFactory
{
      public ORADBAccess Create()
      {
          return new ORADBAccess();
      }
}

<appSettings>
      <add key="factoryName" value="SQLDBFactory"></add>
</appSettings>


public class App
{
      public static void Main(string[] args)
      {
          string strfactoryName = ConfigurationSettings.AppSettings["factoryName"];

          DBFactory factory;
          factory = (DBFactory)Assembly.Load("FactoryMethod").CreateInstance("FactoryMethod." + strfactoryName);

          DBAccesss dBAccesss = factory.Create();
          dBAccesss.Open();
      }
}