c# – 与对象包装类一起使用时的灵活性

public interface iUserInfo
{
    string getUserName(int userId);
    string getUserAge(string username);
}

public class UserDb implements iUserInfo
{
    string getUserName(int userId)
    {
        //code to retrieve user info from the database
    }
}

public class UserTxtFile implements iUserInfo
{
    string getUserName(int userId)
    {
        //code to retrieve user info from the plain text file
    }
}

public class UserManager()
{
    private iUserInfo userInfo;

    public string retrieveUserData(string userName)
    {
        return userInfo.getUserAge(userName);
    }
}

我的教授说

“the above user manager class has a reference variable of type
iUserInfo,so,if this reference is provided by a factory method or by
any other means,the code of the manager class will be the same
regardless of what implementation is provided.

This approach maximizes flexibilty if some implementations need to be
altered during the process of software development,as higher level
layers will not be affected at all.”

我需要了解两件事;灵活性最大化和工厂方法.

后者我猜它可能是这样的

public class UserManager()
{
    private iUserInfo userInfo;
    private UserTxtFile  uTxtFile;
    private UserDb uDB;
    public iUserInfo GetUserInfoObjFactory(bool dbOrTxt)
    {
        return dbOrTxt? uTxtFile:uDB;
    }
    public string retrieveUserData(string userName)
    {
        return userInfo.getUserAge(userName);
    }
}

在演讲期间,我对其他事情做了一些白日梦,现在无法弄清楚究竟是什么意思？我想更深入地了解一下,以防一些采访者用更开放的问题来攻击我,我不知道如何回答.

在高级别图层受到影响时,您是否还可以添加一些代码来破坏上述源代码？非常感谢.

[更新]
我也找到了一个答案,例如“我们需要接口来分组常用方法”并不是很有说服力.仅抽象基类也有帮助.
例如,来自2个对象{Penguin vs Bird}的Fly行为.我不能简单地告诉面试官,根本需要IFly.即使我有数百万个需要Fly的不同对象,我也可以为每个对象实现每个Fly.因为即使我设计了IFly,我仍然需要在实现类中执行此操作.您能否提供更多详细案例或重新解释界面必须如何？