200人1992 X- 200 200 X- 200 200 -40 200 200 -40 200 200 -40 200 200 -40 200 200 200 200： [英] Modelling "I'm a * but I'm also a **"

问题描述

在[这篇文章]中，我正在努力执行@jonp建议的状态模式。我不太了解如何使用他发布的内容，但它导致了这样的想法，也许我试图将一个方形钉插入一个圆孔。所以我的问题：

如果我有一个可以播放多个角色的访问者，即用户可以一个供应商，一个雇主，一个广告客户或全部以上，应该用继承？我已经宣称：

  class Vendor：User {} 
 class Advertiser：User {} 
  

等等，但是当用户既是供应商又是雇主，则不同类的实例确实指向相同的底层对象...我不知道这可以工作。如何建模？

*更新*

感谢大家所有得到一点（这是我可以给的））。我一直在用EF，downcasting和最近几天的状态，把头发拉出来。角色方法更有意义。

解决方案

这听起来像一个属性模式 （或所以我称之为）将是非常合适的。这是一种比简单继承更加松散的方法，可以用于指定多个行为，或者在您的案例类型用户中。这实际上没有比具有另一种对象的标签的对象更复杂。

实现它的最简单的方法是具有一个具体的用户类，具有只读属性 IList< UserRole> （内部可以是列表< T> code>字段）您的 UserRole 类将是抽象的，而 VendorRole / AdvertiserRole /等。将从它导出，允许您将任意数量的不同角色（甚至是相同类型的角色）标记给给定的用户。另外还可以定义一个 GetRole< TRole>

用户类中的code>方法，以便于访问特定类型的角色（假设每个用户仅具有单个角色的特定子类型）。

旁注：您还可以考虑 装饰者patern ，这与上述模式密切相关 - 虽然我个人觉得这在这里是过度的，在灵活性或权力方面真的没有增加。它常常掩盖你想要做的事情;尽管如此，还是要随时调查。

In [this post], I'm struggling to implement a State Pattern as @jonp suggests. I don't quite get how to use what's he's posted but it leads to the thought that maybe I'm trying to fit a square peg into a round hole. So my question:

If I have a visitor to my site that can play multiple roles i.e. a User could be a Vendor, an Employer, an Advertiser, OR all of the above, should I be using inheritance? I've declared:

class Vendor : User {}
class Advertiser : User {}

et cetera, but when a user is both a vendor and an employer then instances of different classes really point to the same underlying object... I'm not sure this can work. How do I model it?

* update *

thanks everyone (you all get a point (it's all I can give)). I've been pulling my hair out over deep-copies with EF, downcasting and the state pattern for the last several days. The role approach makes much more sense.

解决方案

This sounds like a situation to which the attribute pattern (or so I call it) would be very appropriate. It's a much more loosely-coupled approach than simple inheritance that can be used to specify multiple "behaviours" or in your case kinds of User. It's really nothing more complicated than an object having tags of another kind of object.

The easiest way to implement it would be to have a concrete User class, with a read-only property IList<UserRole> (internally this can be a List<T> field perhaps). Your UserRole class would then be abstract, and VendorRole/AdvertiserRole/etc. would derive from it, allowing you to tag on an arbitrary number of different roles (even ones of the same type) onto a given user. These roles can in addition define their own custom behaviours, utility methods, etc.

In addition, you could define a GetRole<TRole> method on your User class to facilitate access to roles of a specific type (assuming each User only has a single Role of a specific subtype).

Side note: you may also consider the decorator patern, which is closely related to the above mentioned pattern -- though personally I feel it is overkill here, and really adds nothing in terms of flexibility or power. It often just obscures what you're trying to do; though feel free to investigate anyway.

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