继承列表< T>实现集合一个坏主意？ [英] Inheriting List<T> to implement collections a bad idea?

问题描述

我曾经读过Imaar Spaanjars的文章，介绍如何构建3层应用程序。 （ http：// imar .spaanjaars.com / 416 / building-layered-web-applications-with-microsoft-aspnet-20-part-1 ），这已经形成了我的编码的基础一段时间。

I once read an article by Imaar Spaanjars on how to build 3 tier applications. (http://imar.spaanjaars.com/416/building-layered-web-applications-with-microsoft-aspnet-20-part-1) which has formed the basis of my coding for a while now.

因此，通过继承一个 List< T> ，可以像他所做的那样实现集合。因此，如果我有一个类Employee，实现一个集合，我也将有一个类Employees如下。

Thus I implement collections as he has done, by inheriting a List<T>. So if I have a class named Employee,to implement a collection I will also have a class Employees as below.

class Employee
{
   int EmpID {get;set;}
   string EmpName {get;set;}  

}

class Employees : List<Employee>
{
   public Employees(){}
}

从来没有真正质疑这是因为它为我做的工作。但现在我开始尝试一些事情，我不知道这是否是正确的方法。

I never really questioned this as it did the work for me. But now that I started trying out a few things I am not sure if this is the correct approach.

例如。如果我想从Employees获取子集，例如

e.g. if I want to get a subset from Employees, such as

 Employees newEmployees = (Employees) AllEmployees.FindAll(emp => emp.JoiningDate > DateTime.Now);

这会抛出System.InvalidCastException异常。但是，如果我使用以下，则没有问题。

This throws a System.InvalidCastException . However, if I use the following then there is no Issue.

List<Employee> newEmployees = AllEmployees.FindAll(emp => emp.JoiningDate > DateTime.Now);

那么如何实现Employees，所以我不必显式使用 List< ; Employee> 在我的DAL或BLL中？或者我该如何摆脱InvalidCastexception？

So how do I implement Employees so that I dont have to explicitly use List<Employee> in my DAL or BLL? Or maybe how do I get rid of the InvalidCastexception?

推荐答案

我不会继承 List< T> - 它引入这样的问题，并没有真正帮助（因为没有 virtual 方法覆盖）。我将使用 List< T> （或更抽象的 IList< T> code>集合< T> 有个虚拟方法。

I wouldn't inherit from List<T> - it introduces issues like these, and doesn't really help (since there are no virtual methods to override). I would either use List<T> (or the more abstract IList<T>), or to introduce polymorphism Collection<T> has virtual methods.

像 FindAll ，你也可以找到LINQ选项（如 .Where（））有用的对手;最值得注意的是，它们将为任何 IList< T> （或 IEnumerable< T> ），而不只是 List< T> 和子类。

As a note; re things like FindAll, you may also find the LINQ options (like .Where()) useful counterparts; most notably, they will work for any IList<T> (or IEnumerable<T>), not just List<T> and subclasses.

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