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

问题描述

我曾经读过关于如何建立3层应用程序的文章Imaar Spaanjars。 （http://imar.spaanjaars.com/416/building-layered-web-applications-with-microsoft-aspnet-20-part-1）现在已经形成我的编码了一段时间的基础。

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​​，我实现的集合，因为他所做的一切，通过继承了列表< T> 。所以，如果我有一个名为Employee类，实现一个集我也会有一类员工如下图所示。

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(){}
}

我从来没有对此提出质疑，因为它所做的工作对我来说。但现在，我开始尝试一些事情，我不知道这是正确的做法。

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。如果我想从员工，一个子集，如

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

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

这将引发System.InvalidCastException。但是，如果我用下面的话就没有问题。

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，如何实现员工让我没有明确地使用名单，LT ;员工> 在我的DAL或BLL？ ？或者，也许我该如何摆脱InvalidCastException的的

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

我也不会从列表与LT继承; T> - 它引入了问题，如这些，并没有真正的帮助（因为没有虚拟方法重写）。我想要么使用列表< T> （或者更抽象的的IList< T> ），或引入多态收藏< T> 的的的虚拟方法

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.

作为一个说明。再比如的FindAll 的事情，你也可能会发现LINQ选项（如。凡（））是有用的同行; T> （或的IEnumerable< T> 任何的的IList<工作>），而不仅仅是列表< T> 和子

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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