Inheritance & BASE Classes in ASP.NET MVC

Question

I’m trying to understand inheritance within an MVC application and I have a couple of things I would like clear up if someone can please help.

Question 1

If I have the following three classes which inherit off each other as follows

Public class StandardPage

Public class ArticlePage : StandardPage

Public class NewsPage : ArticlePage

Is the StandardPage class the only BASE class here or is the ArticlePage class also referred to as a BASE class because the NewsPage class derives from it?

In a nut shell are classes that derive from other classes also referred to as BASE classes (for the class that inherits it)?

Question 2

If I have the above classes defined when working in the strongly typed view file (with Razor), if I write the following code

@foreach (var article in Model.Articles)
{
if (article is StandardPage)
        {
var articlePage = article as StandardPage;

If the object inside the article var is of type StandardPage, ArticlePage or news page then the articlePage var will be populated without any issues because StandardPage is the BASE class which ArticlePage & NewsPage both inherit from, however if I change the code to the following

@foreach (var article in Model.Articles)
{
if (article is StandardPage)
        {
var articlePage = article as ArticlePage;

When the object inside the article var is of type StandardPage I will receive an ‘Object reference not set to an instance of an object’ when trying to populate the articlePage var because StandardPage does not inherit from an ArticlePage (it’s the other way around) and therefore I can’t cast something of type StandardPage into and ArticlePage.

Fingers crossed im nearly there with understanding this but if im way off the mark could someone please point me in the right direction

Thanks in advance

I think if you are looping through your articles and casting them in your view, then you are probably doing something wrong. — Paddy
– Paddy, Commented Jan 21, 2016 at 11:28
Yes but lets say i am supporting an application and this is how it has been written, is my understanding regarding inheritance from within a view correct? The whole point of me asking these questions is to understand how inheritance works. I understand that MVC is meant to provide a way to seperate logic from being inside view etc. Thanks for your response — Paul
– Paul, Commented Jan 21, 2016 at 11:32
@HenkHolterman - I think you might have that wrong - given the code, a NewsPage is an ArticlePage. — Paddy
– Paddy, Commented Jan 21, 2016 at 11:34

lyz · Accepted Answer · 2016-01-21 12:58:34Z

1

I think the notion of base class is relative, a class is base class for the classes which inherits from it, so in your example StandardPage is a base class for ArtcilePage and ArticlePage is also a base class for NewsPage. If you say there is only one base class then the only base class should be Object because all classes inherits from it in the chain.

For the second point, you can not cast an instance of base class to a subclass but you can cast a subclass to a base class, in your example :
NewsPage can be cast to ArticlePage and StandardPage and Object too
AriclePage can be cast to StandardPage and to Object
StandardPage can be cast to Object only

var news = new NewsPage();
var article = new ArticlePage();
var standard = new StandardPage();

news as ArticlePage // works
news as StandardPage // works
news as object // works

article as StandardPage // works
article as object // works
article as NewsPage // won't work

standard as object // works
standard as AriclePages // won't work
standard as NewsPage // won't work

If you could cast base class to subclass then you would be able to cast any class to any other class, because you can cast any class (let's say A)to object as object is base class for all classes and then you cast object to B even if A and B has no reltationship between them.

edited Jan 21, 2016 at 12:58

answered Jan 21, 2016 at 12:47

lyz

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1 Comment

Paul Over a year ago

Hi IYZ, this is exactly what I thought and hence the reason I have raised the question. @paddy has mentioned that ‘Stating that ArticlePage is a 'base class' is not overly helpful’ and reccomended that i should use the terminology ‘is a’ instead. So... with this in mind, to confirm it is correct to state that ArticlePage is a base class for NewsPage but I shouldn’t refer to it this way and rather i should use the terminology 'is a' Again another really helpful answer for my second point ;). I think that I am on the right lines here.

Paddy · Accepted Answer · 2016-01-21 11:45:53Z

1

What you have stated is pretty much correct, however the nomenclature you are trying to use is a bit confusing.

Stating that ArticlePage is a 'base class' is not overly helpful, it's just a class, it is possibly more useful to state that NewsPage 'is a' ArticlePage (and 'is also' a StandardPage).

If you are casting in your view, you are possibly doing something wrong in your build up of your model. Typically, for inheritance like you have above, you might pass a list of 'StandardPage' objects to your view, where they can all be treated the same for display, however if you are having to cast in your view, you may need to re-examine your architecture, to see if you are really getting the benefit from what you have in place.

For example, if you had the following structure:

Public class StandardPage {
    public virtual string Title() { return "A standard page"; }
}

Public class ArticlePage : StandardPage {
    public override string Title() { return "An article page"; }
}

Public class NewsPage : ArticlePage {
    public override string Title() { return "A news page"; }
}

And your view looked like this, then this might make sense (no casting involved and allows for easy adding of new 'StandardPage' types without requiring changes to your front end):

@foreach (var article in Model.Articles)
{
    <h2>@article.Title()</h2>
}

edited Jan 21, 2016 at 11:45

answered Jan 21, 2016 at 11:33

Paddy

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2 Comments

Paul Over a year ago

Ok cool, so i need to improve my terminology regarding BASE classes and instead use 'Is a'. However, the true BASE class here is StandardPage right?, everything else further down the chain uses an 'is a' relationship.

Paul Over a year ago

Thats for the example, that makes a lot of sense ;)

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