Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

class, it would inherit from the Animal class, and all public properties and

methods of Animal would be available to instances of the Dog class.

Similarly, we could create a new class based on the Dog class. In programming

circles, this is called deriving a subclass from Dog. For instance, we might create

a class called AustralianShepherd, and one for my other dog, Amigo, called

Chihuahua, both of which would inherit the properties and methods of the Dog

base class, and define new classes specific to each breed.

Don’t worry too much if this is still a little unclear. The best way to appreciate

inheritance is to see it used in a real program. The most obvious use of inheritance

in ASP.NET is in the technique called code-behind, and we’ll build plenty of ex-

amples using inheritance and code-behind in Chapter 4.

Objects In .NET

If this is the first book in which you’ve read about object oriented programming,

you’ve probably started to dream about objects! Don’t worry, the effect of first

exposure to objects doesn’t usually last for more than a week. Even though this

is yet another discussion about objects, I promise it won’t be boring. Moreover,

in the course of this section, we’ll cover some important concepts that every ser-

ious .NET programmer must know.

So far, we’ve explored various concepts that apply in one form or the other to

almost any truly object oriented language. Every language has its peculiarities,

but the general concepts are the same in all of these languages.

You may already have heard the common mantra of object oriented programmers:

“everything is an object.” This has two meanings. First of all, in C#, every program

consists of a class. In all stages of application development, from design to imple-

mentation, decisions must be made in regard to the way we design and relate

objects and classes to each other. Yes, objects are everywhere.

.NET extends this to yet another level, giving the phrase “everything is an object”

extra meaning. In the world of .NET, every class ultimately derives from a base

class named Object, so “everything is an object” becomes “everything is an Ob-

ject.”

If you look at the documentation for the ASP.NET Page class, you can see the

list of classes from which this class inherits, as shown in Figure 3.7.

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Objects In .NET

Figure 3.7. The Page class’s documentation

You’ll remember from the last section that we said our hypothetical

AustralianShepherd class would inherit from the more general Dog class, which,

in turn, would inherit from the even more general Animal class. This is exactly

the kind of relationship that’s being shown in Figure 3.7—Page inherits methods

and properties from the TemplateControl class, which in turn inherits from a

more general class called Control. In the same way that we say that an Australian

Shepherd is an Animal, we say that a Page is a Control. Control, like all .NET

classes, inherits from Object.

Since Object is so important that every other class derives from it, either directly

or indirectly, it deserves a closer look. Object contains the basic functionality

that the designers of .NET felt should be available in any object. The Object

class contains these public members:

❑

Equals

❑

ReferenceEquals

❑

GetHashCode

❑

GetType

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Chapter 3: VB and C# Programming Basics