Types of Variables

In this article we'll cover the behaviors of variables to Reference Types. In Part I of my article on memory allocation in the .NET Framework,  I covered the basics of the Heap and Stack functionality and where Variable Types and Reference Types are allocated as our program executes.  We also covered the basic idea of what a Pointer is. 

Types of Reference Types

Variables pointing to our reference types are typed. What's that mean? 

If we have a Mammal and a Vegetable class as follows:

public class Mammal
{
    public int Weight;
}

public class Vegetable
{
    public int Weight;
}

And we make a new Mammal:

Mammal programmer = new Mammal();

Our "Programmer" variable expects to be pointing to a Mammal so we can not set our reference to anything other than a Mammal.

programmer = new Vegetable(); // CAN'T DO THIS!

We can't even force our Vegetable to be a Mammal.

programmer = (Mammal) new Vegetable(); // CAN'T DO THIS EITHER!

However, both Mammal and Vegetable inherit from System.Object so we CAN do this:

object programmer = new Mammal();
programmer =  new Vegetable(); // CAN DO THIS!

A reference to a Mammal can also point to anything that is a Mammal, meaning anything that inherits from our Mammal class.  So if we have a Human class as follows.

public class Human:Mammal
{
    public int IQ;
}

We can do this:

Mammal programmer = new Mammal();
programmer =  new Human(); // CAN DO THIS! HUMANS ARE MAMMALS.

But then can't reference the IQ property of our programmer variable because it's only pointing to a Mammal (even though this particular Mammal is a Programmer):

programmer.IQ = 5; // NOT AVAILABLE -- IT'S JUST A MAMMAL

To get to the IQ of our Human programmer, we have to be looking through a reference to a Human:

Mammal programmer = new Mammal();
programmer =  new Human();
Human JohnDoe = programmer as Human;
JohnDoe.IQ = 5; // AVAILABLE -- IT'S A HUMAN

Even though this is a true statement:

programmer.Equals(JohnDoe)


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