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)