Introduction

Before we delve too deeply into how to reason about Object-Orientation and how to utilize it in your programming efforts, it would be useful to understand why object-orientation came to exist. This initial chapter seeks to explore the origins behind object-oriented programming.

Key Terms

Some key terms to learn in this chapter are:

• The Software Crisis
• GOTO statements
• Programming Language Paradigm
• Imperative Programming
• Functional Programming
• Structured Programming
• Object-Orientation

The Growth of Computing

By this point, you should be familiar enough with the history of computers to be aware of the evolution from the massive room-filling vacuum tube implementations of ENIAC, UNIVAC, and other first-generation computers to transistor-based mainframes like the PDP-1, and the eventual introduction of the microcomputer (desktop computers that are the basis of the modern PC) in the late 1970’s. Along with a declining size, each generation of these machines also cost less:

This increase in affordability was also coupled with an increase in computational power. Consider the ENIAC, which computed at 100,000 cycles per second. In contrast, the relatively inexpensive Commodore 64 ran at 1,000,000 cycles per second, while the more pricy IBM PC ran at 4,770,000 cycles per second.

Not surprisingly, governments, corporations, schools, and even individuals purchased computers in larger and larger quantities, and the demand for software to run on these platforms and meet these customers’ needs likewise grew. Moreover, the sophistication expected from this software also grew. Edsger Dijkstra described it in these terms:

The major cause of the software crisis is that the machines have become several orders of magnitude more powerful! To put it quite bluntly: as long as there were no machines, programming was no problem at all; when we had a few weak computers, programming became a mild problem, and now we have gigantic computers, programming has become an equally gigantic problem.

Edsger Dijkstra, The Humble Programmer (EWD340), Communications of the ACM

Coupled with this rising demand for programs was a demand for skilled software developers, as reflected in the following table of graduation rates in programming-centric degrees (the dashed line represents the growth of all bachelor degrees, not just computer-related ones):

Unfortunately, this graduation rate often lagged far behind the demand for skilled graduates, and was marked by several periods of intense growth (the period from 1965 to 1985, 1995-2003, and the current surge beginning around 2010). During these surges, it was not uncommon to see students hired directly into the industry after only a course or two of learning programming (coding boot camps are a modern equivalent of this trend).

All of these trends contributed to what we now call the Software Crisis.

The Software Crisis

At the 1968 NATO Software Engineering Conference held in Garmisch Germany, the term “Software Crisis” was coined to describe the current state of the software development industry, where common problems included:

• Projects that ran over-budget
• Projects that ran over-time
• Software that made inefficient use of calculations and memory
• Software was of low quality
• Software that failed to meet the requirements it was developed to meet
• Projects that became unmanageable and code difficult to maintain
• Software that never finished development

The software development industry sought to counter these problems through a variety of efforts:

• The development of new programming languages with features intended to make it harder for programmers to make errors.
• The development of Integrated Development Environments (IDEs) with developer-centric tools to aid in the software development process, including syntax highlighting, interactive debuggers, and profiling tools
• The development of code repository tools like SVN and GIT
• The development and adoption of code documentation standards
• The development and adoption of program modeling languages like UML
• The use of automated testing frameworks and tools to verify expected functionality
• The adoption of software development practices that adopted ideas from other engineering disciplines

This course will seek to instill many of these ideas and approaches into your programming practice through adopting them in our everyday work. It is important to understand that unless these practices are used, the same problems that defined the software crisis continue to occur!

In fact, some software engineering experts suggest the software crisis isn’t over, pointing to recent failures like the Denver Airport Baggage System in 1995, the Ariane 5 Rocket Explosion in 1996, the German Toll Collect system cancelled in 2003, the rocky healthcare.gov launch in 2013, and the massive vulnerabilities known as the Meltdown and Spectre exploits discovered in 2018.

Language Evolution

One of the strategies that computer scientists employed to counter the software crisis was the development of new programming languages. These new languages would often 1) adopt new techniques intended to make errors harder to make while programming, and 2) remove problematic features that had existed in earlier languages.

A Fortran Example

Let’s take a look at a working (and in current use) program built using Fortran, one of the most popular programming languages at the onset of the software crisis. This software is the Environmental Policy Integrated Climate (EPIC) Model, created by researchers at Texas A&M:

Environmental Policy Integrated Climate (EPIC) model is a cropping systems model that was developed to estimate soil productivity as affected by erosion as part of the Soil and Water Resources Conservation Act analysis for 1980, which revealed a significant need for improving technology for evaluating the impacts of soil erosion on soil productivity. EPIC simulates approximately eighty crops with one crop growth model using unique parameter values for each crop. It predicts effects of management decisions on soil, water, nutrient and pesticide movements, and their combined impact on soil loss, water quality, and crop yields for areas with homogeneous soils and management. EPIC Homepage

You can download the raw source code here (click “EPIC v.1102” under “Source Code”). Open and unzip the source code, and open a file at random using your favorite code editor. See if you can determine what it does, and how it fits into the overall application.

Try this with a few other files. What do you think of the organization? Would you be comfortable adding a new feature to this program?

New Language Features

You probably found the Fortran code in the example difficult to wrap your mind around - and that’s not surprising, as more recent languages have moved away from many of the practices employed in Fortran. Additionally, our computing environment has dramatically changed since this time.

Symbol Character Limits

One clear example is symbol names for variables and procedures (functions) - notice that in the Fortran code they are typically short and cryptic: RT, HU, IEVI, HUSE, and NFALL, for example. You’ve been told since your first class that variable and function names should express clearly what the variable represents or a function does. Would rainFall, dailyHeatUnits, cropLeafAreaIndexDevelopment, CalculateWaterAndNutrientUse(), CalculateConversionOfStandingDeadCropResidueToFlatResidue() be easier to decipher? (Hint: the documentation contains some of the variable notations in a list starting on page 70, and some in-code documentation of global variables occurs in MAIN_1102.f90.).

Believe it or not, there was an actual reason for short names in these early programs. A six character name would fit into a 36-bit register, allowing for fast dictionary lookups - accordingly, early version of FORTRAN enforced a limit of six characters for variable names. However, it is easy to replace a symbol name with an automatically generated symbol during compilation, allowing for both fast lookup and human readability at a cost of some extra computation during compilation. This step is built into the compilation process of most current programming languages, allowing for arbitrary-length symbol names with no runtime performance penalty.

Paradigm Shifts

In addition to these less drastic changes, some evolutionary language changes had sweeping effects, changing the way we approach and think about how programs should be written and executed. These “big ideas” of how programming languages should work are often called paradigms. In the early days of computing, we had two common ones: imperative and functional.

At its core, imperative programming simply means the idea of writing a program as a sequence of commands, i.e. this Python script uses a sequence of commands to write to a file:

f = open("example.txt")
f.write("Hello from a file!")
f.close()

An imperative program would start executing the first line of code, and then continue executing line-by-line until the end of the file or a command to stop execution was reached. In addition to moving one line through the program code, imperative programs could jump to a specific spot in the code and continue execution from there, using a GOTO statement. We’ll revisit that aspect shorty.

In contrast, functional programming consisted primarily of functions. One function was designated as the ‘main’ function that would start the execution of the program. It would then call one or more functions, which would in turn call more functions. Thus, the entire program consisted of function definitions. Consider this Python program:

def concatenateList(str, list):
  if(len(list) == 0):
    return str
  elif(len(list) == 1):
    head = list.pop(0)
    return concatenateList(str + head, list)
  else:
    head = list.pop(0)
    return concatenateList(str + head + ", ", list)

def printToFile(filename, body):
  f = open(filename)
  f.write(body)

def printListToFile(filename, list):
  body = concatenateList("", list)
  printToFile(filename, body)

def main():
  printListToFile("list.txt", ["Dog", "Cat", "Mouse"])

main()

You probably see elements of your favorite higher-order programming language in both of these descriptions. That’s not surprising as modern languages often draw from multiple programming paradigms (after all, both the above examples were written in Python). This, too, is part of language evolution - language developers borrow good ideas as they find them.

But as languages continued to evolve and language creators sought ways to make programming easier, more reliable, and more secure to address the software crisis, new ideas emerged that were large enough to be considered new paradigms. Two of the most impactful of these new paradigms these are structured programming and object orientation. We’ll talk about each next.

Structured Programming

Another common change to programming languages was the removal of the GOTO statement, which allowed the program execution to jump to an arbitrary point in the code (much like a choose-your-own adventure book will direct you to jump to a page). The GOTO came to be considered too primitive, and too easy for a programmer to misuse ¹.

While the GOTO statement is absent from most modern programming languages the actual functionality remains, abstracted into control-flow structures like conditionals, loops, and switch statements. This is the basis of structured programming, a paradigm adopted by all modern higher-order programming languages.

Each of these control-flow structures can be represented by careful use of GOTO statements (and, in fact the resulting assembly code from compiling these languages does just that). The benefit of using structured programming is it promotes “reliability, correctness, and organizational clarity” by clearly defining the circumstances and effects of code jumps ².

You probably aren’t very familiar with GOTO statements because the structured programming paradigm has become so dominant. Before we move on, let’s see how some familiar structured programming patterns were originally implemented using GOTOs:

Conditional (if statement)

In C#, you are probably used to writing if statements with a true branch:

int x = 4;
if(x < 5) 
{
  x = x * 2;
}
Console.WriteLine("The value is:" + x);

With GOTOs, it would look something like:

int x = 4;
if(x < 5) goto TrueBranch;

AfterElse:
  Console.WriteLine("The value is:" + x);
  Environment.Exit(0);

TrueBranch:
  x = x * 2;
  goto AfterElse

Conditional (if-else statement)

Similarly, a C# if statement with an else branch:

int x = 4;
if(x < 5) 
{
  x = x * 2;
}
else 
{
  x = 7;
}
Console.WriteLine("The value is:" + x);

And using GOTOs:

int x = 4;
if(x < 5) goto TrueBranch;
goto FalseBranch;

AfterElse:
  Console.WriteLine("The value is:" + x);
  Environment.Exit(0);

TrueBranch:
  x = x * 2;
  goto AfterElse;

FalseBranch: 
  x = 7;
  goto AfterElse;

Note that with the goto, we must tell the program to stop running explicitly with Environment.Exit(0) or it will continue on to execute the labeled code (we could also place the TrueBranch and FalseBranch before the main program, and use a goto to jump to the main program).

While Loop

Loops were also originally constructed entirely from GOTOs, so the familiar while loop:

int times = 5;
while(times > 0)
{
  Console.WriteLine("Counting Down: " + times);
  times = times - 1;
}

Can be written:

int times = 5;
Test:
  if(times > 0) goto Loop;
  Environment.Exit(0);

Loop: 
  Console.WriteLine("Counting Down: " + times);
  times = times - 1;
  goto Test;

The do while and for loops are implemented similarly. As you can probably imagine, as more control flow is added to a program, using GOTOs and corresponding labels to jump to becomes very hard to follow.

Object-Orientation

The object-orientation paradigm was similarly developed to make programming large projects easier and less error-prone.

The term “Object Orientation” was coined by Alan Kay while he was a graduate student in the late 60’s. Alan Kay, Dan Ingalls, Adele Goldberg, and others created the first object-oriented language, Smalltalk, which became a very influential language from which many ideas were borrowed. To Alan, the essential core of object-orientation was three properties a language could possess: ¹

• Encapsulation & Information Hiding
• Message passing
• Dynamic binding

Let’s break down each of these ideas, and see how they helped address some of the problems we’ve identified in this chapter.

Encapsulation refers to breaking programs into smaller units that are easier to read and reason about. In an object-oriented language these units are classes and objects, and the data contained in these units is protected from being changed by code outside the unit through information hiding.

Message Passing allows us to send well-defined messages between objects. This gives us a well-defined and controlled method for accessing and potentially changing the data contained in an encapsulated unit. In an object oriented language, calling a method on an object is a form of message passing, as are events.

Dynamic Binding means we can have more than one possible way to handle messages and the appropriate one can be determined at run-time. This is the basis for polymorphism, an important idea in many object-oriented languages.

Remember these terms and pay attention to how they are implemented in the languages you are learning. They can help you understand the ideas that inspired the features of these languages.

We’ll take a deeper look at each of these in the next few chapters. But before we do, you might want to see how language popularity has fared since the onset of the software crisis, and how new languages have appeared and grown in popularity in this animated chart from Data is Beautiful:

Interestingly, the four top languages in 2019 (Python, JavaScript, Java, and C#) all adopt the object-oriented paradigm - though the exact details of how they implement it vary dramatically.

Summary

In this chapter, we’ve discussed the environment in which object-orientation emerged. Early computers were limited in their computational power, and languages and programming techniques had to work around these limitations. Similarly, these computers were very expensive, so their purchasers were very concerned about getting the largest possible return on their investment. In the words of Niklaus Wirth:

Tricks were necessary at this time, simply because machines were built with limitations imposed by a technology in its early development stage, and because even problems that would be termed "simple" nowadays could not be handled in a straightforward way. It was the programmers' very task to push computers to their limits by whatever means available.

As computers became more powerful and less expensive, the demand for programs (and therefore programmers) grew faster than universities could train new programmers. Unskilled programmers, unwieldy programming languages, and programming approaches developed to address the problems of older technology led to what became known as the “software crisis” where many projects failed or floundered.

This led to the development of new programming techniques, languages, and paradigms to make the process of programming easier and less error-prone. Among the many new programming paradigms was structured programming paradigm, which introduced control-flow structures into programming languages to help programmers reason about the order of program execution in a clear and consistent manner.

Also developed during this time was the object-oriented paradigm, which brings together four big ideas: encapsulation & information hiding, message passing, and dynamic binding. We will be studying this paradigm, its ideas, and implementation in the C# language throughout this course.

Introduction

A signature aspect of object-oriented languages is (as you might expect from the name), the existence of objects within the language. In this chapter, we take a deep look at objects, exploring why they were created, what they are at both a theoretical and practical level, and how they are used.

Key Terms

Some key terms to learn in this chapter are:

• Encapsulation

• Information Hiding

• Message Passing

• State

• Class

• Object

• Field

• Method

• Constructor

• Parameterless Constructor

• Property

• Public

• Private

• Static

To begin, we’ll examine the term encapsulation.

Encapsulation

The first criteria that Alan Kay set for an object-oriented language was encapsulation. In computer science, the term encapsulation refers to organizing code into units. This provides a mechanism for organizing complex software.

A second related idea is information hiding, which provides mechanisms for controlling access to encapsulated data and how it can be changed.

Think back to the FORTRAN EPIC model we introduced earlier. All of the variables in that program were declared globally, and there were thousands. How easy was it to find where a variable was declared? Initialized? Used? Are you sure you found all the spots it was used?

Also, how easy was it to determine what part of the system a particular block of code belonged to? If I told you the program involved modeling hydrology (how water moves through the soils), weather, erosion, plant growth, plant residue decomposition, soil chemistry, planting, harvesting, and chemical applications, would you be able to find the code for each of those processes?

Remember from our discussion on the growth of computing the idea that as computers grew more powerful, we wanted to use them in more powerful ways? The EPIC project grew from that desire - what if we could model all the aspects influencing how well a crop grows? Then we could use that model to help us make better decisions in agriculture. Or, what if we could model all the processes involved in weather? If we could do so, we could help save lives by predicting dangerous storms! A century ago, you knew a tornado was coming when you heard its roaring winds approaching your home. Now we have warnings that conditions are favorable to produce one hours in advance. This is all thanks to using computers to model some very complex systems.

But how do we go about writing those complex systems? I don’t know about you, but I wouldn’t want to write a model the way the EPIC programmers did. And neither did most software developers at the time - so computer scientists set out to define better ways to write programs. David Parnas formalized some of the best ideas emerging from those efforts in his 1972 paper “On the Criteria To Be Used in Decomposing Systems into Modules”. ¹

A data structure, its internal linkings, accessing procedures and modifying procedures are part of a single module.

Here he suggests organizing code into modules that group related variables and the procedures that operate upon them. For the EPIC module, this might mean all the code related to weather modeling would be moved into its own module. That meant that if we needed to understand how weather was being modeled, we only had to look at the weather module.

They are not shared by many modules as is conventionally done.

Here he is laying the foundations for the concept we now call scope - the idea of where a specific symbol (a variable or function name) is accessible within a program’s code. By limiting access to variables to the scope of a particular module, only code in that module can change the value. That way, we can’t accidentally change a variable declared in the weather module from the soil chemistry module (which would be a very hard error to find, as if the weather module doesn’t seem to be working, that’s what we would probably focus on trying to fix).

Programmers of the time referred to this practice as information hiding, as we ‘hid’ parts of the program from other parts of the program. Parnas and his peers pushed for not just hiding the data, but also how the data was manipulated. By hiding these implementation details, they could prevent programmers who were used to the globally accessible variables of early programming languages from looking into our code and using a variable that we might change in the future.

The sequence of instructions necessary to call a given routine and the routine itself are part of the same module.

As the actual implementation of the code is hidden from other parts of the program, a mechanism for sharing controlled access to some part of that module in order to use it needed to be made. An interface, if you will, that describes how the other parts of the program might trigger some behavior or access some value.

C# Encapsulation Examples

Let’s start by focusing on encapsulation’s benefits to organizing our code by exploring some examples of encapsulation you may already be familiar with.

Namespaces

The C# libraries are organized into discrete units called namespaces. The primary purpose of this is to separate code units that potentially use the same name, which causes name collisions where the interpreter isn’t sure which of the possibilities you mean in your program. This means you can use the same name to refer to two different things in your program, provided they are in different namespaces.

For example, there are two definitions for a Point Struct in the .NET core libraries: System.Drawing.Point and System.Windows.Point. The two have a very different internal structures (the former uses integers and the latter doubles), and we would not want to mix them up. If we needed to create an instance of both in our program, we would use their fully-quantified name to help the interpreter know which we mean:

System.Drawing.Point pointA = new System.Drawing.Point(500, 500);
System.Windows.Point pointB = new System.Windows.Point(300.0, 200.0);

The using directive allows you reference the type without quantification, i.e.:

using System.Drawing;
Point pointC = new Point(400, 400);

You can also create an alias with the using directive, providing an alternative (and usually abbreviated) name for the type:

using WinPoint = System.Windows.Point;
WinPoint pointD = new WinPoint(100.0, 100.0);

We can also declare our own namespaces, allowing us to use namespaces to organize our own code just as Microsoft has done with its .NET libraries.

Encapsulating code within a namespace helps ensure that the types defined within are only accessible with a fully qualified name, or when the using directive is employed. In either case, the intended type is clear, and knowing the namespace can help other programmers find the type’s definition.

Structs

In the discussion of namespaces, we used a struct. A C# struct is what computer scientists refer to as a compound type, a type composed from other types. This too, is a form of encapsulation, as it allows us to collect several values into a single data structure. Consider the concept of a vector from mathematics - if we wanted to store three-dimensional vectors in a program, we could do so in several ways. Perhaps the easiest would be as an array:

double[] vectorA = {3, 4, 5};

However, other than the variable name, there is no indication to other programmers that this is intended to be a three-element vector. And, if we were to accept it in a function, say a dot product:

public double DotProduct(double[] a, double[] b) {
    if(a.Length < 3 || b.Length < 3) throw new ArgumentException();
    return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
}

We would need to check that both arrays were of length three… A struct provides a much cleaner option, by allowing us to define a type that is composed of exactly three doubles:

/// <summary>
/// A 3-element vector 
/// </summary>
public struct Vector3 {
    public double x;
    public double y;
    public double z;

    public Vector3(double x, double y, double z) {
        this.x = x;
        this.y = y;
        this.z = z;
    }
}

Then, our DotProduct can take two arguments of the Vector3 struct:

public double DotProduct(Vector3 a, Vector3 b) {
    return a.x * b.x + a.y * b.y + a.z * b.z;
}

There is no longer any concern about having the wrong number of elements in our vectors - it will always be three. We also get the benefit of having unique names for these fields (in this case, x, y, and z).

Thus, a struct allows us to create structure to represent multiple values in one variable, encapsulating the related values into a single data structure. Variables, and compound data types, represent the state of a program. We’ll examine this concept in detail next.

Modules

You might think that the kind of modules that Parnas was describing don’t exist in C#, but they actually do - we just don’t call them ‘modules’. Consider how you would raise a number by a power, say 10 to the 8th power:

Math.Pow(10, 8);

The Math class in this example is actually used just like a module! We can’t see the underlying implementation of the Pow() method, it provides to us a well-defined interface (i.e. you call it with the symbol Pow and two doubles for parameters), and this method and other related math functions (Sin(), Abs(), Floor(), etc.) are encapsulated within the Math class.

We can define our own module-like classes by using the static keyword, i.e. we could group our vector math functions into a static VectorMath class:

/// <summary>
/// A library of vector math functions
/// </summary>
public static class VectorMath() {
    
    /// <summary>
    /// Computes the dot product of two vectors 
    /// </summary>
    public static double DotProduct(Vector3 a, Vector3 b) {
        return a.x * b.x + a.y * b.y + a.z * b.z;
    }

    /// <summary>
    /// Computes the magnitude of a vector
    /// </summary>
    public static double Magnitude(Vector3 a) {
        return Math.Sqrt(Math.Pow(a.x, 2) + Math.Pow(a.y, 2) + Math.Pow(a.z, 2));
    }

}

Classes

But what most distinguishes C# is that it is an object-oriented language, and as such, it’s primary form of encapsulation is classes and objects. The key idea behind encapsulation in an object-oriented language is that we encapsulate both state and behavior in the class definition. Let’s explore that idea more deeply in the next section.

State and Behavior

The data stored in a program at any given moment (in the form of variables, objects, etc.) is the state of the program. Consider a variable:

int a = 5;

The state of the variable a after this line is 5. If we then run:

a = a * 3;

The state is now 15. Consider the Vector3 struct we defined in the last section:

public struct Vector3 {
    public double x;
    public double y;
    public double z;

    // constructor
    public Vector3(double x, double y, double z) {
        this.x = x;
        this.y = y;
        this.z = z;
    }
}

If we create an instance of that struct in the variable b:

Vector3 b = new Vector3(1.2, 3.7, 5.6);

The state of our variable b is $\{1.2, 3.7, 5.6\}$. If we change one of b’s fields:

b.x = 6.0;

The state of our variable b is now $\{6.0, 3.7, 5.6\}$.

We can also think about the state of the program, which would be something like: $\{a: 5, b: \{x: 6.0, y: 3.7, z: 5.6\}\}$, or a state vector like: $|5, 6.0, 3.7, 5.6|$. We can therefore think of a program as a state machine. We can in fact, draw our entire program as a state table listing all possible legal states (combinations of variable values) and the transitions between those states. Techniques like this can be used to reason about our programs and even prove them correct!

This way of reasoning about programs is the heart of Automata Theory, a subject you may choose to learn more about if you pursue graduate studies in computer science.

What causes our program to transition between states? If we look at our earlier examples, it is clear that assignment is a strong culprit. Expressions clearly have a role to play, as do control-flow structures decide which transformations take place. In fact, we can say that our program code is what drives state changes - the behavior of the program.

Thus, programs are composed of both state (the values stored in memory at a particular moment in time) and behavior (the instructions to change that state).

Now, can you imagine trying to draw the state table for a large program? Something on the order of EPIC?

On the other hand, with encapsulation we can reason about state and behavior on a much smaller scale. Consider this function working with our Vector3 struct:

/// <summary>
/// Returns the the supplied vector scaled by the provided scalar
/// </summary>
public static Vector3 Scale(Vector3 vec, double scale) {
    double x = vec.x * scale;
    double y = vec.y * scale;
    double z = vec.z * scale;
    return new Vector3(x, y, z);
}

If this method was invoked with a vector $\langle4.0, 1.0, 3.4\rangle$ and a scalar $2.0$ our state table would look something like:

Because the parameters vec and scale, as well as the variables x, y, z, and the unnamed Vector3 we return are all defined only within the scope of the method, we can reason about them and the associated state changes independently of the rest of the program. Essentially, we have encapsulated a portion of the program state in our Vector3 struct, and encapsulated a portion of the program behavior in the static Vector3 library. This greatly simplifies both writing and debugging programs.

However, we really will only use the Vector3 library in conjunction with Vector3 structures, so it makes a certain amount of sense to define them in the same place. This is where classes and objects come into the picture, which we’ll discuss next.

Classes and Objects

The module-based encapsulation suggested by Parnas and his contemporaries grouped state and behavior together into smaller, self-contained units. Alan Kay and his co-developers took this concept a step farther. Alan Kay was heavily influenced by ideas from biology, and saw this encapsulation in similar terms to cells.

Biological cells are also encapsulated - the complex structures of the cell and the functions they perform are all within a cell wall. This wall is only bridged in carefully-controlled ways, i.e. cellular pumps that move resources into the cell and waste out. While single-celled organisms do exist, far more complex forms of life are made possible by many similar cells working together.

This idea became embodied in object-orientation in the form of classes and objects. An object is like a specific cell. You can create many, very similar objects that all function identically, but each have their own individual and different state. The class is therefore a definition of that type of object’s structure and behavior. It defines the shape of the object’s state, and how that state can change. But each individual instance of the class (an object) has its own current state.

Let’s re-write our Vector3 struct as a class using this concept:

/// <summary>A class representing a 3-element vector</summary>
public class Vector3 {

    /// <summary>The X component of the vector</summary>
    public double X;
    /// <summary>The Y component of the vector</summary>
    public double Y;
    /// <summary>The Z component of the vector</summary>
    public double Z;

    /// <summary>Constructs a new vector</summary>
    /// <param name="x">The value of the vector's x component</param>
    /// <param name="y">The value of the vector's y component</param>
    /// <param name="z">The value of the vector's z component</param>
    public Vector3(double x, double y, double z) {
        X = x;
        Y = y;
        Z = z;
    }

    /// <summary>Computes the dot product of this and <paramref name="other"/> vector</summary>
    /// <param name="other">The other vector to compute with</param>
    /// <returns>The dot product</returns>
    public double DotProduct(Vector3 other) {
        return X * other.X + Y * other.Y + Z * other.Z;
    }

    /// <summary>Scales this vector by <paramref name="scalar"/></summary>
    /// <paramref name="scalar">The value to scale by</paramref>
    public void Scale(double scalar) {
        X *= scalar;
        Y *= scalar;
        Z *= scalar;
    }
}

Here we have defined:

1. The structure of the object state - three doubles, X, Y, and Z
2. How the object is constructed - the Vector3() constructor that takes a value for the object’s initial state
3. Instructions for how that object state can be changed, i.e. our Scale() method

We can create as many objects from this class definition as we might want:

Vector3 one = new Vector3(1.0, 1.0, 1.0);
Vector3 up = new Vector3(0.0, 1.0, 0.0);
Vector3 a = new Vector3(5.4, -21.4, 3.11);

Conceptually, what we are doing is not that different from using a compound data type like a struct and a module of functions that work upon that struct. But practically, it means all the code for working with Vectors appears in one place. This arguably makes it much easier to find all the pertinent parts of working with vectors, and makes the resulting code better organized and easier to maintain and add features to.

Classes also provide additional benefits over structs in the form of polymorphism, which we’ll discuss in Chapter 2.

Information Hiding

Now let’s return to the concept of information hiding, and how it applies in object-oriented languages.

Unanticipated changes in state are a major source of errors in programs. Again, think back to the EPIC source code we looked at earlier. It may have seemed unusual now, but it used a common pattern from the early days of programming, where all the variables the program used were declared in one spot, and were global in scope (i.e. any part of the program could reassign any of those variables).

If we consider the program as a state machine, that means that any part of the program code could change any part of the program state. Provided those changes were intended, everything works fine. But if the wrong part of the state was changed problems would ensue.

For example, if you were to make a typo in the part of the program dealing with water run-off in a field which ends up assigning a new value to a variable that was supposed to be used for crop growth, you’ve just introduced a very subtle and difficult-to-find error. When the crop growth modeling functionality fails to work properly, we’ll probably spend serious time and effort looking for a problem in the crop growth portion of the code… but the problem doesn’t lie there at all!

Access Modifiers

There are several techniques involved in data hiding in an object-oriented language. One of these is access modifiers, which determine what parts of the program code can access a particular class, field, property, or method. Consider a class representing a student:

public class Student {
    private string _first;
    private string _last;
    private uint _wid;

    public Student(string first, string last, uint wid) {
        this._first = first;
        this._last = last;
        this._wid = wid;
    }
}

By using the access modifier private, we have indicated that our fields _first, _last, and _wid cannot be accessed (seen or assigned to) outside of the code that makes up the Student class. If we were to create a specific student:

Student willie = new Student("Willie", "Wildcat", 888888888);

We would not be able to change his name, i.e. willie._first = "Bob" would fail, because the field _first is private. In fact, we cannot even see his name, so Console.WriteLine(willie._first); would also fail.

If we want to allow a field or method to be accessible outside of the object, we must declare it public. While we can declare fields public, this violates the core principles of encapsulation, as any outside code can modify our object’s state in uncontrolled ways.

Accessor Methods

Instead, in a true object-oriented approach we would write public accessor methods, a.k.a. getters and setters (so called because they get or set the value of a field). These methods allow us to see and change field values in a controlled way. Adding accessors to our Student class might look like:

/// <summary>A class representing a K-State student</summary>
public class Student
{
    private string _first;
    private string _last;
    private uint _wid;

    /// <summary>Constructs a new student object</summary>
    /// <param name="first">The new student's first name</param>
    /// <param name="last">The new student's last name</param>
    /// <param wid="wid">The new student's Wildcat ID number</param>
    public Student(string first, string last, uint wid)
    {
        _first = first;
        _last = last;
        _wid = wid;
    }

    /// <summary>Gets the first name of the student</summary>
    /// <returns>The student's first name</returns>
    public string GetFirst()
    {
        return _first;
    }

    /// <summary>Sets the first name of the student</summary>
    public void SetFirst(string value)
    {
        if (value.Length > 0) _first = value;
    }

    /// <summary>Gets the last name of the student</summary>
    /// <returns>The student's last name</returns>
    public string GetLast()
    {
        return _last;
    }

    /// <summary>Sets the last name of the student</summary>
    /// <param name="value">The new name</summary>
    /// <remarks>The <paramref name="value"/> must be a non-empty string</remarks>
    public void SetLast(string value)
    {
        if (value.Length > 0) _last = value;
    }

    /// <summary>Gets the student's Wildcat ID Number</summary>
    /// <returns>The student's Wildcat ID Number</returns>
    public uint GetWid()
    {
        return _wid;
    }

    /// <summary>Gets the full name of the student</summary>
    /// <returns>The first and last name of the student as a string</returns>
    public string GetFullName()
    {
        return $"{_first} {_last}"
    }
}

Notice how the SetFirst() and SetLast() method check that the provided name has at least one character? We can use setters to make sure that we never allow the object state to be set to something that makes no sense.

Also, notice that the _wid field only has a getter. This effectively means once a student’s Wid is set by the constructor, it cannot be changed. This allows us to share data without allowing it to be changed outside of the class.

Finally, the GetFullName() is also a getter method, but it does not have its own private backing field. Instead it derives its value from the class state. We sometimes call this a derived getter for that reason.

C# Properties

While accessor methods provide a powerful control mechanism in object-oriented languages, they also require a lot of typing the same code syntax over and over (we often call this boilerplate). Many languages therefore introduce a mechanism for quickly defining basic accessors. In C#, we have Properties. Let’s rewrite our Student class with Properties:

public class Student {

    private string _first;
    /// <summary>The student's first name</summary>
    public string First {
        get { return _first; }
        set { if(value.Length > 0) _first = value;}
    }

    private string _last;
    /// <summary>The student's last name</summary>
    public string Last {
        get { return _last; }
        set { if(value.Length > 0) _last = value; }
    }

    private uint _wid;
    /// <summary>The student's Wildcat ID number</summary>
    public uint Wid {
        get { return this._wid; }
    }

    /// <summary>The student's full name</summary>
    public string FullName 
    {
      get 
      {
        return $"{First} {Last}"
      }
    }

    /// <summary>The student's nickname</summary>
    public string Nickname { get; set; }

    /// <summary>Constructs a new student object</summary>
    /// <param name="first">The new student's first name</param>
    /// <param name="last">The new student's last name</param>
    /// <param name="nick">The new student's nickname</param>
    /// <param wid="wid">The new student's Wildcat ID number</param>
    public Student(string first, string last, string nick, uint wid) {
        _first = first;
        _last = last;
        Nickname = nick;
        _wid = wid;
    }
}

If you compare this example to the previous one, you will note that the code contained in bodies of the get and set are identical to the corresponding getter and setter methods. Essentially, C# properties are shorthand for writing out the accessor methods. In fact, when you compile a C# program it transforms the get and set back into methods, i.e. the get in first is used to generate a method named get_First().

While properties are methods, the syntax for working with them in code is identical to that of fields, i.e. if we were to create and then print a Student’s identifying information, we’d do something like:

Student willie = new Student("Willie", "Wildcat", "WillieCat", 99999999);
Console.Write("Hello, ")
Console.WriteLine(willie.FullName);
Console.Write("Your WID is:");
Console.WriteLine(willie.Wid);

Note too that we can declare properties with only a get or a set body, and that properties can be derived from other state rather than having a private backing field.

public string FullName 
    {
        get 
        {
            return $"{First} {Last}"
        }
    }

Auto-Property Syntax

Not all properties need to do extra logic in the get or set body. Consider our Vector3 class we discussed earlier. We used public fields to represent the X, Y, and Z components, i.e.:

public double X = 0;

If we wanted to switch to using properties, the X property would end up like this:

private double _x = 0;
public double X 
{
  get 
  {
    return _x;
  }
  set 
  {
    _x = value;
  }
}

Which seems like a lot more work for the same effect. To counter this perception and encourage programmers to use properties even in cases like this, C# also supports auto-property syntax. An auto-property is written like:

public double X {get; set;} = 0;

Note the addition of the {get; set;} - this is what tells the compiler we want a property and not a field. When compiled, this code is transformed into a full getter and setter whose bodies match the basic get and set in the example above. The compiler even creates a private backing field (but we cannot access it in our code, because it is only created at compile time). Any time you don’t need to do any additional logic in a get or set, you can use this syntax.

Note that in the example above, we set a default value of 0. You can omit setting a default value. You can also define a get-only autoproperty that always returns the default value (remember, you cannot access the compiler-generated backing field, so it can never be changed):

public double Pi {get;} = 3.14;

In practice, this is effectively a constant field, so consider carefully if it is more appropriate to use that instead:

public const PI = 3.14;

While it is possible to create a set-only auto-property, you will not be able access its value, so it is of limited use.

Expression-Bodied Members

Later versions of C# introduced a concise way of writing functions common to functional languages known as lambda syntax, which C# calls Expression-Bodied Members.

Properties can be written using this concise syntax. For example, our FullName get-only derived property in the Student written as an expression-bodied read-only property would be:

public FullName => $"{FirstName} {LastName}"

Note the use of the arrow formed by an equals and greater than symbol =>. Properties with both a getter and setter can also be written as expression-bodied properties. For example, our FirstName property could be rewritten:

public FirstName 
{
  get => _first;
  set => if(value.Length > 0) _first = value;
}

This syntax works well if your property bodies are a single expression. However, if you need multiple lines, you should use the regular property syntax instead (you can also mix and match, i.e. use an expression-bodied get with a regular set).

Different Access Levels

It is possible to declare your property as public and give a different access level to one of the accessors, i.e. if we wanted to add a GPA property to our student:

public double GPA { get; private set; } = 4.0;

In this case, we can access the value of the GPA outside of the student class, but we can only set it from code inside the class. This approach works with all ways of defining a property.

Init Property Accessor

C# 9.0 introduced a third accessor, init. This also sets the value of the property, but can only be used when the class is being initialized, and it can only be used once. This allows us to have some properties that are immutable (unable to be changed).

Our student example treats the Wid as immutable, but we can use the init keyword with an auto-property for a more concise representation:

public uint Wid {get; init;}

And in the constructor, replace setting the backing field (_wid = wid) with setting the property (Wid = wid). This approach is similar to the public property/private setter, but won’t allow the property to ever change once declared.

Programs in Memory

Before we move on to our next concept, it is helpful to explore how programs use memory. Remember that modern computers are stored program computers, which means the program as well as the data are stored in the computer’s memory. A Universal Turing Machine, the standard example of stored program computer, reads the program from the same paper tape that it reads its inputs to and writes its output to. In contrast, to load a program in the ENIAC, the first electronic computer in the United States, programmers had to physically rewire the computer (it was later modified to be a stored-program computer).

When a program is run, the operating system allocates part of the computer’s memory (RAM) for the program to use. This memory is divided into three parts - the static memory, the stack, and the heap.

The program code itself is copied into the static section of that memory, along with any literals (1, "Foo"). Additionally, the space to hold any variables that are declared static is allocated here. The reason this memory space is called static is that it is allocated when the program begins, and remains allocated for as long as the program is running. It does not grow or shrink (though the value of static variables may change).

The stack is where the space for scoped variables is allocated. We often call it the stack because functionally it is used like the stack data structure. The space for global variables is allocated first, at the “bottom” of the stack. Then, every time the program enters a new scope (i.e. a new function, a new loop body, etc.) the variables declared there are allocated on the stack. When the program exits that scope (i.e. the function returns, the loop ends), the memory that had been reserved for those values is released (like the stack pop operation).

Thus, the stack grows and shrinks over the life of the program. The base of the stack is against the static memory section, and it grows towards the heap. If it grows too much, it runs out of space. This is the root cause of the nefarious stack overflow exception. The stack has run out of memory, most often because an infinite recursion or infinite loop.

Finally, the heap is where dynamically allocated memory comes from - memory the programmer specifically reserved for a purpose. In C programming, you use the calloc(), malloc(), or realloc() to allocate space manually. In Object-Oriented languages, the data for individual objects are stored here. Calling a constructor with the new keyword allocates memory on the heap, the constructor then initializes that memory, and then the address of that memory is returned. We’ll explore this process in more depth shortly.

This is also where the difference between a value type and a reference type comes into play. Value types include numeric objects like integers, floating point numbers, and also booleans. Reference types include strings and classes. When you create a variable that represents a value type, the memory to hold its value is created in the stack. When you create a variable to hold a reference type, it also has memory in the stack - but this memory holds a pointer to where the object’s actual data is allocated in the heap. Hence the term reference type, as the variable doesn’t hold the object’s data directly - instead it holds a reference to where that object exists in the heap!

This is also where null comes from - a value of null means that a reference variable is not pointing at anything.

The objects in the heap are not limited to a scope like the variables stored in the stack. Some may exist for the entire running time of the program. Others may be released almost immediately. Accordingly, as this memory is released, it leaves “holes” that can be re-used for other objects (provided they fit). Many modern programming languages use a garbage collector to monitor how fragmented the heap is becoming, and will occasionally reorganize the data in the heap to make more contiguous space available.

Objects in Memory

We often talk about the class as a blueprint for an object. This is because classes define what properties and methods an object should have, in the form of the class definition. An object is created from this blueprint by invoking the class’ constructor. Consider this class representing a planet:

/// <summary>
/// A class representing a planet
// </summary>
public class Planet {

    /// <summary>
    /// The planet's mass in Earth Mass units (~5.9722 x 10^24kg)
    /// </summary>
    private double mass;
    public double Mass 
    {
        get { return mass; }
    }

    /// <summary>
    /// The planet's radius in Earth Radius units (~6.738 x 10^6m)
    /// </summary>
    private double radius;
    public double Radius 
    {
        get { return radius; }
    }

    /// <summary>
    /// Constructs a new planet
    /// <param name="mass">The planet's mass</param>
    /// <param name="radius">The planet's radius</param>
    public Planet(double mass, double radius) 
    {
        this.mass = mass;
        this.radius = radius;
    }

}

It describes a planet as having a mass and a radius. But a class does more than just labeling the properties and fields and providing methods to mutate the state they contain. It also specifies how memory needs to be allocated to hold those values as the program runs. In memory, we would need to hold both the mass and radius values. These are stored side-by-side, as a series of bits that are on or off. You probably remember from CIS 115 that a double is stored as a sign bit, mantissa and exponent. This is also the case here - a C# double requires 64 bits to hold these three parts, and we can represent it with a memory diagram:

We can create a specific planet by invoking its constructor, i.e.:

new Planet(1, 1);

This allocates (sets aside) the memory to hold the planet, and populates the mass and radius with the supplied values. We can represent this with a memory diagram:

With memory diagrams, we typically write the values of variables in their human-readable form. Technically the values we are storing are in binary, and would each be 0000000000010000000000000000000000000000000000000000000000000001, so our overall object would be the bits: 00000000000100000000000000000000000000000000000000000000000000010000000000010000000000000000000000000000000000000000000000000001.

And this is exactly how it is stored in memory! The nice boxes we drew in our memory diagram are a tool for us to reason about the memory, not something that actually exists in memory. Instead, the compiler determines the starting point for each double by looking at the structure defined in our class, i.e. the first field defined is mass, so it will be the first 64 bits of the object in memory. The second field is radius, so it starts 65 bits into the object and consists of the next (and final) 64 bits.

If we assign the created Planet object to a variable, we allocate memory for that variable:

Planet earth = new Planet(1, 1);

Unlike our double and other primitive values, this allocated memory holds a reference (a starting address of the memory where the object was allocated). We indicate this with a box and arrow connecting the variable and object in our memory diagram:

A reference is either 32 bits (on a computer with a 32-bit CPU) or 64 bits (on a computer with a 64-bit CPU), and essentially is an offset from the memory address $0$ indicating where the object will be located in memory (in the computer’s RAM). You’ll see this in far more detail in CIS 450 - Computer Architecture and Operations, but the important idea for now is the variable stores where the object is located in memory not the object’s data itself. This is also why if we define a class variable but don’t assign it an object, i.e.:

Planet mars;

The value of this variable will be null. It’s because it doesn’t point anywhere!

Returning to our Earth example, earth is an instance of the class Planet. We can create other instances, i.e.

Planet mars = new Planet(0.107, 0.53);

We can even create a Planet instance to represent one of the exoplanets discovered by NASA’s TESS:

Planet hd21749b = new Planet(23.20, 2.836);

Let’s think more deeply about the idea of a class as a blueprint. A blueprint for what, exactly? For one thing, it serves to describe the state of the object, and helps us label that state. If we were to check our variable mars’ radius, we do so based on the property Radius defined in our class:

mars.Radius

This would follow the mars reference to the Planet object it represents, and access the second group of 64 bits stored there, interpreting them as a double (basically it adds 64 to the reference address and then reads the next 64 bits)

State and memory are clearly related - the current state is what data is stored in memory. It is possible to take that memory’s current state, write it to persistent storage (like the hard drive), and then read it back out at a later point in time and restore the program to exactly the state we left it with. This is actually what Windows does when you put it into hibernation mode.

The process of writing out the state is known as serialization, and it’s a topic we’ll revisit later.

public class Simple {
    public static int A;
    public int B;

    public Simple(int a, int b) {
        this.A = a;
        this.B = b;
    }
}

Simple first = new Simple(10, 12);
Simple second = new Simple(8, 5);

first.A = 3;

C# Object Initialization

With our broader understanding of objects in memory, let’s re-examine something you’ve been working with already, how the values in that memory are initialized (set to their initial values). In C#, there are four primary ways a value is initialized:

1. By zeroing the memory
2. By setting a default value
3. By the constructor
4. With Initialization syntax

This also happens to be the order in which these operations occur - i.e. the default value can be overridden by code in the constructor. Only after all of these steps are completed is the initialized object returned from the constructor.

Zeroing the Memory

This step is actually done for you - it is a feature of the C# language. Remember, allocated memory is simply a series of bits. Those bits have likely been used previously to represent something else, so they will already be set to 0s or 1s. Once you treat it as a variable, it will have a specific meaning. Consider this statement:

int foo;

That statement allocates the space to hold the value of foo. But what is that value? In many older languages, it would be whatever is specified by how the bits were set previously - i.e. it could be any integer within the available range. And each time you ran the program, it would probably be a different value! This is why it is always a good idea to assign a value to a variable immediately after you declare it.

The creators of C# wanted to avoid this potential problem, so in C# any memory that is allocated by a variable declaration is also zeroed (all bits are set to 0). Exactly what this means depends on the variable’s type. Essentially, for numerics (integers, floating points, etc) the value would be 0, for booleans it would be false. And for reference types, the value would be null.

Default Values

A second way a field’s value can be set is by assigning a default value after it is declared. Thus, if we have a private backing _count in our CardDeck class, we could set it to have a default value of 52:

public class CardDeck
{
  private int _count = 52;

  public int Count 
  {
    get 
    {
      return _count;
    }
    set 
    {
      _count = value;
    }
  }
}

This ensures that _count starts with a value of 52, instead of 0.

We can also set a default value when using auto-property syntax:

public class CardDeck
{
  public int Count {get; set;} = 52;
}

public class CardDeck
{
  public int Count {get; set;} = 52;
  public int PricePerCard {get;} = 5m / Count; 
}

Constructors

This brings us to the constructor, the standard way for an object-oriented program to initialize the state of the object as it is created. In C#, the constructor always has the same name as the class it constructs and has no return type. For example, if we defined a class Square, we might type:

public class Square {
    public float length;

    public Square(float length) {
        this.length = length;
    }

    public float Area() {
        return length * length;
    }
}

Note that unlike the regular method, Area(), our constructor Square() does not have a return type. In the constructor, we set the length field of the newly constructed object to the value supplied as the parameter length. Note too that we use the this keyword to distinguish between the field length and the parameter length. Since both have the same name, the C# compiler assumes we mean the parameter, unless we use this.length to indicate the field that belongs to this - i.e. this object.

Parameterless Constructors

A parameterless constructor is one that does not have any parameters. For example:

public class Ball {
    private int x;
    private int y;

    public Ball() {
        x = 50;
        y = 10;
    }
}

Notice how no parameters are defined for Ball() - the parentheses are empty.

If we don’t provide a constructor for a class the C# compiler automatically creates a parameterless constructor for us, i.e. the class Bat:

public class Bat {
    private bool wood = true;
}

Can be created by invoking new Bat(), even though we did not define a constructor for the class. If we define any constructors, parameterless or otherwise, then the C# compiler will not create an automatic parameterless constructor.

Initializer Syntax

Finally, C# introduces some special syntax for setting initial values after the constructor code has run, but before initialization is completed - Object initializers. For example, if we have a class representing a rectangle:

public class Rectangle 
{
  public int Width {get; set;}
  public int Height {get; set;}
}

We could initialize it with:

Rectangle r = new Rectangle() {
  Width = 20,
  Height = 10
};

The resulting rectangle would have its width and height set to 20 and 10 respectively before it was assigned to the variable r.

Message Passing

The second criteria Alan Kay set for object-oriented languages was message passing. Message passing is a way to request that a unit of code engage in a behavior, i.e. changing its state, or sharing some aspect of its state.

Consider the real-world analogue of a letter sent via the postal service. Such a message consists of: an address the message needs to be sent to, a return address, the message itself (the letter), and any data that needs to accompany the letter (the enclosures). A specific letter might be a wedding invitation. The message includes the details of the wedding (the host, the location, the time), an enclosure might be a refrigerator magnet with these details duplicated. The recipient should (per custom) send a response to the host addressed to the return address letting them know if they will be attending.

In an object-oriented language, message passing primarily takes the form of methods. Let’s revisit our example Vector3 class:

public class Vector3 {
    public double X {get; set;}
    public double Y {get; set;}
    public double Z {get; set;}

    /// <summary> 
    /// Creates a new Vector3 object
    /// </summary>
    public Vector3(double x, double y, double z) {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }

    /// <summary>
    /// Computes the dot product of this vector and another one 
    /// </summary>
    /// <param name="other">The other vector</param>
    public double DotProduct(Vector3 other) {
        return this.X * other.X + this.Y * other.Y + this.Z * other.Z;
    }
}

And let’s use its DotProduct() method:

Vector3 a = new Vector3(1, 1, 2);
Vector3 b = new Vector3(4, 2, 1);
double c = a.DotProduct(b);

Consider the invocation of a.DotProduct(b) above. The method name, DotProduct provides the details of what the message is intended to accomplish (the letter). Invoking it on a specific variable, i.e. a, tells us who the message is being sent to (the recipient address). The return type indicates what we need to send back to the recipient (the invoking code), and the parameters provide any data needed by the class to address the task (the enclosures).

Let’s define a new method for our Vector3 class that emphasizes the role message passing plays in mutating object state:

public class Vector3 {
    public double X {get; set;}
    public double Y {get; set;}
    public double Z {get; set;}

    /// <summary> 
    /// Creates a new Vector3 object
    /// </summary>
    public Vector3(double x, double y, double z) {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }

    public void Normalize() {
        var magnitude = Math.Sqrt(Math.pow(this.X, 2) + Math.Pow(this.Y, 2) + Math.Pow(this.Z, 2));
        this.X /= magnitude;
        this.Y /= magnitude;
        this.Z /= magnitude;
    }
}

We can now invoke the Normalize() method on a Vector3 to mutate its state, shortening the magnitude of the vector to length 1.

Vector3 f = new Vector3(9.0, 3.0, 2.0);
f.Normalize();

Note how here, f is the object receiving the message Normalize. There is no additional data needed, so there are no parameters being passed in. Our earlier DotProduct() method took a second vector as its argument, and used that vector’s values to mutate its state.

Message passing therefore acts like those special molecular pumps and other gate mechanisms of a cell that control what crosses the cell wall. The methods defined on a class determine how outside code can interact with the object. An extra benefit of this approach is that a method becomes an abstraction for the behavior of the code, and the associated state changes it embodies. As a programmer using the method, we don’t need to know the exact implementation of that behavior - just what data we need to provide, and what it should return or how it will alter the program state. This makes it far easier to reason about our program, and also means we can change the internal details of a class (perhaps to make it run faster) without impacting the other aspects of the program.

This is also the reason we want to use getters and setters (or properties in C#) instead of public fields in an object-oriented language. Getters, setters, and C# properties are all methods, and therefore are a form of message passing, and they ensure that outside code is not modifying the state of the object (rather, the outside code is requesting the object to change its state). It is a fine distinction, but one that can be very important.

Summary

In this chapter, we looked at how Object-Orientation adopted the concept of encapsulation to combine related state and behavior within a single unit of code, known as a class. We also discussed the three key features found in the implementation of classes and objects in Object-Oriented languages:

1. Encapsulation of state and behavior within an object, defined by its class
2. Information hiding applied to variables defined within that class to prevent unwanted mutations of object state
3. Message passing to allow controlled mutations of object state in well-defined ways

We explored how objects are instances of a class created through invoking a constructor method, and how each object has its own independent state but shares behavior definitions with other objects constructed from the same class. We discussed several different ways of looking at and reasoning about objects - as a state machine, and as structured data stored in memory. We saw how the constructor creates the memory to hold the object state and initializes its values. We saw how access modifiers and accessor methods can be used to limit and control access to the internal state of an object through message passing.

Finally, we explored how all of these concepts are implemented in the C# language.

Introduction

The term polymorphism means many forms. In computer science, it refers to the ability of a single symbol (i.e. a function or class name) to represent multiple types. Some form of polymorphism can be found in nearly all programming languages.

While encapsulation of state and behavior into objects is the most central theoretical idea of object-oriented languages, polymorphism - specifically in the form of inheritance is a close second. In this chapter we’ll look at how polymorphism is commonly implemented in object-oriented languages.

Key Terms

Some key terms to learn in this chapter are:

• Polymorphism
• Type
• Type Checking
• Casting
• Implicit Casting
• Explicit Casting
• Interface
• Inheritance
• Superclass
• Subclass
• Abstract Classes

C# Keywords:

• Interface
• protected
• abstract
• virtual
• override
• sealed
• as
• is
• dynamic

Types

Before we can discuss polymorphism in detail, we must first understand the concept of types. In computer science, a type is a way of categorizing a variable by its storage strategy, i.e., how it is represented in the computer’s memory.

You’ve already used types extensively in your programming up to this point. Consider the declaration:

int number = 5;

The variable number is declared to have the type int. This lets the .NET interpreter know that the value of number will be stored using a specific scheme. This scheme will use 32 bits and contain the number in Two’s complement binary form. This form, and the number of bytes, allows us to represent numbers in the range -2,147,483,648 to 2,147,483,647. If we need to store larger values, we might instead use a long which uses 64 bits of storage. Or, if we only need positive numbers, we might instead use a uint, which uses 32 bits and stores the number in regular base 2 (binary) form.

This is why languages like C# provide multiple integral and float types. Each provides a different representation, representing a tradeoff between memory required to store the variable and the range or precision that variable can represent.

In addition to integral and float types, most programming languages include types for booleans, characters, arrays, and often strings. C# is no exception - you can read about its built-in value types in the documentation.

User-Defined Types

In addition to built-in types, most programming languages support user-defined types, that is, new types defined by the programmer. For example, if we were to define a C# enum:

public enum Grade {
  A,
  B,
  C,
  D,
  F
}

Defines the type Grade. We can then create variables with that type:

Grade courseGrade = Grade.A;

Similarly, structs provide a way of creating user-defined compound data types.

Classes are Types

In an object-oriented programming language, a Class also defines a new type. As we discussed in the previous chapter, the Class defines the structure for the state (what is represented) and memory (how it is represented) for objects implementing that type. Consider the C# class Student:

public class Student {
  // backing variables
  private float creditPoints = 0;
  private uint creditHours = 0;

  /// <summary>
  /// Gets and sets first name.
  /// </summary>
  public string First { get; set; }

  /// <summary>
  /// Gets and sets last name.
  /// </summary>
  public string Last { get; set; }

  /// <summary>
  /// Gets the student's GPA
  /// </summary>
  public float GPA {
    get {
      return creditPoints / creditHours;
    }
  }

  /// <summary>
  /// Adds a final grade for a course to the
  // student's GPA.
  /// </summary>
  /// <param name="grade">The student's final letter grade in the course</param>
  /// <param name="hours">The course's credit hours</param>
  public void AddCourseGrade(Grade grade, uint hours) {
    this.creditHours += hours;
    switch(grade) {
      case Grade.A:
        this.creditPoints += 4.0 * hours;
        break;
      case Grade.B:
        this.creditPoints += 3.0 * hours;
        break;
      case Grade.C:
        this.creditPoints += 2.0 * hours;
        break;
      case Grade.D:
        this.creditPoints += 1.0 * hours;
        break;
      case Grade.F:
        this.creditPoints += 0.0 * hours;
        break;
    }
  }
}

If we want to create a new student, we would create an instance of the class Student which is an object of type Student:

Student willie = new Student("Willie", "Wildcat");

Hence, the type of an object is the class it is an instance of. This is a staple across all object-oriented languages.

Static vs. Dynamic Typed Languages

A final note on types. You may hear languages being referred to as statically or dynamically typed. A statically typed language is one where the type is set by the code itself, either explicitly:

int foo = 5;

or implicitly (where the compiler determines the type based on the assigned value):

var bar = 6;

In a statically typed language, a variable cannot be assigned a value of a different type, i.e.:

foo = 8.3;

Will fail with an error, as a float is a different type than an int. Similarly, because bar has an implied type of int, this code will fail:

bar = 4.3;

However, we can cast the value to a new type (changing how it is represented), i.e.:

foo = (int)8.9;

For this to work, the language must know how to perform the cast. The cast may also lose some information - in the above example, the resulting value of foo is 8 (the fractional part is discarded).

In contrast, in a dynamically typed language the type of the variable changes when a value of a different type is assigned to it. For example, in JavaScript, this expression is legal:

var a = 5;
a = "foo";

and the type of a changes from int (at the first assignment) to string (at the second assignment).

C#, Java, C, C++, and Kotlin are all statically typed languages, while Python, JavaScript, and Ruby are dynamically typed languages.

Interfaces

If we think back to the concept of message passing in object-oriented languages, it can be useful to think of the collection of public methods available in a class as an interface, i.e., a list of messages you can dispatch to an object created from that class. When you were first learning a language (and probably even now), you find yourself referring to these kinds of lists, either in the language documentation, or via Intellisense in Visual Studio.

Essentially, programmers use these ‘interfaces’ to determine what methods can be invoked on an object. In other words, which messages can be passed to the object. This ‘interface’ (note the lowercase i) is determined by the class definition, specifically what methods it contains.

In dynamically typed programming languages, like Python, JavaScript, and Ruby, if two classes accept the same message, you can treat them interchangeably, i.e. the Kangaroo class and Car class both define a jump() method, you could populate a list with both, and call the jump() method on each:

var jumpables = [new Kangaroo(), new Car(), new Kangaroo()];
for(int i = 0; i < jumpables.length; i++) {
    jumpables[i].jump();
}

This is sometimes called duck typing, from the sense that “if it walks like a duck, and quacks like a duck, it might as well be a duck.”

However, for statically typed languages we must explicitly indicate that two types both possess the same message definition, by making the interface explicit. We do this by declaring an interface. I.e., the interface for classes that possess a parameter-less jump method might be:

/// <summary>
/// An interface indicating an object's ability to jump
/// </summary>
public interface IJumpable {
    
    /// <summary>
    /// A method that causes the object to jump
    /// </summary>
    void Jump();
}

In C#, it is common practice to preface interface names with the character I. The interface declaration defines an ‘interface’ - the shape of the messages that can be passed to an object implementing the interface - in the form of a method signature. Note that this signature does not include a body, but instead ends in a semicolon (;). An interface simply indicates the message to be sent, not the behavior it will cause! We can specify as many methods in an interface declaration as we want.

Also note that the method signatures in an interface declaration do not have access modifiers. This is because the whole purpose of defining an interface is to signify methods that can be used by other code. In other words, public access is implied by including the method signature in the interface declaration.

This interface can then be implemented by other classes by listing it after the class name, after a colon :. Any Class declaration implementing the interface must define public methods whose signatures match those were specified by the interface:

/// <summary>A class representing a kangaroo</summary>
public class Kangaroo : IJumpable 
{
    /// <summary>Causes the Kangaroo to jump into the air</summary>
    public void Jump() {
        // TODO: Implement jumping...
    }
}

/// <summary>A class representing an automobile</summary>
public class Car : IJumpable 
{
    /// <summary>Helps a stalled car to start by providing electricity from another car's battery</summary>
    public void Jump() {
        // TODO: Implement jumping a car...
    }

    /// <summary>Starts the car</summary>
    public void Start() {
        // TODO: Implement starting a car...
    }
}

We can then treat these two disparate classes as though they shared the same type, defined by the IJumpable interface:

List<IJumpable> jumpables = new List<IJumpable>() {new Kangaroo(), new Car(), new Kangaroo()};
for(int i = 0; i < jumpables.Count; i++)
{
    jumpables[i].Jump();
}

Note that while we are treating the Kangaroo and Car instances as IJumpable instances, we can only invoke the methods defined in the IJumpable interface, even if these objects have other methods. Essentially, the interface represents a new type that can be shared amongst disparate objects in a statically-typed language. The interface definition serves to assure the static type checker that the objects implementing it can be treated as this new type - i.e. the Interface provides a mechanism for implementing polymorphism.

We often describe the relationship between the interface and the class that implements it as a is-a relationship, i.e. a Kangaroo is an IJumpable (i.e. a Kangaroo is a thing that can jump). We further distinguish this from a related polymorphic mechanism, inheritance, by the strength of the relationship. We consider the relationship between interfaces and classes implementing them to be weak is-a connections. For example, other than the shared interface, a Kangaroo and a Car don’t have much to do with one another.

A C# class can implement as many interfaces as we want, they just need to be separated by commas, i.e.:

public class Frog : IJumpable, ICroakable, ICatchFlies
{
    // TODO: Implement frog class...
}

Object Inheritance

In an object-oriented language, inheritance is a mechanism for deriving part of a class definition from another existing class definition. This allows the programmer to “share” code between classes, reducing the amount of code that must be written.

Consider a Student class:

/// <summary>
/// A class representing a student
/// </summary>
public class Student {

  // private backing variables
  private double hours;
  private double points;

  /// <summary>
  /// Gets the students' GPA
  /// </summary>
  public double GPA {
    get {
      return points / hours;
    }
  }

  /// <summary>
  /// Gets or sets the first name
  /// </summary>
  public string First { get; set; }

  /// <summary>
  /// Gets or sets the last name
  /// </summary>
  public string Last { get; set; }

  /// <summary>
  /// Constructs a new instance of Student
  /// </summary>
  /// <param name="first">The student's first name </param>
  /// <param name="last">The student's last name</param>
  public Student(string first, string last) {
    this.First = first;
    this.Last = last;
  }

  /// <summary>
  /// Adds a new course grade to the student's record.
  /// </summary>
  /// <param name="creditHours">The number of credit hours in the course </param>
  /// <param name="finalGrade">The final grade earned in the course</param>
  ///
  public void AddCourseGrade(uint creditHours, Grade finalGrade) {
    this.hours += creditHours;
    switch(finalGrade) {
      case Grade.A:
        this.points += 4 * creditHours;
        break;
      case Grade.B:
        this.points += 3 * creditHours;
        break;
      case Grade.C:
        this.points += 2 * creditHours;
        break;
      case Grade.D:
        this.points += 1 * creditHours;
        break;
    }
  }
}

This would work well for representing a student. But what if we are representing multiple kinds of students, like undergraduate and graduate students? We’d need separate classes for each, but both would still have names and calculate their GPA the same way. So it would be handy if we could say “an undergraduate is a student, and has all the properties and methods a student has” and “a graduate student is a student, and has all the properties and methods a student has.” This is exactly what inheritance does for us, and we often describe it as a is a relationship. We distinguish this from the interface mechanism we looked at earlier by saying it is a strong is a relationship, as an Undergraduate student is, for all purposes, also a Student.

Let’s define an undergraduate student class:

/// <summary>
/// A class representing an undergraduate student
/// </summary>
public class UndergraduateStudent : Student {

  /// <summary>
  /// Constructs a new instance of UndergraduateStudent
  /// </summary>
  /// <param name="first">The student's first name </param>
  /// <param name="last">The student's last name</param>
  public UndergraduateStudent(string first, string last) : base(first, last) {
  }
}

In C#, the : indicates inheritance - so public class UndergraduateStudent : Student indicates that UndergraduateStudent inherits from (is a) Student. Thus, it has properties First, Last, and GPA that are inherited from Student. Similarly, it inherits the AddCourseGrade() method.

In fact, the only method we need to define in our UndergraduateStudent class is the constructor - and we only need to define this because the base class has a defined constructor taking two parameters, first and last names. This Student constructor must be invoked by the UndergraduateStudent constructor - that’s what the :base(first, last) portion does - it invokes the Student constructor with the first and last parameters passed into the UndergraduateStudent constructor.

Inheritance, State, and Behavior

Let’s define a GraduateStudent class as well. This will look much like an UndergraduateStudent, but all graduates have a bachelor’s degree:

/// <summary>
/// A class representing a graduate student
/// </summary>
public class GraduateStudent : Student {

  /// <summary>
  /// Gets the student's bachelor degree
  /// </summary>
  public string BachelorDegree {
    get; private set;
  }

  /// <summary>
  /// Constructs a new instance of GraduateStudent
  /// </summary>
  /// <param name="first">The student's first name </param>
  /// <param name="last">The student's last name</param>
  /// <param name="degree">The student's bachelor degree</param>
  public GraduateStudent(string first, string last, string degree) : base(first, last) {
    BachelorDegree = degree;
  }
}

Here we add a property for BachelorDegree. Since it’s setter is marked as private, it can only be written to by the class, as is done in the constructor. To the outside world, it is treated as read-only.

Thus, the GraduateStudent has all the state and behavior encapsulated in Student, plus the additional state of the bachelor’s degree title.

The protected Keyword

What you might not expect is that any fields declared private in the base class are inaccessible in the derived class. Thus, the private fields points and hours cannot be used in a method defined in GraduateStudent. This is again part of the encapsulation and data hiding ideals - we’ve encapsulated and hidden those variables within the base class, and any code outside that assembly, even in a derived class, is not allowed to mess with it.

However, we often will want to allow access to such variables in a derived class. C# uses the access modifier protected to allow for this access in derived classes, but not the wider world.

Inheritance and Memory

What happens when we construct an instance of GraduateStudent? First, we invoke the constructor of the GraduateStudent class:

GraduateStudent bobby = new GraduateStudent("Bobby", "TwoSocks", "Economics");

This constructor then invokes the constructor of the base class, Student, with the arguments "Bobby" and "TwoSocks". Thus, we allocate space to hold the state of a student, and populate it with the values set by the constructor. Finally, execution returns to the derived class of GraduateStudent, which allocates the additional memory for the reference to the BachelorDegree property. Thus, the memory space of the GraduateStudent contains an instance of the Student, somewhat like nesting dolls.

Because of this, we can treat a GraduateStudent object as a Student object. For example, we can store it in a list of type Student, along with UndergraduateStudent objects:

List<Student> students = new List<Student>();
students.Add(bobby);
students.Add(new UndergraduateStudent("Mary", "Contrary"));

Because of their relationship through inheritance, both GraduateStudent class instances and UndergraduateStudent class instances are considered to be of type Student, as well as their supertypes.

Nested Inheritance

We can go as deep as we like with inheritance - each base type can be a superclass of another base type, and has all the state and behavior of all the inherited base classes.

This said, having too many levels of inheritance can make it difficult to reason about an object. In practice, a good guideline is to limit nested inheritance to two or three levels of depth.

Abstract Classes

If we have a base class that only exists to inherit from (like our Student class in the example), we can mark it as abstract with the abstract keyword. An abstract class cannot be instantiated (that is, we cannot create an instance of it using the new keyword). It can still define fields and methods, but you can’t construct it. If we were to re-write our Student class as an abstract class:

/// <summary>
/// A class representing a student
/// </summary>
public abstract class Student {

  // private backing variables
  private double hours;
  private double points;

  /// <summary>
  /// Gets the students' GPA
  /// </summary>
  public double GPA {
    get {
      return points / hours;
    }
  }

  /// <summary>
  /// Gets or sets the first name
  /// </summary>
  public string First { get; set; }

  /// <summary>
  /// Gets or sets the last name
  /// </summary>
  public string Last { get; set; }

  /// <summary>
  /// Constructs a new instance of Student
  /// </summary>
  /// <param name="first">The student's first name </param>
  /// <param name="last">The student's last name</param>
  public Student(string first, string last) {
    this.First = first;
    this.Last = last;
  }

  /// <summary>
  /// Adds a new course grade to the student's record.
  /// </summary>
  /// <param name="creditHours">The number of credit hours in the course </param>
  /// <param name="finalGrade">The final grade earned in the course</param>
  ///
  public void AddCourseGrade(uint creditHours, Grade finalGrade) {
    this.hours += creditHours;
    switch(finalGrade) {
      case Grade.A:
        this.points += 4 * creditHours;
        break;
      case Grade.B:
        this.points += 3 * creditHours;
        break;
      case Grade.C:
        this.points += 2 * creditHours;
        break;
      case Grade.D:
        this.points += 1 * creditHours;
        break;
    }
  }
}

Now with Student as an abstract class, attempting to create a Student instance i.e. Student mark = new Student("Mark", "Guy") would fail with an exception. However, we can still create instances of the derived classes GraduateStudent and UndergraduateStudent, and treat them as Student instances. It is best practice to make a class abstract if it serves only as a base class for derived classes and will never be created directly.

Sealed Classes

Conversely, C# also offers the sealed keyword, which can be used to indicate that a class should not be inheritable. For example:

/// <summary>
/// A class that cannot be inherited from
/// </summary>
public sealed class DoNotDerive {

}

Derived classes can also be sealed. I.e., we could seal our UndergraduateStudent class to prevent further derivation:

/// <summary>
/// A sealed version  of the class representing an undergraduate student
/// </summary>
public sealed class UndergraduateStudent : Student {

  /// <summary>
  /// Constructs a new instance of UndergraduateStudent
  /// </summary>
  /// <param name="first">The student's first name </param>
  /// <param name="last">The student's last name</param>
  public UndergraduateStudent(string first, string last) : base(first, last) {
  }
}

Many of the library classes provided with the C# installation are sealed. This helps prevent developers from making changes to well-known classes that would make their code harder to maintain. It is good practice to seal classes that you expect will never be inherited from.

Interfaces and Inheritance

A class can use both inheritance and interfaces. In C#, a class can only inherit one base class, and it should always be the first after the colon (:). Following that we can have as many interfaces as we want, all separated from each other and the base class by commas (,):

public class UndergraduateStudent : Student, ITeachable, IEmailable 
{
  // TODO: Implement student class 
}

Casting

You have probably used casting to convert numeric values from one type to another, i.e.:

int a = 5;
double b = a;

And

int c = (int)b;

What you are actually doing when you cast is transforming a value from one type to another. In the first case, you are taking the value of a (5), and converting it to the equivalent double (5.0). If you consider the internal representation of an integer (a 2’s complement binary number) to a double (an IEEE 754 standard representation), we are actually applying a conversion algorithm to the binary representations.

We call the first operation an implicit cast, as we don’t expressly tell the compiler to perform the cast. In contrast, the second assignment is an explicit cast, as we signify the cast by wrapping the type we are casting to in parenthesis before the variable we are casting. We have to perform an explicit cast in the second case, as the conversion has the possibility of losing some precision (i.e. if we cast 7.2 to an integer, it would be truncated to 7). In any case where the conversion may lose precision or possibly throw an error, an explicit cast is required.

Custom Casting Conversions

We can actually extend the C# language to add additional conversions to provide additional casting operations. Consider if we had Rectangle and Square structs:

/// <summary>A struct representing a rectangle</summary>
public struct Rectangle {
    
    /// <summary>The length of the short side of the rectangle</summary>
    public int ShortSideLength;
    
    /// <summary>The length of the long side of the rectangle</summary>
    public int LongSideLength;
    
    /// <summary>Constructs a new rectangle</summary>
    /// <param name="shortSideLength">The length of the shorter sides of the rectangle</param>
    /// <param name="longSideLength">The length of the longer sides of the rectangle</param>
    public Rectangle(int shortSideLength, int longSideLength){
        ShortSideLength = shortSideLength;
        LongSideLength = longSideLength;
    }
}

/// <summary>A struct representing a square</summary>
public struct Square {

    /// <summary> The length of the square's sides</summary>
    public int SideLength;

    /// <summary>Constructs a new square</summary>
    /// <param name="sideLength">The length of the square's sides</param>
    public Square(int sideLength){
        SideLength = sideLength;
    }
}

Since we know that a square is a special case of a rectangle (where all sides are the same length), we might define an implicit casting operator to convert it into a Rectangle (this would be placed inside the Square struct definition):

    /// <summary>Casts the <paramref name="square"/> into a Rectangle</summary>
    /// <param name="square">The square to cast</param>
    public static implicit operator Rectangle(Square square) 
    {
        return new Rectangle(square.SideLength, square.SideLength);
    }

Similarly, we might create a cast operator to convert a rectangle to a square. But as this can only happen when the sides of the rectangle are all the same size, it would need to be an explicit cast operator , and throw an exception when that condition is not met (this method is placed in the Rectangle struct definition):

    /// <summary>Casts the <paramref name="rectangle"/> into a Square</summary>
    /// <param name="rectangle">The rectangle to cast</param>
    /// <exception cref="System.InvalidCastOperation">The rectangle sides must be equal to cast to a square</exception>
    public static explicit operator Square(Rectangle rectangle){
        if(rectangle.LongSideLength != rectangle.ShortSideLength) throw new InvalidCastException("The sides of a square must be of equal lengths");
        return new Square(rectangle.LongSideLength);
    }

Casting and Inheritance

Casting becomes a bit more involved when we consider inheritance. As you saw in the previous discussion of inheritance, we can treat derived classes as the base class, i.e. the code:

Student sam = new UndergraduateStudent("Sam", "Malone");

Is actually implicitly casting the undergraduate student “Sam Malone” into a student class. Because an UndergraduateStudent is a Student, this cast can be implicit. Moreover, we don’t need to define a casting operator - we can always implicitly cast a class to one of its ancestor classes, it’s built into the inheritance mechanism of C#.

Going the other way requires an explicit cast as there is a chance that the Student we are casting isn’t an undergraduate, i.e.:

UndergraduateStudent u = (UndergraduateStudent)sam;

If we tried to cast sam into a graduate student:

GraduateStudent g = (GraduateStudent)sam;

The program would throw an InvalidCastException when run.

Casting and Interfaces

Casting interacts similarly with interfaces. A class can be implicitly cast to an interface it implements:

IJumpable roo = new Kangaroo();

But must be explicitly cast to convert it back into the class that implemented it:

Kangaroo k = (Kangaroo)roo;

And if that cast is illegal, we’ll throw an InvalidCastException:

Car c = (Car)roo;

The as Operator

When we are casting reference and nullable types, we have an additional casting option - the as casting operator.

The as operator performs the cast, or evaluates to null if the cast fails (instead of throwing an InvalidCastException), i.e.:

UndergraduateStudent u = sam as UndergraduateStudent; // evaluates to an UndergraduateStudent 
GraduateStudent g = sam as GraduateStudent; // evaluates to null
Kangaroo k = roo as Kangaroo; // evaluates to a Kangaroo 
Car c = roo as Kangaroo; // evaluates to null

The is Operator

Rather than performing a cast and catching the exception (or performing a null check when using the as operator), it is often useful to know if a cast is possible. This can be checked for with the is operator. It evaluates to a boolean, true if the cast is possible, false if not:

sam is UndergraduateStudent; // evaluates to true
sam is GraduateStudent; // evaluates to false
roo is Kangaroo; // evaluates to true
roo is Car; // evaluates to false

Square s = new Square(10);
bool test = s is Rectangle;

The is operator is commonly used to determine if a cast will succeed before performing it, i.e.:

if(sam is UndergraduateStudent) 
{
    Undergraduate samAsUGrad = sam as UndergraduateStudent;
    // TODO: Do something undergraduate-ey with samAsUGrad
}

This pattern was so commonly employed, it led to the addition of the is type pattern matching expression in C# version 7.0:

if(sam is UndergraduateStudent samAsUGrad) 
{
    // TODO: Do something undergraduate-y with samAsUGrad
}

If the cast is possible, it is performed and the result assigned to the provided variable name (in this case, samAsUGrad). This is another example of syntactic sugar.

Message Dispatching

The term dispatch refers to how a language decides which polymorphic operation (a method or function) a message should trigger.

Consider polymorphic functions in C# (aka Method Overloading, where multiple methods use the same name but have different parameters) like this one for calculating the rounded sum of an array of numbers:

int RoundedSum(int[] a) {
  int sum = 0;
  foreach(int i in a) {
    sum += i;
  }
  return sum;
}

int RoundedSum(float[] a) {
    double sum = 0;
    foreach(int i in a) {
        sum += i;
    }
    return (int)Math.Round(sum);
}

How does the interpreter know which version to invoke at runtime? It should not be a surprise that it is determined by the arguments - if an integer array is passed, the first is invoked, if a float array is passed, the second.

Object-Oriented Polymorphism

However, inheritance can cause some challenges in selecting the appropriate polymorphic form. Consider the following fruit implementations that feature a Blend() method:

/// <summary>
/// A base class representing fruit
/// </summary>
public class Fruit
{
    /// <summary>
    /// Blends the fruit
    /// </summary>
    /// <returns>The result of blending</returns>
    public string Blend() {
      return "A pulpy mess, I guess";
    }
}

/// <summary>
/// A class representing a banana
/// </summary>
public class Banana : Fruit
{
    /// <summary>
    /// Blends the banana
    /// </summary>
    /// <returns>The result of blending the banana</returns>
    public string Blend()
    {
        return "yellow mush";
    }
}

/// <summary>
/// A class representing a Strawberry
/// </summary>
public class Strawberry : Fruit
{
    /// <summary>
    /// Blends the strawberry
    /// </summary>
    /// <returns>The result of blending a strawberry</returns>
    public string Blend()
    {
        return "Gooey Red Sweetness";
    }
}

Let’s add fruit instances to a list, and invoke their Blend() methods:

List<Fruit> toBlend = new List<Fruit>();
toBlend.Add(new Banana());
toBlend.Add(new Strawberry());

foreach(Fruit item in toBlend) {
  Console.WriteLine(item.Blend());
}

You might expect this code to produce the lines:

yellow mush
Gooey Red Sweetness

As these are the return values for the Blend() methods for the Banana and Strawberry classes, respectively. However, we will get:

A pulpy mess, I guess?
A pulpy mess, I guess?

Which is the return value for the Fruit base class Blend() implementation. The line forEach(Fruit item in toBlend) explicitly tells the interpreter to treat the item as a Fruit instance, so of the two available methods (the base or super class implementation), the Fruit base class one is selected.

C# 4.0 introduced a new keyword, dynamic to allow variables like item to be dynamically typed at runtime. Hence, changing the loop to this:

forEach(dynamic item in toBlend) {
  Console.WriteLine(item.Blend());
}

Will give us the first set of results we discussed.

Method Overriding

Of course, part of the issue in the above example is that we actually have two implementations for Blend() available to each fruit. If we wanted all bananas to use the Banana class’s Blend() method, even when the banana was being treated as a Fruit, we need to override the base method instead of creating a new one that hides it (in fact, in Visual Studio we should get a warning that our new method hides the base implementation, and be prompted to add the new keyword if that was our intent).

To override a base class method, we first must mark it as abstract or virtual. The first keyword, abstract, indicates that the method does not have an implementation (a body). The second, virtual, indicates that the base class does provide an implementation. We should use abstract when each derived class will define its own implementation, and virtual when some derived classes will want to use a common base implementation. Then, we must mark the method in the derived class with the override keyword.

Considering our Fruit class, since we’re providing a unique implementation of Blend() in each derived class, the abstract keyword is more appropriate:

/// <summary>
/// A base class representing fruit
/// </summary>
public abstract class Fruit : IBlendable
{
    /// <summary>
    /// Blends the fruit
    /// </summary>
    /// <returns>The result of blending</returns>
    public abstract string Blend();
}

As you can see above, the Blend() method does not have a body, only the method signature.

Also, note that if we use an abstract method in a class, the class itself must also be declared abstract. The reason should be clear - an abstract method cannot be called, so we should not create an object that only has the abstract method. The virtual keyword can be used in both abstract and regular classes.

Now we can override the Blend() method in Banana class:

/// <summary>
/// A class representing a banana
/// </summary>
public class Banana : Fruit
{
    /// <summary>
    /// Blends the banana
    /// </summary>
    /// <returns>The result of blending the banana</returns>
    public override string Blend()
    {
        return "yellow mush";
    }
}

Now, even if we go back to our non-dynamic loop that treats our fruit as Fruit instances, we’ll get the result of the Banana class’s Blend() method.

We can override any method marked abstract, virtual, or override (this last will only occur in a derived class whose base class is also derived, as it is overriding an already-overridden method).

Sealed Methods

We can also apply the sealed keyword to overridden methods, which prevents them from being overridden further. Let’s apply this to the Strawberry class:

/// <summary>
/// A class representing a Strawberry
/// </summary>
public class Strawberry : Fruit
{
    /// <summary>
    /// Blends the strawberry
    /// </summary>
    /// <returns>The result of blending a strawberry</returns>
    public sealed override string Blend()
    {
        return "Gooey Red Sweetness";
    }
}

Now, any class inheriting from Strawberry will not be allowed to override the Blend() method.

C# Collections

Collections in C# are a great example of polymorphism in action. Many collections utilize generics to allow the collection to hold an arbitrary type. For example, the List<T> can be used to hold strings, integers, or even specific objects:

List<string> strings = new List<string>();
List<int> ints = new List<int>();
List<Person> persons = new List<Person>();

We can also use an interface as the type, as we did with the IJumpable interface as we discussed in the generics section, i.e.:

List<IJumpable> jumpables = new List<IJumpable>();
jumpables.Add(new Kangaroo());
jumpables.Add(new Car());
jumpables.Add(new Kangaroo());

Collection Interfaces

The C# language and system libraries also define a number of interfaces that apply to custom collections. Implementing these interfaces allows different kinds of data structures to be utilized in a standardized way.

The IEnumerable<T> Interface

The first of these is the IEnumerable<T> interface, which requires the collection to implement one method:

• public IEnumerator<T> GetEnumerator()

Implementing this interface allows the collection to be used in a foreach loop.

The ICollection<T> Interface

C# Collections also typically implement the ICollection<T> interface, which extends the IEnumerable<T> interface and adds additional methods:

• public void Add<T>(T item) adds item to the collection
• public void Clear() empties the collection
• public bool Contains(T item) returns true if item is in the collection, false if not.
• public void CopyTo(T[] array, int arrayIndex) copies the collection contents into array, starting at arrayIndex.
• public bool Remove(T item) removes item from the collection, returning true if item was removed, false otherwise

Additionally, the collection must implement the following properties:

• int Count the number of items in the collection
• bool IsReadOnly the collection is read-only (can’t be added to or removed from)

The IList<T> Interface

Finally, collections that have an implied order and are intended to be accessed by a specific index should probably implement the IList<T> interface, which extends ICollection<T> and IEnumerable<T>. This interface adds these additional methods:

• public int IndexOf(T item) returns the index of item in the list, or -1 if not found
• public void Insert(int index, T item) Inserts item into the list at position index
• public void RemoveAt(int index) Removes the item from the list at position index

The interface also adds the property:

• Item[int index] which gets or sets the item at index.

Collection Implementation Strategies

When writing a C# collection, there are three general strategies you can follow to ensure you implement the corresponding interfaces:

1. Write the entire class by scratch
2. Implement the interface methods as a pass-through to a system library collection
3. Inherit from a system library collection

Writing collections from scratch was the strategy you utilized in CIS 300 - Data Structures and Algorithms. While this strategy gives you the most control, it is also the most time-consuming.

The pass-through strategy involves creating a system library collection, such as a List<T>, as a private field in your collection class. Then, when you implement the necessary interface methods, you simply pass through the call to the private collection. I.e.:

public class PassThroughList<T> : IList<T>
{
  private List<T> _list = new List<T>;

  public IEnumerator<T> GetEnumerator() 
  {
    return _list.GetEnumerator();
  } 

  // TODO: Implement remaining methods and properties...
}

Using this approach, you can add whatever additional logic your collection needs into your pass-through methods without needing to re-implement the basic collection functionality.

Using inheritance gives your derived class all of the methods of the base class, so if you extend a class that already implements the collection interfaces, you’ve already got all the methods!

public class InheritedList<T> : List<T>
{
  // All IList<T>, ICollection<T>, and IEnumerable<T> methods 
  // from List<T> are already defined on InheritedList<T>
}

However, most system collection class methods are not declared as virtual, so you cannot override them to add custom functionality.

Summary

In this chapter, we explored the concept of types and discussed how variables are specific types that can be explicitly or implicitly declared. We saw how in a statically-typed language (like C#), variables are not allowed to change types (though they can do so in a dynamically-typed language). We also discussed how casting can convert a value stored in a variable into a different type. Implicit casts can happen automatically, but explicit casts must be indicated by the programmer using a cast operator, as the cast could result in loss of precision or the throwing of an exception.

We explored how class declarations and interface declarations create new types. We saw how polymorphic mechanisms like interface implementation and inheritance allow objects to be treated as (and cast to) different types. We also introduced the as and is casting operators, which can be used to cast or test the ability to cast, respectively. We saw that if the as cast operator fails, it evaluates to null instead of throwing an exception. We also saw the is type pattern expression, which simplifies a casting test and casting operation into a single expression.

Next, we looked at how C# collections leverage the use of interfaces, inheritance, and generic types to quickly and easily make custom collection objects that interact with the C# language in well-defined ways.

Finally, we explored how messages are dispatched when polymorphism is involved. We saw that the method invoked depends on what Type we are currently treating the object as. We saw how the C# modifiers protected, abstract, virtual, override, and sealed interacted with this message dispatch process. We also saw how the dynamic type could delay determining an object’s type until runtime.

Introduction

As part of the strategy for tackling the challenges of the software crisis, good programming practice came to include writing clear documentation to support both the end-users who will utilize your programs, as well as other programmers (and yourself) in understanding what that code is doing so that it is easy to maintain and improve.

Key Terms

Some key terms to learn in this chapter are:

• User documentation
• Developer documentation
• Markdown
• XML
• Autodoc tools
• Intellisense

Key Skills

The key skill to learn in this chapter is how to use C# XML code comments to document the C# code you write.

Documentation

Documentation refers to the written materials that accompany program code. Documentation plays multiple, and often critical roles. Broadly speaking, we split documentation into two categories based on the intended audience:

• User Documentation is meant for the end-users of the software
• Developer Documentation is meant for the developers of the software

As you might expect, the goals for these two styles of documentation are very different. User documentation instructs the user on how to use the software. Developer documentation helps orient the developer so that they can effectively create, maintain, and expand the software.

Historically, documentation was printed separately from the software. This was largely due to the limited memory available on most systems. For example, the EPIC software we discussed had two publications associated with it: a User Manual, which explains how to use it, and Model Documentation which presents the mathematic models that programmers adapted to create the software. There are a few very obvious downsides to printed manuals: they take substantial resources to produce and update, and they are easily misplaced.

User Documentation

As memory became more accessible, it became commonplace to provide digital documentation to the users. For example, with Unix (and Linux) systems, it became commonplace to distribute digital documentation alongside the software it documented. This documentation came to be known as man pages based on the man command (short for manual) that would open the documentation for reading. For example, to learn more about the linux search tool grep, you would type the command:

$ man grep 

Which would open the documentation distributed with the grep tool. Man pages are written in a specific format; you can read more about it here.

While a staple of the Unix/Linux filesystem, there was no equivalent to man pages in the DOS ecosystem (the foundations of Windows) until Powershell was introduced, which has the Get-Help tool. You can read more about it here.

However, once software began to be written with graphical user interfaces (GUIs), it became commonplace to incorporate the user documentation directly into the GUI, usually under a “Help” menu. This served a similar purpose to man pages of ensuring user documentation was always available with the software. Of course, one of the core goals of software design is to make the software so intuitive that users don’t need to reference the documentation. It is equally clear that developers often fall short of that mark, as there is a thriving market for books to teach certain software.

Not to mention the thousands of YouTube channels devoted to teaching specific programs!

Developer Documentation

Developer documentation underwent a similar transformation. Early developer documentation was often printed and placed in a three-ring binder, as Neal Stephenson describes in his novel Snow Crash: ¹

Fisheye has taken what appears to be an instruction manual from the heavy black suitcase. It is a miniature three-ring binder with pages of laser-printed text. The binder is just a cheap unmarked one bought from a stationery store. In these respects, it is perfectly familiar to Him: it bears the earmarks of a high-tech product that is still under development. All technical devices require documentation of a sort, but this stuff can only be written by the techies who are doing the actual product development, and they absolutely hate it, always put the dox question off to the very last minute. Then they type up some material on a word processor, run it off on the laser printer, send the departmental secretary out for a cheap binder, and that's that.

Shortly after the time this novel was written, the internet became available to the general public, and the tools it spawned would change how software was documented forever. Increasingly, web-based tools are used to create and distribute developer documentation. Wikis, bug trackers, and autodocumentation tools quickly replaced the use of lengthy, and infrequently updated word processor files.

Documentation Formats

Developer documentation often faces a challenge not present in other kinds of documents - the need to be able to display snippets of code. Ideally, we want code to be formatted in a way that preserves indentation. We also don’t want code snippets to be subject to spelling- and grammar-checks, especially auto-correct versions of these algorithms, as they will alter the snippets. Ideally, we might also apply syntax highlighting to these snippets. Accordingly, a number of textual formats have been developed to support writing text with embedded program code, and these are regularly used to present developer documentation. Let’s take a look at several of the most common.

HTML

Since its inception, HTML has been uniquely suited for developer documentation. It requires nothing more than a browser to view - a tool that nearly every computer is equipped with (in fact, most have two or three installed). And the <code> element provides a way of styling code snippets to appear differently from the embedded text, and <pre> can be used to preserve the snippet’s formatting. Thus:

<p>This algorithm reverses the contents of the array, <code>nums</code></p>
<pre>
<code>
for(int i = 0; i < nums.Length/2; i++) {
    int tmp = nums[i];
    nums[i] = nums[nums.Length - 1 - i];
    nums[nums.Length - 1 - i] = tmp;
}
</code>
</pre>

Will render in a browser as:

for(int i = 0; i < nums.Length/2; i++) {
    int tmp = nums[i];
    nums[i] = nums[nums.Length - 1 - i];
    nums[nums.Length - 1 - i] = tmp;
}

JavaScript and CSS libraries like highlight.js, prism, and others can provide syntax highlighting functionality without much extra work.

Of course, one of the strongest benefits of HTML is the ability to create hyperlinks between pages. This can be invaluable in documenting software, where the documentation about a particular method could include links to documentation about the classes being supplied as parameters, or being returned from the method. This allows developers to quickly navigate and find the information they need as they work with your code.

Markdown

However, there is a significant amount of boilerplate involved in writing a webpage (i.e. each page needs a minimum of elements not specific to the documentation to set up the structure of the page). The extensive use of HTML elements also makes it more time-consuming to write and harder for people to read in its raw form. Markdown is a markup language developed to counter these issues. Markdown is written as plain text, with a few special formatting annotations, which indicate how it should be transformed to HTML. Some of the most common annotations are:

• Starting a line with hash (#) indicates it should be a <h1> element, two hashes (##) indicates a <h2>, and so on…
• Wrapping a statement with underscores (_) or asterisks (*) indicates it should be wrapped in a <i> element
• Wrapping a statement with double underscores (__) or double asterisks (**) indicates it should be wrapped in a <b> element
• Links can be written as [link text](url), which is transformed to <a href="url">link text</a>
• Images can be written as ![alt text](url), which is transformed to <img alt="alt text" src="url"/>

Code snippets are indicated with backtick marks (`). Inline code is written surrounded with single backtick marks, i.e. `int a = 1` and in the generated HTML is wrapped in a <code> element. Code blocks are wrapped in triple backtick marks, and in the generated HTML are enclosed in both <pre> and <code> elements. Thus, to generate the above HTML example, we would use:

This algorithm reverses the contents of the array, `nums`
```
for(int i = 0; i < nums.Count/2; i++) {
    int tmp = nums[i];
    nums[i] = nums[nums.Count - 1 - i];
    nums[nums.Count - 1 - i] = tmp;
}
```

Most markdown compilers also support specifying the language (for language-specific syntax highlighting) by following the first three backticks with the language name, i.e.:

```csharp
List = new List;
```

Nearly every programming language features at least one open-source library for converting Markdown to HTML. Microsoft even includes a C# one in the Windows Community Toolkit. In addition to being faster to write than HTML, and avoiding the necessity to write boilerplate code, Markdown offers some security benefits. Because it generates only a limited set of HTML elements, which specifically excludes some most commonly employed in web-based exploits (like using <script> elements for script injection attacks), it is often safer to allow users to contribute markdown-based content than HTML-based content. Note: this protection is dependent on the settings provided to your HTML generator - most markdown converters can be configured to allow or escape HTML elements in the markdown text

In fact, this book was written using Markdown, and then converted to HTML using the Hugo framework, a static website generator built using the Go programming language.

Additionally, chat servers like RocketChat and Discord support using markdown in posts!

GitHub even incorporates a markdown compiler into its repository displays. If your file ends in a .md extension, GitHub will evaluate it as Markdown and display it as HTML when you navigate your repo. If your repository contains a README.md file at the top level of your project, it will also be displayed as the front page of your repository. GitHub uses an expanded list of annotations known as GitHub-flavored markdown that adds support for tables, task item lists, strikethroughs, and others.

XML

Extensible Markup Language (XML) is a close relative of HTML - they share the same ancestor, Standard Generalized Markup Language (SGML). It allows developers to develop their own custom markup languages based on the XML approach, i.e. the use of elements expressed via tags and attributes. XML-based languages are usually used as a data serialization format. For example, this snippet represents a serialized fictional student:

<student>
    <firstName>Willie</firstName>
    <lastName>Wildcat</lastName>
    <wid>8888888</wid>
    <degreeProgram>BCS</degreeProgram>
</student>

While XML is most known for representing data, it is one of Microsoft’s go-to tools. For example, they have used it as the basis of Extensible Application Markup Language (XAML), which is used in Windows Presentation Foundation as well as cross-platform Xamrin development. So it shouldn’t be a surprise that Microsoft also adopted it for their autodocumentation code commenting strategy. We’ll take a look at this next.

Autodocs

One of the biggest innovations in documenting software was the development of autodocumentation tools. These were programs that would read source code files, and combine information parsed from the code itself and information contained in code comments to generate documentation in an easy-to-distribute form (often HTML). One of the earliest examples of this approach came from the programming language Java, whose API specification was generated from the language source files using JavaDoc.

This approach meant that the language of the documentation was embedded within the source code itself, making it far easier to update the documentation as the source code was refactored. Then, every time a release of the software was built (in this case, the Java language), the documentation could be regenerated from the updated comments and source code. This made it far more likely developer documentation would be kept up-to-date.

Microsoft adopted a similar strategy for the .NET languages, known as XML comments. This approach was based on embedding XML tags into comments above classes, methods, fields, properties, structs, enums, and other code objects. These comments are set off with a triple forward slash (///) to indicate the intent of being used for autodoc generation. Comments using double slashes (//) and slash-asterisk notation (/* */) are ignored in this autodoc scheme.

For example, to document an Enum, we would write:

/// <summary>
/// An enumeration of fruits used in pies 
/// </summary>
public enum Fruit {
    Cherry,
    Apple,
    Blueberry,
    Peach
}

At a bare minimum, comments should include a <summary> element containing a description of the code structure being described.

Let’s turn our attention to documenting a class:

public class Vector2 {

    public float X {get; set;}

    public float Y {get; set;}

    public Vector2(float x, float y) {
        X = x;
        Y = y;
    }

    public void Scale(float scalar) {
        X *= scalar;
        Y *= scalar;
    }

    public float DotProduct(Vector2 other) {
        return this.X * other.X + this.Y * other.Y;
    }

    public float Normalize() {
        float magnitude = Math.Sqrt(Math.Pow(this.X, 2), Math.Pow(this.Y, 2));
        if(magnitude == 0) throw new DivideByZeroException();
        X /= magnitude;
        Y /= magnitude;
    }
}