In this article we will understand some of the concepts of object-oriented programming in C# like objects and classes. To read this article you must have C# programming basics.
NOTE: read the whole article because there aresome concepts you may will not get the best of it until you finish the article.And we will revisit all the concepts more than one time when I see it’sappropriate in future articles so don’t worry at all.
Introduction:
OOP stands for Object-Oriented Programming. OOP isrelatively a new way to program computer applications. Before OOP programmersused to creating computer applications using procedural-programming (orstructure-programming) but when OOP solved a lot of the problems of theprocedural-programming so almost all of the programmers and developers beganusing OOP languages. In procedural- programming all the program functionalitywritten in a few modules of code or maybe one module (depending on the program)and these modules depend on one another and maybe if you change a line of codeyou will have to rewrite the whole module again and maybe the whole program butin Object-Oriented Programming programmers write independent parts of a programcalled classes each class represent a part of the program functionality andthese classes can be assembled to form a program and when you need to changesome of the program functionality all what you have to do is to replace thetarget class which may contain a problem. So in OOP applications create by theuse of classes and these applications can contain any number of classes. Thatwill get us to discuss the Class and Object concept.
Classes and objects:
You may find it not easy to understand the class and objectstory but I will try to do my best explaining it. Actually the class and objectconcept is related to each other and some beginners don’t care aboutunderstanding it clear so I think they will have a hard times learning C#.
Object-Oriented concept takes the most of its functionalityfrom the real-life concepts. For example, I will discuss the concept of Classesand Objects of the world first and then you will understand the computer’sClasses and Objects before I even write anything about it.
World’s Classes and Objects:
In our world we have a classes and objects for thoseclasses. Everything in our world considered to be an object. For example,people are objects, animals are objects too, minerals are objects, everythingin the world are objects. Easy right ? but what about classes. In our world wehave to differentiate between objects that we are living with. So we mustunderstand that there are a classifications (this is how they get the nameand the concepts of the Class) for all of those objects. For example, I’man object, David is object too, Maria is another object so we are from apeople class (or type). I have a dog called Ricky so it’s an object, Myfriend’s dog called Doby is also an object so they are from a Dogs class (ortype). A third example, I have a computer Pentium III this is object, Myfriend have a computer Pentium IIII so this is another object and they are froma Computers class (or type). Now I think you got the concept of theClass and Object but let me crystallize it for you. In our world we have aclassifications for objects and every object must be from some classification.so a Class is a way for describing some properties and functionalities orbehaviors of a group of objects. In other words, The class considered to be atemplate for some objects. So maybe I will create a class called person so thisis a template of the functionality and the properties of persons. I explainedit by more than a way so wait until you see the first example and I think youwill grasp it completely.
Computer’s Classes and Objects:
Computer’s Classes discussion is similar to what you graspfrom the last section with some modifications to become computerized.
A C# Class Considered being the primary building block ofthe language. What I mean by the primary building block of the language is thatevery time you work with C# you will create Classes to form a program. We useClasses as a template to put the properties and functionalities or behaviors inone building block for some group of objects and after that we use thattemplate to create the objects we need. For example, We need to have personsobjects in our program so the first thing to do here is to create a Classcalled Person that contains all the functionalities or behaviors and propertiesof any person and after that we will use that Class or template to create asmany objects as we need. Creating object of a specific class type called”instance of the class”. Don’t worry if you didn’t grasp it 100% anddon’t worry if you don’t know what’s the Class and Object’s properties andfunctionalities or behaviors because we still in the beginning and until now Ididn’t give any code examples. So let’s take a brief of what’s the Class andwhat’s an object ?
The Class : Is abuilding block that contains the properties and functionalities that
describesome group of objects, We can create a class Person that contains:
1- The properties of any normal person on the earth like :Hair Color, Age, Height, Weight, Eyes Color.
2- The functionalities or behaviors of any normal person onthe earth like : Drink water, Eat, Go to the work and later we will see how wecan implement the functionalities or behaviors and properties.
There are 2 kinds of classes : The built-it classes thatcome with the .NET Framework and called Framework Class Library. And theprogrammer defined-classes which we create it.
The class contains data (in the form of variables andproperties) and behaviors (in the form of methods to process these data). Wewill understand this concept more later in the article.
When we declare a variable in a class we call it membervariables or instance variables. The name instance come from the fact that whenwe create an object we instance a class to create that object so instance of aclass means object of that class and instance variable means variable thatexists in that class.
The Object : It’s objectof some classification (or class, or type. All means the same thing) and whenyou create the object you can specify the properties of that object. What Imean here is me as an object can have a different properties (Hair Color, Age,Height, Weight) of you as another object. For example, I have a brown eyes andyou have a green eyes so when I create 2 objects I will specify a browncolor for my object’s Eyes Color property and I will specify a green color foryour object’s Eyes Color property.
So to complete my introduction to Classes we must discussProperties and Variables.
Properties and Variables:
Variables declared in a class store the data for eachinstance, What that means ? means that when you instantiate this class (thatis, When you create an object of this class) the object will allocate a memorylocations to store the data of its variables. Let’s take an example tounderstand it well.
class Person
{
public int Age;
public string HairColor;
}
This is our simple class which contains 2 variables. Don’tworry about public keyword now because we willtalk about it later . Now we will instantiate this class (that is, When youcreate an object of this class).
static void Main(string[]args)
{
Person Michael = newPerson();
Person Mary = new Person();
// Specify some values for the instance variables
Michael.Age = 20;
Michael.HairColor = “Brown”;
Mary.Age = 25;
Mary.HairColor = “Black”;
// print the console’s screen someof the variable’s values
Console.WriteLine(“Michael’s age = {0}, and Mary’s age= {1}”,Michael.Age,
Mary.Age);
Console.ReadLine();
}
So we begin our Main method by creating 2 objects of Persontype. After creating the 2 objects we initialize the instance variables forobject Michael and then for object Mary. Finally we print some values to theconsole. here when you create Michael object C# compiler allocate a memorylocation for the 2 instance variables to put the values there. Also the samething with Mary object the compiler will create 2 variables in the memory forMary object. So each object now contains a different data. Note that wedirectly accessed the variables and we put any values we want, Right ? so maybesomeone doesn’t like me will put in my object’s variable Age value of 120 yearsso I will not get any kind of jobs. But wait there are a solution for thisproblem. We will use properties.
Properties:
Properties is a way to access the variables of the class ina secure manner. Let’s see the same example using properties.
class Person
{
private int age;
private string hairColor;
public int Age
{
get
{
returnage;
}
set
{
if(value <= 65 && value >= 18)
{
age = value;
}
else
age = 18;
}
}
public string HairColor
{
get
{
return hairColor;
}
set
{
hairColor = value;
}
}
}
I made some modifications but please just care about the new2 properties that I created it here. So the property consists of 2 accessor.The get accessor which is responsibleof retrieving the variable value, And the set accessor which is responsible of modifying the variable’svalue. So The get accessor code is very simple wejust use the keyword return withthe variable name to return its value. so the following code:
get
{
return hairColor;
}
return the value stored in hairColor.
Note :the keyword value isa reserved keyword by C# (that is, reserved keywords means that these keywordsown only by C# and you can’t create it for other purpose. For example, Youcan’t create a variable called value .Ifyou did that C# compiler will generate an error and to make things easierVisual Studio.NET will color the reserved keywords to blue.)
Let’s put this code at work and after that discuss theset accessor.
static void Main(string[]args)
{
Person Michael = newPerson();
Person Mary = new Person()
;
// Specify some values for the instance variables
Michael.Age = 20;
Michael.HairColor = “Brown”;
Mary.Age = 25;
Mary.HairColor = “Black”;
// print the console’s screen some of the variable’s values
Console.WriteLine(“Michael’s age = {0}, and Mary’s age= {1}”,Michael.Age,
Mary.Age);
Console.ReadLine();
}
Here I created the same objects from last example themodifications that I used only properties to access the variable instead ofaccess it directly. Look at the following line of code
Michael.Age = 20;
When you assign a value to the property like that C# willuse the set accessor. The great thing withthe set accessor is that we can controlthe assigned value and test it and maybe change to in some cases. When youassign a value to a property C# change the value in a variable and you canaccess the variable’s value using the reserved keyword value exactly as I did in the example. Let’s see it again here.
set
{
if(value <= 65 && value >= 18)
{
age = value;
}
else
age = 18;
}
Here in the code I used if statement to test the assigned value because for some reasonI want any object of type Person to be in age between 18 and 65. Here I testthe value and if it in the range then simply I will store it in the variableage and it it’s not in the range I will put 18 as a value to age. It was just asimple example for the properties but there is a complete article about propertiessoon.
How we create objects and classes ?
We create classes by define it like that:
using the keyword class followedby the class name like that
class Person
then we open a left brace “{” and after we writeour methods and properties we close it by a right brace “}”. That’show we create a class. Let’s see how we create an instance of that class.
In the same way as we declare a variable of type int we create an object variable of Person type with some modifications:
int age;
Person Michael = new Person();
In the first line of code we specified integer variablecalled age. In the second line we specified first the type of Object we need tocreate followed by the object’s name followed by a reserved operator called new and we end by typing the class name again followed byparenthesis “()”.
Let’s understand it step-by-step. Specifying the class nameat the beginning tell the C# Compiler to allocate a memory location for thattype (C# compiler knows all the variables and properties and methods of theclass so it will allocate the right amount of memory). Then we followed theclass name by out object variable name that we want it. The rest of the code”=new Person();” callthe object’s constructor. We will talk about constructor later but for nowunderstand that the constructor is a way to initialize your object’s variablewhile you are creating it not after you create it. For example, The Michaelobject we created it in the last section can be written as following :
Person Michael = new Person(20, “Brown”);
here I specified the variable’s values in the parameter listso I initialized the variables while I’m creating the object. But for this codeto work we will need to specify the constructor in the Person class and I willnot do that here because constructor section will come in later articles. Ithink you got a good introduction about Classes and Objects not I will completein in my next article and I will talk about constructors and building blockscoping. I hope you got a new thing from my article.
About the Author:
Michael Youssef is 20 years old and he’sworking with Microsoft technologies since he was 17. He is working with VB,VB.NET, C#,ASP, ASP. NET, XML Web services, COM+, SQL Server 2000, Windows 2000Server, Active Directory, Networks design. He is MCSD.NET, MCDBA, MCSE, MCSAand he’s working now as a .NET Trainer in Egypt. Michael can be reached atmichaellabibat@hotmail.com .
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