During this lesson we’ll review what Object-oriented programming is. Also we’ll learn C# types and create a first program.


Object oriented programming

OOP is a way of organizing the source code in a program by grouping it into objects. Object is an individual element that includes information (data values) and functionality.

Picture below demonstrates “objects” in a real life.

And here is an example how it can be coded in programming language:

class Eye
    public string Color { get; set; }

class Ear
    public void Listen()

class Finger
    public string NailColor { get; set; }

class Hand
    public Finger[] Fingers { get; set; } = new Finger[5];

class Leg
    public Finger[] Fingers { get; set; } = new Finger[5];

class Nose
    public void ToSniff()
    public void ToSneeze()

class Mouth
    public bool IsMouthOpened { get; set; }

    public void Open()
        Console.WriteLine("Mouth is opened.");

    public void Close()
        Console.WriteLine("Mouth is closed.");

class Head
    public Eye LeftEye { get; set; }
    public Eye RightEye { get; set; }
    public Mouth Mouth { get; set; }
    public Ear LeftEar { get; set; }
    public Ear RightEar { get; set; }
    public Nose Nose { get; set; }

class Body
    public string Name { get; set; }
    public int Weight { get; set; }
    public int High { get; set; }
    public Head Head { get; set; }
    public Hand[] Hands { get; set; } = new Hand[2];
    public Leg[] Legs { get; set; } = new Leg[2];

In C# even a simplest program is a class. Function Main is an entry point in application. It means, that function Main will be called first when application starts.

using System;

namespace Lesson1
    class Program
        static void Main(string[] args)
            Console.WriteLine("Hello World");

Stack and Heap

  • Value Type - stores its contents in memory allocated on the stack. When you created a Value Type, a single space in memory is allocated to store the value and that variable directly holds a value.
  • Reference Types - are used by a reference which holds a reference (address) to the object but not the object itself. Because reference types represent the address of the variable rather than the data itself, assigning a reference variable to another doesn’t copy the data. Instead it creates a second copy of the reference, which refers to the same location of the heap as the original value.

Value types in C#

Type Represents Range Default Value
bool Boolean value True or False False
byte 8-bit unsigned integer 0 to 255 0
char 16-bit Unicode character U +0000 to U +ffff '\0'
decimal 128-bit precise decimal values with 28-29 significant digits (-7.9 x 1028 to 7.9 x 1028) / 100 to 28 0.0M
double 64-bit double-precision floating point type (+/-)5.0 x 10-324 to (+/-)1.7 x 10308 0.0D
float 32-bit single-precision floating point type -3.4 x 1038 to + 3.4 x 1038 0.0F
int 32-bit signed integer type -2,147,483,648 to 2,147,483,647 0
long 64-bit signed integer type -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 0L
sbyte 8-bit signed integer type -128 to 127 0
short 16-bit signed integer type -32,768 to 32,767 0
uint 32-bit unsigned integer type 0 to 4,294,967,295 0
ulong 64-bit unsigned integer type 0 to 18,446,744,073,709,551,615 0
ushort 16-bit unsigned integer type 0 to 65,535 0

Boxing and unboxing

System.Object is a parent type for all other types. I.e. all types are inherited from Object type.

Boxing is the process of converting a value type to the type object. The reverse process called unboxing.

int i = 123;
// The following line boxes i.
object o = i;
o = 123;
// unboxing
i = (int)o;

Mutable and Immutable objects

  • Mutable type – multiple values can be assigned to mutable object many times and its state can be altered
  • Immutable type – immutable classes are read-only. Once declared their value cannot be changed

String is a good example of immutable type:

string city = "Vinnytsia";
Console.WriteLine(city); //nothing changed
city = city.ToUpper();
// new value assigned

StringBuilder is mutable type:

StringBuilder builder = new StringBuilder();
builder.Append("Vinnytsia ");
builder.Append("is a good city!");
//Vinnytsia is a good city!

Nullable type

C# provides a special data types, the nullable types, to which you can assign normal range of values as well as null values.

int? value = null;
// Assign the nullable integer to a constant integer.
// ... The HasValue property is now true.
// ... You can access the Value property as well.
value = 1;



Lesson1 from Alex Honcharuk