Object-Oriented Programming
Chapter 4
Ebook: Beginning Visual C# 2010, part 1, chapter 8,9,10,13
Reference: DEITEL - CSharp How to Program
Contents

Review concepts in OOP

Write classes in C#

Interface

Inheritance

Polymorphism

Relationships between objects
Slide 2
Review concepts in OOP

Review concepts in OOP

class

object

field

property

method


event

Other problems

Static members: field, property and method (p.191)

Static constructor (p.191)

Static class (p.192)
Slide 3
ClassName
- Fields
+ Methods
Class diagram
Contents

Review concepts in OOP

Write classes in C#

Interface

Inheritance

Polymorphism

Relationships between objects
Slide 4
Create a class in C#

Slide 7
Member access modifiers

Without a
access modifier
: can be accessed by any class in
the same namespace

private: can only be accessed from inside the class

public: can be accessed from anywhere

protected: can be accessed from inside the child class
or any class in the same namespace
Slide 8
Writing properties

Properties
(accessors):

Contain two blocks:
get
and
set
the value of the fields

One of these blocks can be omitted to create read-only or
write-only properties

You can use

Encapsulate Field
function to properties
Slide 9
Writing constructors

A constructor is a special method in a class that is invoked
when the object gets instantiated

A constructor is used to initialize the properties of the
object

Note to write a constructor:

The name of the constructor and the name of the class are
the same

A constructor does not return value, not even void

A class can have multiple constructors (overloaded
constructors)
public ClassName (parameterList)
{
}
Slide 10
Instantiating an object

To instantiate an object, using the new keyword

Example:
Lop l = new Lop();

Lop c;
c = new Lop(“NCTH2K”, “Cao dang nghe 2K”);
ClassName object = new ClassName (…);
ClassName object;
object = new ClassName (…);
Slide 11
Using properties, methods of a class

Call methods or properties of a class

Using
dot
operator

Accessors:

get: variable = object.propertyName

set: object.propertyName = variable

Example:
Lop c = new Lop();
c.MaLop = "NCTH2K"; // call set{}
c.TenLop = "Cao dang nghe 2K"; // call set{}
Slide 12
OOP tools in VS

The Class View Window (p.222)

Menu View\Class View


The Object Browser (p.224)

Menu View\Object Browser

Class Diagrams (p.227)

The class diagram editor in VS enables you to generate UML-
like diagrams of your code and use them to modify projects
Slide 13
static class members (p.191)

Every object of a class has its own copy of all instance
variables

How can all objects of a class share the same copy of a
variable?

Declare variables using keyword static

Static members can be fields, properties, methods

When using static members, don’t need to instantiate an
object
Slide 14
static class members (cont.)
public class Employee
{
private string firstName;
private string lastName;

private static int count; // Employee objects in memory
public Employee( string fName, string lName )
{
firstName = fName;
lastName = lName;
count++;
}
public static int Count
{
get { return count; }
}
}
Slide 15
Employee e1=new Employee("A","AA");
Employee e2=new Employee("B","BB");
e2 = new Employee("C", "CC");
MessageBox.Show("S employee: " + ố
Employee.Count);
static constructors (p.191)

Static constructors
must have no access modifiers and
cannot have any parameters

In
static constructors
, cannot access nonstatic member
variables

Static constructors

only be called once

Static constructors
will run before any instance of your
class is created or static members are accessed

Example:
public class ABC{
private static string name;
static ABC() {
name = “static member”;
}
}
Slide 16
const and readonly members

To declare constant members (members whose value will
never change) using:

the keyword const

const members are implicitly static

const members must be initialized when they are declared

the keyword readonly

readonly members will be initialized in the constructor but not
change after that
Slide 17

const and readonly members (cont.)
public class Constants
{
// PI là m t h ng s constộ ằ ố
public const double PI = 3.14159;
// radius là m t h ng s ch a đ c kh i t oộ ằ ố ư ượ ở ạ
public readonly int radius;
public Constants( int radiusValue )
{
radius = radiusValue;
}
}
Slide 18
const and readonly members (cont.)
public class UsingConstAndReadOnly
{
static void Main( string[] args )
{
Random random = new Random();
Constants constantValues = new Constants( random.Next( 1, 20 ) );
MessageBox.Show( "Radius = " + constantValues.radius +
"\nCircumference = " + 2 * Constants.PI * constantValues.radius,
"Circumference" );
}
}
Slide 19
Contents

Review concepts in OOP


Write classes in C#

Interface

Inheritance

Polymorphism

Relationships between objects
Slide 22
Inheritance (p.194)

Inheritance enables to create a new class that extends an
existing class

The existing class is called the
parent class,
or
super class
, or
base class

The new class is called the
child class
or
subclass, derived class

The child class inherits the properties and methods of the
parent class


A programmer can tailor a child class as needed by adding
new variables
or
methods
, or by modifying the inherited ones

C# provides a common base class for all objects called
Object class
Slide 23
Inheritance (cont.)

Declare inheritance class

Classes in C# may derive only from a single base class
directly

The protected modifier allows a child class to reference
a variable or method directly in the parent class

A protected variable is visible to any class in the same
namespace
Slide 24
ParentClass
ChildClass
public class ChildClass : ParentClass
{
//
}
The base reference


Constructors are not inherited

To invoke the parent's
constructor

The base reference can also be used to reference other
variables
and
methods
defined in the parent’s class
Slide 25
: base (parameters);
base.VariableName
base.MethodName( parameters );
The base reference (cont.)
public class NegativeNumberException : ApplicationException
{
public NegativeNumberException()
: base( "Phai nhap vao so khong am" )
{
}
public NegativeNumberException( string message ) : base( message )
{
}
public NegativeNumberException( string message, Exception inner )
: base( message, inner )
{
}
}
Slide 26

Overriding methods

A child class can redefine a base-class method; this method
is called
overriding
method

To be overridden, a base-class method must be declared
virtual

To write an overriding method, using override keyword in
the method header

To view the method header for a method, using Object
Browser

Example: view Object’s methods
Slide 27
Object class (p.215)

All classes inherit from Object

All classes have access to the protected and public members
of this class (table p.215)

You can override some methods of Object class (if a method
is being overridden)

Example: public override string ToString()


Some Object’s methods:

ToString(): example

Equals(object): example

GetHashCode(): example
Slide 28
Example: class Point
public class Point
{
private int x, y;
public Point()
{
}
public Point( int xValue, int yValue )
{
X = xValue; // use property X
Y = yValue; // use property Y
}
public int X
{
get { return x; }
set { x = value; }
}
Slide 29
public int Y
{
get { return y; }
set { y = value; }

}
public override string ToString()
{
return "[" + x + ", " + y + "]";
}
}