C# Starter L02-Classes and Objects

Mohammad Shaker
mohammadshaker.com
C# Programming Course
@ZGTRShaker
2011, 2012, 2013, 2014
C# Starter
L02 – Classes, Polymorphism, Versioning,
Interfaces, Reference and Value Types

Today’s Agenda
• Classes
• Inheritance
• Polymorphism
• Versioning
• Interfaces
• Reference types VS Value types

Object oriented programming (OOP) is a way
of programming where you create chunks of
code that match up with real world objects.

A Class
• Let’s create a Player
• And a Constructor
class Player
{
private string name;
private int score;
private int livesLeft;
}
class Player
{
private string name;
private int score;
private int livesLeft;
public Player(string name)
{
this.name = name;
}
public Player(string name, int startingLives)
{
this.name = name;
livesLeft = startingLives;
}
}

Classes and Objects
Who is Who?

Objects appear only at Runtime

this Keyword
A pointer pointing on the object itself at runtime

this Keyword Usage
• Name hiding and clarity
• Passing Player instance at runtime to other object.
• Cloning the Player object (instance at runtime) into another Player object (in
constructor.)
public Player(string name)
{
this.name = name;
}
public Player(string name, int startingLives)
{
this.name = name;
livesLeft = startingLives;
}

Destructor
• Correspond to finalizers in Java.
• Called for an object before it is removed by the garbage collector.
• No public or private.
• Is dangerous (object resurrection) and should be avoided
class Test
{
~Test()
{
... finalization work ...
// automatically calls the destructor of the base class
}
}

using System;
public class ParentClass
{
public ParentClass()
{
Console.WriteLine("Parent Constructor.");
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}
public class ChildClass : ParentClass
{
public ChildClass()
{
Console.WriteLine("Child Constructor.");
}
public static void Main()
{
ChildClass child = new ChildClass();
child.print();
}
}
Inheritance

using System;
public class ParentClass
{
public ParentClass()
{
}
public void print()
{
}
}
public class ChildClass : ParentClass
{
public ChildClass()
{
}
{
ChildClass child = new ChildClass();
child.print();
}
}
Parent Constructor.
Child Constructor.
I'm a Parent Class.
Inheritance

public class Parent
{
string parentString;
public Parent()
{
}
public Parent(string myString)
{
parentString = myString;
Console.WriteLine(parentString);
}
public void print()
{
}
}
public class Child : Parent
{
public Child() : base("From Derived")
{
}
public new void print()
{
base.print();
Console.WriteLine("I'm a Child Class.");
}
{
Child child = new Child();
child.print();
((Parent)child).print();
}
}
Inheritance

public class Parent
{
string parentString;
public Parent()
{
}
public Parent(string myString)
{
parentString = myString;
Console.WriteLine(parentString);
}
public void print()
{
}
}
public class Child : Parent
{
public Child() : base("From Derived")
{
}
public new void print()
{
base.print();
Console.WriteLine("I'm a Child Class.");
}
{
Child child = new Child();
child.print();
((Parent)child).print();
}
}
From Derived
Child Constructor.
I'm a Parent Class.
I'm a Child Class.
I'm a Parent Class.
Inheritance

public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine(
"I'm just a generic drawing object.");
}
}
public class Line : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Line.");
}
}
public class Circle : DrawingObject
{
{
Console.WriteLine("I'm a Circle.");
}
}
public class Square : DrawingObject
{
{
Console.WriteLine("I'm a Square.");
}
}
Polymorphism

class Program
{
static void Main(string[] args)
{
DrawingObject[] dObj = new DrawingObject[4];
dObj[0] = new Line();
dObj[1] = new Circle();
dObj[2] = new Square();
dObj[3] = new DrawingObject();
foreach (DrawingObject drawObj in dObj)
{
drawObj.Draw();
}
}
}
Polymorphism

class Program
{
static void Main(string[] args)
{
DrawingObject[] dObj = new DrawingObject[4];
dObj[0] = new Line();
dObj[1] = new Circle();
dObj[2] = new Square();
dObj[3] = new DrawingObject();
foreach (DrawingObject drawObj in dObj)
{
drawObj.Draw();
}
}
}
I'm a Line.
I'm a Circle.
I'm a Square.
I'm just a generic drawing object.
Press any key to continue...
Polymorphism

{
public DrawingObject(string objectName)
{
}
{
Console.WriteLine("I'm just a generic drawing object.");
}
}
{
{
}
}
Polymorphism

{
{
Console.WriteLine(objectName);
}
{
Console.WriteLine("I'm just a generic drawing
object.");
}
}
{
public Line():base("ForBaseClass,
DrawingObject")
{
Console.WriteLine(this.ToString());
}
{
}
}
Polymorphism

{
{
}
{
object.");
}
}
{
DrawingObject")
{
}
{
}
}
ForBaseClass, DrawingObject
ConsoleApplicationCourseTest.Line
Polymorphism

{
{
}
{
object.");
}
}
{
DrawingObject")
{
}
public override string ToString()
{
return "just another Line object on runtime!";
}
{
}
}
Polymorphism

{
{
}
{
object.");
}
}
{
DrawingObject")
{
}
public override string ToString()
{
return "just another Line object on runtime!";
}
{
}
}
ForBaseClass, DrawingObject
just another Line object on runtime!
Polymorphism

Abstract Classes
• Abstract methods do not have an implementation.
• Abstract methods are implicitly virtual.
• If a class has abstract methods it must be declared abstract itself.
• One cannot create objects of an abstract class.
abstract class Stream
{
public abstract void Write(char ch);
public void WriteString(string s) { foreach (char ch in s) Write(s); }
}
class File: Stream
{
public override void Write(char ch) {... write ch to disk...}
}

sealed and internal classes
sealed: can’t be extended (Java’s final)
internal: can’t be used in other namespaces

class DerivedClass: BaseClass
{
public override string Meth1()
{
return "MyDerived-Meth1";
}
public new string Meth2()
{
}
public string Meth3()
{
}
{
DerivedClassmD = new MyDerived();
BaseClass mB = (BaseClass)mD;
System.Console.WriteLine(mB.Meth1());
}
}
Versioning
public class BaseClass
{
public virtual string Meth1()
{
return "BaseClass-Meth1";
}
{
}
{
}
}

{
{
}
{
}
{
}
{
}
}
Versioning
Overrides the virtual method
Meth1 using the override
keyword
{
{
}
{
}
{
}
}

{
{
}
{
}
{
}
{
}
}
Versioning
Explicitly hide the virtual
method Meth2 using the new
keyword
{
{
}
{
}
{
}
}

{
{
}
{
}
{
}
{
}
}
Versioning
Because no keyword is specified
in the following declaration a
warning will be issued to alert
the programmer that the method
hides the inherited member
BaseClass.Meth3()
{
{
}
{
}
{
}
}

{
{
}
{
}
{
}
{
}
}
Versioning
{
{
}
{
}
{
}
}
MyDerived-Meth1
BaseClass-Meth2
BaseClass-Meth3

Multiple Inheritance?
C#.NET doesn't allow it, Why?

Multiple Inheritance?
C#.NET doesn't allow it
C++.NET doesn’t allow it
Java doesn’t allow it
C++, as you know, allows it

However, C# allow
multiple interfaces

However, C# allow
multiple interfaces
But what are they?

Interfaces VS Abstract Classes

Interfaces
• An interface contains only the signatures of methods, delegates or events.
• The implementation of the methods is done in the class that implements the
interface.
• Can’t contain Fields!

Consider a Human, an Animal and a Car Class, where they all implement a crazy method called
ConsumeWater().
If we have many objects of each type of Human, Animal and Car and we want to call
ConsumeWater() for all objects of Human, Animal and Car; we have to call it like this:
human1.ConsumeWater();
animal1.ConsumeWater();
car1.ConsumeWater();

SomeObject
Human Animal Car
And they they can’t be subclassed from one particular abstract/base class like this:

SomeObject
Human Animal Car
And they they can’t be subclassed from one particular abstract/base class like this:
Because they are not the same and they share some common properties!

But if we can implement a common functionalities from a common place, that would be nice!
interface
Human Animal Car
Implementation
and not
inheritance!

But if we can implement a common functionalities from a common place, that would be nice!
IWaterable
Human Animal Car
Implementation
and not
inheritance!

Now we can add all objects to a common list of IWaterable and just call ConsumeWater()
for each IWaterable object (they are all Waterable now!)
List<IWaterable> waterables = new List<IWaterable>() {“human1”, “human2”, “human3”,
“animal1”, “animal2”, “car1”, “car2”};
foreach(IWaterable waterable in waterables)
waterable.ConsumeWater();
Look how nice the code is and how clear the relation is. When we implement an interface we are
just saying that this interface provides a certain functionality for us (and others may freely have this
functionality as well.)
IWaterable
Human Animal Car
Implementation
and not
inheritance!

Interfaces – the Code
Public interface IWaterable
{
public void ConsumeWater();
}

{
}
Public class Human: IWaterable
{
public void ConsumeWater()
{
}
}
Public class Animal: IWaterable
{
{
}
}
Public class Car: IWaterable
{
{
}
}

{
}
{
{
//Drinking Water
}
}
Public class Animal: IWaterable
{
{
//Drinking Water
}
}
Public class Car: IWaterable
{
{
//Cooling the engine
}
}

{
}
{
{
//Drinking Water
}
}
Public class Animal:IWaterable,
INosiable
{
{
//Drinking Water
}
public void MakeNoise()
{
//Mew, Roar or Moo!
}
}
Public class Car: IWaterable,
INosiable
{
{
//Cooling the engine
}
public void MakeNoise()
{
//Rev the engine!
}
}
Public interface INoisable
{
public void MakeNoise();
}

Interfaces
• An interface can be a member of a namespace or a class and can contain
signatures of the following members:
– Methods
– Properties
– Indexers
– Events
• “No” Fields!

using System;
class Program
{
static void Main()
{
float lengthFloat = 7.35f;
// lose precision - explicit conversion
int lengthInt = (int)lengthFloat;
// no problem - implicit conversion
double lengthDouble = lengthInt;
Console.WriteLine("lengthInt = " + lengthInt);
Console.WriteLine("lengthDouble = " + lengthDouble);
Console.ReadKey();
}
}
Reference VS Value Types

using System;
class Program
{
static void Main()
{
float lengthFloat = 7.35f;
// lose precision - explicit conversion
int lengthInt = (int)lengthFloat;
// no problem - implicit conversion
double lengthDouble = lengthInt;
Console.WriteLine("lengthInt = " + lengthInt);
Console.WriteLine("lengthDouble = " + lengthDouble);
Console.ReadKey();
}
}
lengthInt = 7
lengthDouble = 7

• Reference type
• variables are named appropriately (reference) because the variable holds a reference to an
object.
• In C and C++, we have something similar that which is “a pointer”, which points to an object.
While you can modify a pointer, you can't modify the value of a reference - it simply points at
the object in memory.

using System;
class Employee
{
private string _name;
public string Name
{
get { return _name; }
set { _name = value; }
}
}

Reference Types
class Program
{ static void Main()
{
Employee joe = new Employee();
joe.Name = "Joe";
Employee bob = new Employee();
bob.Name = "Bob";
Console.WriteLine("Original Employee Values:");
Console.WriteLine("joe = " + joe.Name);
Console.WriteLine("bob = " + bob.Name);
// assign joe reference to bob variable
bob = joe;
Console.WriteLine("Values After Reference Assignment:");
joe.Name = "Bobbi Jo";
Console.WriteLine("Values After Changing One Instance:");
Console.ReadKey();
}
}

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
Original Employee Values:
joe = Joe
bob = Bob
Values After Reference Assignment:
joe = Joe
bob = Joe
Values After Changing One Instance:
joe = Bobbi Jo
bob = Bobbi Jo

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
joe = Joe
bob = Bob
joe = Joe
bob = Joe
joe = Bobbi Jo
bob = Bobbi Jo
How is that?!

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
Emp Emp
joe bob

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
Emp
joe
Emp
bob

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
Emp
joe bob

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
joe bob
Emp
Name =
“Joe”

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
joe bob
Emp
Name =
“Bobbi Jo”

Reference Types
class Program
{
joe.Name = "Joe";
bob.Name = "Bob";
bob = joe;
Console.ReadKey();
}
}
joe bob
Emp
Name =
“Bobbi Jo”
joe = Joe
bob = Bob
joe = Joe
bob = Joe
joe = Bobbi Jo
bob = Bobbi Jo

Reference Types
• The following types are reference types:
• arrays
• class
• delegates
• interfaces

Value Types
• A value type
– variable holds its own copy of an object and when you perform assignment from one value
type variable to another, both the left-hand-side and right-hand-side of the assignment hold
two separate copies of that value.

Value Types
• A value type
– variable holds its own copy of an object and when you perform assignment from one value
type variable to another, both the left-hand-side and right-hand-side of the assignment hold
two separate copies of that value.
• An important fact you need to understand is that when you are assigning one
reference type variable to another, only the reference is copied, not the
object. The variable holds the reference and that is what is being copied.

struct Height
{
private int m_inches;
public int Inches
{
get { return m_inches; }
set { m_inches = value; }
}
}
Value Types

class Program
{
static void Main()
{
Height joe = new Height();
joe.Inches = 71;
Height bob = new Height();
bob.Inches = 59;
Console.WriteLine("Original Height Values:");
Console.WriteLine("joe = " + joe.Inches);
Console.WriteLine("bob = " + bob.Inches);
bob = joe;
Console.WriteLine("Values After Value Assignment:");
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
Height Height
joe bob

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
Height Height
joe bob
Height

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
Height Height
joe bob
Exactly
the
same

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
71 71
joe bob
Exactly
the
same

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
71 71
joe bob

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
65 71
joe bob

class Program
{
static void Main()
{
joe.Inches = 71;
bob.Inches = 59;
bob = joe;
joe.Inches = 65;
Console.ReadKey();
}
}
Value Types
65 71
joe bob
Original Height Values:
joe = 71
bob = 59
Values After Value Assignment:
joe = 71
bob = 71
joe = 65
bob = 71

Value Types
• The following types are value types:
– enum
– struct

Classes and Structs
Classes
• Reference Types
• (objects stored on the heap)
• support inheritance
• (all classes are derived from object)
• can implement interfaces
• may have a destructor
Structs
• Value Types
• (objects stored on the stack)
• no inheritance
• (but compatible with object)
• can implement interfaces
• no destructors allowed

Creating a Class Library Project for Your
Project’s Logic

Now write all your code in the Class Library project and reference it in your presentation layer project

Adding References to Other Projects to
Your Project

Adding References to Your Project

The Principles
• Single Responsibility Principle: design your classes so that each has a single purpose
• Open / Closed Principle: Open for extension but closed for modification
• Liskov Substitution Principle (LSP): functions that use pointers or references to base classes must be
able to use objects of derived classes without knowing it
• Interface Segregation Principle (ISP): clients should not be forced to depend upon interfaces that they
do not use.
• Dependency Inversion Principle (DIP): high level modules should not depend upon low level modules.
Both should depend upon abstractions.
abstractions should not depend upon details. Details should depend upon abstractions.

is the process of validating the correctness of a small section of code. The target code may be a method within
a class, a group of members or even entire components that are isolated from all or most of their dependencies.

C# Starter L02-Classes and Objects

Question #1
{
{
}
{
}
{
}
}
{
{
}
{
}
{
}
{
BaseClass mB = mD;
}
}

Question #1
{
{
}
{
}
{
}
}
{
{
}
{
}
{
}
{
BaseClass mB = mD;
}
}
MyDerived-Meth1
BaseClass-Meth2
BaseClass-Meth3

Question #2
class Class1 { }
class Class2 : Class1{ }
class Class3 { }
public class TestingClass
{
public static void Test(object o)
{
Class1 a;
Class2 b;
Class3 c;
if (o is Class1)
{
Console.WriteLine("obj is Class1");
a = (Class1)o;
}
else if (o is Class2)
{
b = (Class2)o;
}
{
c = (Class3)o;
}
else if((Class3)o!= null)
{}
}
{
try
{
Class1 c1 = new Class1();
Test(c1);
Test(c2);
Test(c3);
Test("a string");
}
catch(Exception e)
{
Console.WriteLine("Sth wrong happened!");
}
}
}

Question #2
class Class1 { }
class Class2 : Class1{ }
class Class3 { }
public class TestingClass
{
public static void Test(object o)
{
Class1 a;
Class2 b;
Class3 c;
if (o is Class1)
{
a = (Class1)o;
}
{
b = (Class2)o;
}
{
c = (Class3)o;
}
else if((Class3)o!= null)
{}
}
{
try
{
Test(c1);
Test(c2);
Test(c3);
Test("a string");
}
catch(Exception e)
{
Console.WriteLine("Sth wrong happened!");
}
}
}
obj is Class1
obj is Class1
obj is Class3
Sth wrong happened!

public interface IsBaseTest
{
void Point1(object obj);
}
public class IsTest
{
public static void Point1(object obj)
{
Console.WriteLine(obj.ToString());
Point2("That's the point");
}
public static void Point2(string str)
{
Console.WriteLine(str.ToString());
}
{
if (obj.ToString() == " Passed String")
{
Console.WriteLine("In Point3");
}
}
{
Point1("Passed String");
}
}
Question #3

public interface IsBaseTest
{
void Point1(object obj);
}
public class IsTest
{
{
Console.WriteLine(obj.ToString());
}
public static void Point2(string str)
{
Console.WriteLine(str.ToString());
}
{
if (obj.ToString() == " Passed String")
{
Console.WriteLine("In Point3");
}
}
{
Point1("Passed String");
}
}
Question #3
Passed String
That's the point
In Point3

That’s it for today!
Hope you enjoy it!

C# Starter L02-Classes and Objects

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C# Starter L02-Classes and Objects