The document discusses several key differences between programming concepts: - Abstraction vs encapsulation focuses on design vs implementation levels and hiding unwanted vs internal details. - Composition vs aggregation defines strong relationships where entities need each other vs weak relationships where entities can exist independently. - Private classes are only accessible within their definition vs sealed classes which can be accessed by any class but not inherited from. - Static classes contain only static members and no instances vs sealed classes which can contain static and nonstatic members and instances. - Virtual methods allow overriding but not require it, while abstract methods require derived classes to override them.

1. Difference between Abstraction and Encapsulation
Abstraction Encapsulation
Abstraction solves the problem in the Encapsulation solves the problem in the
design level. implementation level.
Abstraction is used for hiding the Encapsulation means hiding the code
unwanted data and giving relevant data. and data into a single unit to protect the
data from outside world.
Abstraction allows us to focus on what Encapsulation means hiding the internal
the object does instead of how it does it details or mechanism of how an object
does something.
Abstraction- Outer layout, used in Encapsulation- Inner layout, used in
terms of design. terms of implementation.
For Example:- For Example:- Inner Implementation
Outer Look of a Mobile Phone, like it detail of a Mobile Phone, how keypad
has a display screen and keypad buttons button and Display Screen are
to dial a number.
connected with each other using circuits
Difference between Composition and Aggregation
Composition Aggregation
Defines a strong-coupled relationship Defines a weak-coupled relationship
between two entities, where the one between two entities, where one entity
entity is part of another, and both need could be part of another, but either can
each other for their existence. exist without the other, independantly.
e.g. Human body and the Heart. e.g.School and teacher.
Composition implies real ownership of Aggregation does not necessarily own
its components any of its aggregates.
Composition has a stronger bond of its Aggregation has weaker or looser bonds
components. with its aggregates.
Composition has components that exist Aggregation has aggregates that live at
at the inner level. the outer level.

2. Difference between Private Class and Sealed Class
Private Class Sealed Class
A Private class can only be accessed by A Sealed class can be accessed by any
the class it is defined and contain within class.Private Constructor of a Private
- it is completely inaccessible to outside Class = Sealed Class.
classes.
In private Class,we can create a In Sealed class we can not create a
constructor and therefore we can create constructor of that class, so no instance
an instance of that class. of that class is possible.
public class A public sealed class A
{ {
private class B }
{ public class B : A //ERROR
} {
B b = new B(); }
}
public class C
{
A.B b = new A.B(); // ERROR
}
The main use of Private class is to create The sealed classes are mainly used to
a user-defined type, which we want to be prevent inheritance features of object
accessible to that class only. oriented programming.
Private class(i.e private constructor) is
also used to implement singleton
classes(pattern). Singleton means "A
single-instance object, and it simplify
complex code. Singletons have a static
property that we must access to get the
object reference."
Difference between Static Class and Sealed Class
Static Class Sealed Class
We can neither create their instances, We can create their instances, but cannot
nor inherit them inherit them
They can have static members only. They can contain static as well as
nonstatic members.
ex: ex:
static class Program

3. { sealed class demo
{
}
}
class abc:demo
{
--Wrong
}
Static classes are used when a class The sealed classes are mainly used to
provides functionality that is not specific prevent inheritance features of object
to any unique instance. oriented programming.
Difference between Virtual method and Abstract method
Feature Virtual method Abstract method
Overriding Virtual method may or may An abstract method should be
not override by inherited class. overriden by inherited class.
i.e.,Virtual method provide i.e.,Abstract method forces
the derived class with the the derived class to
option of overriding it. override it.
Virtual = = Overridable abstract == MustOverride
Implementation Virtual method has an Abstract method does not
implementation. provide an implementation.
Necessity to Virtual methods allow Abstract methods in a
Implement subclasses to provide their class contain no method
own implementation of that body, and are implicitly
method using the override virtual
keyword
Scope Virtual methods scope to Abstract method's scope to
members only. members and classes
Instancing Virtual methods - Not Abstract method - direcly
applicable, as we can't NO, but other way, Yes.
create instance for We can create an instance
members, it is possible only of a class that derives from
with classes. an abstract class. And we
can declare a type Abstract
class and instantiate that
as a derived class.

4. Example:
public abstract class Test
{
public abstract void A(); // Abstract method
public virtual void B()
{
Console.WriteLine("Test.B"); } // Virtual Method
}
public class InheritedTest : Test
{
public override void A()
{
Console.WriteLine("InheritedTest.A");
}
//Method B implementation is optional
public override void B()
{
Console.WriteLine("InheritedTest.B");
}
}
Difference between Class and Static Class
Class Static Class
Class has Instance Members Static class does not have Instance
Members
In Class, Constructor has Access In Static Class, Constructor does not
Specifier. have Access Specifier.
In Class Constructor, initiation is done In Static Class ,Constructor will be
every time when an object is created called only one time .
for the class
In Class, Class members can be In Static Class, members can be
accessed through class object. accessed through its Class name only

5. Difference between Method Overloading and Method overriding in C#
Method Overloading Method overriding
Method Overloading is passing same Method Overriding is redifining parent
message for different functionality class function to the child class
Method Overloading is between the Method Overriding is between the same
same function name with the different method.
signature
Method Overloading does not check for Method Overriding checks the return
the return type. type.
Method Overloading takes place in the Method Overriding takes place between
same class. parent and child classes
Method Overloading is static binding Method Overriding is a dynamic
binding.