Part 32 - C# Tutorial - Abstract classes in c#.avi Video Lecture - C sharp

hello welcome to presume technologies
and bankit this is part 32 abstract
classes in this session we learn the
basics about abstract classes and in a
later session we will learn way to use
these abstract classes in a real time
project to create an abstract class we
use abstract keyword let's look at an
example okay usually to create a class
we use the class keyword for example if
I want to create a customer class I
would say class customer so this is a
non abstract class to make it abstract
we just use an abstract keyword so now
customer class is an abstract class so
what do we mean by abstract class
abstract classes are incomplete meaning
they have abstract members now within a
class any member can basically be an
abstract member of for example
properties methods events etc okay so
how do we make a member abstract again
just by using abstract keyword for
example let's say this customer class is
going to have a print method and I want
that print method to be an abstract
method the way we do it is public and
the method doesn't return anything and
then print now if we provide a method
like this inside a class then this is a
non abstract method if you want to make
this an abstract method all you do is
you use the abstract keyword now when
you use the abstract keyword the method
cannot have implementation okay because
an abstract method is an incomplete
method and it is there for any class
that's going to derive from this
abstract class you know to provide
implementation
okay so if I go ahead and build this we
will actually get a compiler error
stating that you know print cannot
declare a body because it is marked as
abstract so any abstract member cannot
have an implementation
it's basically incomplete so now if you
look at this customer class is an
abstract class and it has a print method
now I don't do abstract classes are
incomplete because they don't have
implementation
for all of its members because they are
abstract so since they're incomplete
classes it doesn't make sense to be able
to create an instance of this class and
use those members to do something for
you that's why you cannot create an
instance of an abstract class abstract
classes cannot be instantiated they can
only be used as base classes for other
classes okay so for example for this I
mean this program class can inherit from
customer class so when and non-si if you
look at this program class it's a non
abstract class because why it doesn't
have the abstract keyword so when an on
abstract class inherits an abstract
class then your non-abstract class has
to provide implementation for all the
abstract members within the base class
if it doesn't and if I try to compile
then I get a compiler warning okay I
mean let's put the access modifier here
that's what the error is about we'll
talk about access modifiers in a later
session so let's go ahead and build the
solution so program does not program
class does not implement abstract member
customer dot print so since this class
is inheriting from the customer class
then this class has to provide
implementation for that abstract method
if the program class doesn't wish to
provide the implementation then you can
mark this class abstract as well so if I
go ahead and build this now it will
happily you know succeed the bill that's
because you know you're marking this
program class is abstract meaning it's
it has got some abstract members so
program is inheriting from customer so
this abstract print method which this
class has increment you know inherited
it's not implementing that so we are
turning this class also an abstract
class okay so the bottom line is if a
class inherits an abstract class then
the class has got two options okay let's
look the slide option one provide
implementation for all the abstract
members inherited from the base abstract
class or if the class does not wish to
provide implementations or all the
abstract members and head
from the base abstract class then the
class has to be marked as abstract so
you have to do one of these two options
if your class is going to hand headed
from an abstract class okay here we
decided to mark up a class as abstract
but on the other hand if you don't want
that to be abstract then you have to
provide the implementation for this
print abstract method okay and to do
that say public void print so now we are
actually providing the implementation
for the abstract method all right so now
if we go ahead and build this oh yeah in
order to provide the implementation you
have to use the override keyword so now
if we go ahead and build Cobell solution
the build succeeds okay and obviously if
you want to call that method you have
two ways you can either create an
instance of the program class like so
program P equals new program and then
you can say P dot print method P dot
print you know you can either do this or
you can create an instance of your
abstract class remember you cannot
create an instance of an abstract class
but an abstract class reference variable
can point to the derived class object so
what we can do here instead of creating
when a program P you can say customer C
is equal to new program this is possible
you know from the concepts of
inheritance we know that a derived class
object you know a a parent class
reference variable can point to a
derived class object okay so similarly
here we can just say seed or print and
if we go ahead and run that you know
print method alright so let's go back to
the slides so we have seen that we use
the ABS abstract keyword to create an
abstract class and
abstract class basically is incomplete
and hence cannot be instantiated an
abstract class can only be used as a
base class you cannot you know create an
instance of that class and you use that
an abstract class cannot be sealed this
is a very important point to remember
we know that okay in the inheritance
chapter we learnt that if you want to
prevent your class from being inherited
by another class then we use a sealed
keyword okay so if we use the sealed
keyword then we know that that class
cannot be used as base class okay but
whereas by definition an abstract class
can only be used as a base class so what
you cannot do is you cannot say okay my
class is going to be abstract and sealed
at the same time why because they
contradict each other
abstract means this class can only be
used as base class sealed means it
cannot be used as a base class so they
do contradict that's why a class cannot
be an an abstract class cannot be sealed
or a seal the class cannot be abstract
an abstract class may contain abstract
members you know methods properties
indexers
events all of these can be abstract but
not mandatory for example I can have a
class okay I am I'm creating this
customer class and it has a print method
and its abstract but I can create a
class without abstract members and still
mark my class as abstract that's
possible and if the class is an abstract
class there's no guarantee that it's
going to have some abstract members your
class can have all members implemented
and it can and it can still be abstract
that's fine let's look at that so when I
say console dot right okay just hit
print and don't let's comment that let's
comment this as well now if you look at
this class customer it's marked as
abstract but it doesn't need it haven't
I mean it didn't get any abstract
members if I try to go ahead and do that
it builds successfully with
any issues so keep the point in mind an
abstract class may contain abstract
members but not managed we okay and a
non-abstract class derived from an
abstract class must provide
implementation for all inherited
abstract members okay basically we know
that if a class you know whether its
abstract or non abstract if a class
inherits an abstract class then there
are two options available for that class
you know the class can either wish to
provide implementation for all of its
members and and remain to be a non
abstract class or you can turn the class
into non abstract and it may not provide
implementation for you know inherited
abstract members in the next session we
will actually talk about you know way we
use this abstract classes in a real time
project on this slide you can find
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interview questions that's it for today
thank you for listening have a great