Part 52 C# Tutorial Attributes in C# Video Lecture | C sharp: Enhance your Skills (English) - Back-End Programming

hello welcome to presume technologies I
am venket this is part 52 attributes in
this session we'll understand the
purpose of attributes using an attribute
in example and customizing attributes
using parameters now let's understand
what an attribute is and what is the
purpose of attributes in life basically
attributes allow us to add declarative
information to our programs and this
information can then be retrieved at
runtime using reflection let's get an
example which will make this clear to us
now let's say I have a very simple
calculator class so public class
calculator and all this class does is it
has got an ADD method which can
basically add two numbers so first
number and a second number so what this
method does it adds those two numbers
and return their sum so return the sum
of first and second numbers okay now if
people want to use this method all they
do is since this is a static method they
can invoke it on the name of the class
so calculate a class dot add and then
maybe I want to add ten and twenty okay
so here if I want to add two numbers
then this method is useful let's say for
example there are users who want to add
three numbers so is this function useful
now so what we have to do is probably
add another overloaded version of add
that can add three numbers so if I just
make a copy of this so maybe I will have
something like this maybe another third
number the end
third number and then what we will do we
will return the sum of that as well okay
all right so if somebody wants to add
three numbers they can do that now
they can also pass it maybe another
third number okay fine but let's say
there is another user who wants to add
five numbers now if you look at this we
might add another method which takes in
five parameters but what if there is a
user who wants to add twenty parameters
do you create a method with 20
parameters now so basically the way we
have thought about this you know
initially when we are developing the
calculator class you know we didn't
think about the requirements of the
future and we just provided an ADD
method which can add two numbers but the
requirements over the period of time
have changed and users want to add more
number of numbers and Allah uses to do
that okay we are adding an enhancement
to a class and probably what you can do
is I am adding maybe a list of integers
and by the way list is a generic
collection which is present in system
dot collections dot generics so input
that name space and then what I want to
basically do is list of end and then we
can pass in numbers so now if you look
at this method whoever wants to add like
three or four or five or even two you
know they can just pass in those two
numbers as a list of integers and your
method will be capable of doing that so
let's quickly implement this so for each
end number n numbers so what we want to
do we want to add those numbers and
let's create a simple sum variable to
hold the sum so in the sum is equal to
zero and what we can do sum is equal to
sum plus number and basically this is
the implementation of this
method and finally we want to return the
sum okay now if you look at this method
this method is pretty useful now this
method is this method can basically add
two numbers three numbers five numbers
you know depends on the user how many
numbers he wants to add because it is
take again a list of numbers and why did
we introduce this method because of the
limitations of this number this method
okay it can only add two numbers this
method but whereas this method can add
any number of numbers okay so as we go
forward you know this calculator class
and this calculator class if you look at
this this method is an outdated method
but whereas this is a new method so
anybody who is using your calculator you
want the users to use the new method
that you have introduced here instead of
the old add method okay so how do you
tell the users to do that now you might
be thinking okay instead of doing that
what if I am to remove this method okay
if you remove this method you know there
might be users who are using your add
method before and they will break okay
that's why you have to leave that old
method there for you know backward
compatibility people who are already
using you am an ADD method shouldn't
break and they should continue to work
the way they are but new people who ever
are going to use your calculator you
know you want to tell them to use the
new method how do you do that you know
an example would be to use an attribute
okay so within the dotnet framework
there's an attribute called
obsolete attribute which we can use now
using that attribute I can add
declarative information to this method
for example the absolute attribute the
way we add it is now look at this this
user is using this add method and we
don't get any compilation error
I mean error or a warning message you
get this when I try to build the
solution and if I go into the output
window it says okay we don't have errors
or warnings so let's say rebuild
solution and if you now go to the window
so one succeeded zero phase
Elda zero skip there are no errors and
there are no warnings okay now what I
want to basically do is if somebody uses
this method ad which takes in two
numbers instead of the list one I want
to show a warning message in the
compiler window is that possible
absolutely that's possible using
attributes and we can make use of
obsolete attribute and you look at the
name obsolete the name itself says
something is obsolete outdated this
method is outdated and this is the one
which people should be using so now what
I can do is I can use the obsolete
attribute so just decorate your method
with obsolete and look at this the
moment we have done that you see a green
squiggly line there indicating that
calculated not ad which takes into
integer parameters is obsolete look at
that window okay so so this I'm using
this attribute to add some declarative
information to this method okay so if
you look at the definition so what's an
attribute and attributes allow you to
add the clarity of information to your
program this information can then be
queried at runtime using reflection so
using reflection what what this visual
studio is doing is okay this add method
has got obsolete okay so it's it's
checking that when I am using that add
method it's actually checking okay there
is an obsolete attribute and the purpose
of this one is to generate compiler
warnings and errors and it it displays
that information to us and how is that
possible because of this attribute so
attributes basically allows you to add
declarative information to your methods
or types or even at the assembly level
you can apply attributes to properties
methods classes etc and the way you
apply them is you apply let's say for
example I want to add an attribute to
this method I just specify it just
before that you know using square
brackets okay fine now let us say how to
customize this attribute now if you look
at this attribute okay if I build my
solution
okay build succeeded on the status bar
if I go to the output window you should
see that you know there is zero errors
and one warning and if you look at that
warning it clearly says okay calculated
dot add int comma end is obsolete right
that's fine it's obsolete okay if I am
not supposed to use this add method then
which method am I supposed to use you
know the way you are using the obsolete
attribute here it's not that useful to
that to the developer who is using your
class because you are telling him this
add method is obsolete but you're not
providing him with an alternative and to
do that you can actually customize this
okay so what I basically can do look at
this to this attribute I can pass in
look at this there are three overloaded
you know obsolete attribute constructors
okay so one of three one version which
does not take any parameter if you look
at the other one it takes a string
message and if you look at another one
it takes a boolean error okay so which
means using this overloaded version of
this attribute and look at this when I
right click on this absolute attribute
and say go to definition
what's an attribute an attribute is
nothing but a class look at this you
know the name of this class is obsolete
attribute and this is inheriting from
the base system would attribute class so
keep that in mind all attributes
directly or indirectly inherit from
system to attribute class and what are
attributes they're nothing but they're
also classes and if you look at this
class it has got three constructors and
when you are using that attribute you're
actually creating an instance of that
attribute class and you are using one of
the overloaded constructors and passing
in parameters to customize the message
that you want to have displayed okay in
this case let's say I want to tell the
user to use this method instead of add
in to end so how do I do that
there is one of the overloaded
constructor of this obsolete attribute
class which takes in a string message so
I can pass my string message here so
what I'm saying is use a dove list which
takes a list of integers okay so what we
are doing now is basically we are
passing in this message into this
attribute now if I hover my mouse over
here okay let's fill this and now if you
look at this look at that
it has calculated that add that takes in
two integer parameters is obsolete but
whereas ad list of int I mean it says
use add list of int numbers method okay
so this method is you obsolete
use add of list I mean add method that
takes in a list of parameters so now if
I just say instead of this one okay
let's pass an list of integers so list
of integers and let's say I want to pass
in 10 20 and 40 and let's see what
happens now if we build a solution okay
do I have the compiler warning now I
don't
so obviously this has served my purpose
now I want to tell to my users of this
calculator class you know this method is
obviously they shouldn't be using that
how am i doing that using the obsolete
attribute we have also seen how to
customize the message that is shown to
the user using parameters and we also
understood that an attribute is nothing
but a class that inherits from system
dot attribute base class and another
thing if you look at this if I right
click on that and say go to definition
the name of the class is actually
obsolete attribute but if you look at
the way we are using it we are saying
obsolete so how is it possible actually
you can also use attribute here so this
attribute part of the attribute is
optional okay now the behavior is
exactly the same except that you this is
optional you can either use it or not
okay so let's get back to the slides so
we have seen how to you know use
attributes custom as a message they
display to the user and with a dotnet
framework you know actually there are
two types of attributes one is the
predefined attributes which are actually
provided by dotnet framework like
obsolete attributes we have seen an
example of how to use that and there is
a web method attribute for example if
you are an asp.net programmer and if you
are developing web services and as part
of that web services you want to expose
a method as a web service method then
you have to decorate that with web
method attribute probably when we talk
about asp.net that's when we will see
how to use this web method attribute and
similarly for example if you're working
across application domains for example
if you're developing a remoting
application or a WCF application where
the objects will be crossing application
domain boundaries then those object
needs to be serialized and one of the
simple ways of serializing a class is to
make use of serializable attribute okay
so there are several attributes like
this and it is also possible to create
our own custom attributes okay but
creating custom attributes women we know
we'll talk about that in a later session
but just understand that you know there
are lots and lots of predefined
dotnet attributes that we can make use
of for a variety of purposes okay and we
have also understood an attribute is a
class that inherits from system dot
attribute base class on this slide you
can find resources for asp.net and C
sharp interview questions and by the way
I just forgot to tell you one more thing
I mean this attribute class you can
customize it to the next level as well
if you look at this absolute attribute
it has got another constructor which
takes it takes in another parameter
boolean true or false and it says error
okay now by default that's false meaning
look at this I can pass the second
parameter into this method and what does
it say the boolean value that indicates
whether the obsolete element usage is
considered an error now if you want if
you want the users to prevent using this
method completely then you can say I
mean - this parameter you can pass in
true so now when you pass in true to
this parameter you're basically telling
okay this method is obsolete if anybody
tries to use this method you know just
raise an error that's what this
parameter does so if I go ahead and use
the older version you know from 10 comma
50 and look at the moment I do that the
moment I flip to using this method I
already have a red squiggly there
indicating you know calculator to pad is
obsolete and it's red squiggly meaning
an error if I try to go to the output
window you should see I mean error list
you should already see that ok
calculator don't add int comma int is
obsolete use this method instead okay so
it's also possible to raise error
message so it's completely attributes
using parameters you can completely
customize them and then use it to the T
or need all right so on the slide you
can find resources phrase we don't need
C sharp interview questions that's it
for today thank you for listening have a
great day