Part 80 Some useful methods of List collection class Video Lecture | C sharp: Enhance your Skills (English) - Back-End Programming

this is part 80 of c-sharp tutorial in
this video we'll discuss some useful
methods of list collection class this is
continuation to part 79 so please watch
about 79 before proceeding with this
video in this video we'll be discussing
these three functions true for all
Athlete only trim access let's look at
true for all this method returns true or
false depending on whether if every
element within the list matches the
conditions that are defined by the
specified per a decade let's understand
this with an example we'll be using this
customer class that we have been working
with in the previous sessions and within
the main method we are creating an
instance I mean three instances of this
customer class customer one customer two
and customer three and then here we have
a list of customers and then to that
list we are adding the three customer
objects ok let's look at using this true
for all method now let's say I want to
check if all the customer objects within
the list have got a salary of five
thousand how do we normally do that we
loop through each customer object and
then check their salary but instead of
that we can use this true for all
function and then specify our condition
here so true for all let's pass the
predicate X such that X dot salary
greater than five thousand and if you
look at this true for all method what is
it returning it's returning a boolean
true or false okay so if all the objects
within that list matches that condition
this method returns true otherwise it
returns false
so let's actually print the message
console dot readline or all
salaries greater than 5,000
all right so let's go ahead and run this
and if you look at the customer objects
that we have here this customer one
object has got a salary less than five
thousand so not all objects meeting that
condition so it's false on the other
hand if we say are all salaries greater
than three thousand and then let's
change this condition to three thousand
so are all these customer objects having
salary greater than three thousand
that's true so let's go ahead and run
this now true for all should return true
okay so very handy method in case if you
want to check if all the elements of a
collection matches your conditions all
right another useful function is this as
a read-only so this function returns a
read-only wrapper for the current
collection now we use this method if we
don't want the client code to modify the
collection that is we don't want the
client code to add or remove any
elements from that collection so the
read-only collection will not have
methods to add a remove items from the
collection you can only read items from
this collection let's understand this
with an example at the moment if you
look at this list customers okay so this
collection has got add method so
basically you can use this method and
add a customer object similarly if you
want to remove a customer object there
are several ways you can use remove
remove all remove add etc so basically
this collection is allowing you to
modify the collection that is you can
add and remove elements okay but then
let's say our requirement is such that
we are going to pass a collection object
to you know some of the client
application and that we don't want that
application to be modifying our
collection if that's your requirement
then you can use this as a read-only
method on this collection and if you
look at what this method is returning it
is returning a read-only collection
object okay so look at that read-only
collection of customer ok and if this
read-only collection class is present in
system that object collection start
object model name
space so make sure you have included
that namespace and so what are we
getting back we are only we are getting
a read-only collection of customer and
maybe let's call this read-only
customers equals that and if you look at
this collection look at that I don't
have admitted similarly I don't have
remove or remove all any of those
methods which allows you know to add or
remove elements from the collection okay
but then you can retrieve the items from
the collection using the index look at
that I can pass an index and look at
what it is returning it's returning a
customer object back okay
again if you look at the implementation
of this index it has got only the cat
axis a meaning you can only retrieve an
item from this collection but you cannot
set an item meaning you cannot replace
an existing item within that correction
okay but then you can retrieve the items
definitely similarly if you want to get
the count you can use the count property
so let's say we want to print the total
count of items within the collection we
can simply say items equals whatever is
the count
let's see if we get that you know the
read-only collection should also have
three elements within that look at that
we get that three elements all right so
very useful function the other method
that we will discuss is trim access so
what is this method going to do it's
going to set the capacity to the actual
number of elements in the list if that
number is less than a threshold value so
basically if you look at this list when
we have created this list V you have
used a constructor you know where we
have said the initial capacity to four
but let's set the initial capacity to
hundred so at the moment we have said
the capacity of the list 200 but how
many elements do we have it in the list
we have only three elements now let's go
ahead and print the capacity and to
print the capacity less
customers that capacity property will
give us the capacity of that list let's
go and print that using console dot
write line and let's say capacity before
trimming that is before invoking the
trim access function okay so let's go
ahead and run this now and look at the
capacity
so what's capacity capacity is hundred
at the moment
okay now let's invoke trim access
function and what is this method going
to do it's going to set the capacity to
the actual number of elements how many
actual number of elements do we have
within the list three so it's going to
set the capacity to that number
after we invoke this function so
capacity after trimming let's run this
look at that
capacity before trimming is hundred but
capacity after trimming is three so what
is the advantage of invoking this method
this method can be used to minimize a
collections memory overhead if we know
that we are not going to add new
elements to that collection and the but
then one thing that we have to keep in
mind is the cost of reallocating and
copying a large list can be considerable
so basically what does this trim access
function doing here here we have set the
capacity to hundred while we have only
three elements within the list okay so
when we invoke the stream access what it
has done is it has reallocated and
copied all those elements okay so that
now the list capacity is only three okay
so this can improve the performance of
our application a bit but then if I have
100 I mean if we have set the capacity
to hundred and if we already have 90
elements and then when we invoke trim
access and if it has to do that
reallocation again then that process
will be much expensive than the
performance gain that we get that's why
the threshold value at the moment
according to MSDN is 90% so if the list
has
you know the capacity of 90% already
full then in working trim access is
going to do nothing okay
so that's something that has to be kept
in mind but the threshold of 90% can
change in the future all right that's it
for today thank you for listening have a
great day