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In object-oriented programming,

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the class that uses another class
is called the client of the class.

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Don't confuse the term client here
with client server or business client.

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A client is simply a class that
uses another class in some way.

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We often want to make it so
that clients of a class can

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access the items inside of a collection
that's stored in the class.

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Let's make a property in the SchoolRoll
class that exposes the students list.

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Let's add a property named Students
here to return the students list.

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So say public List of student,
call it students and

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have the getter return
the private students list.

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Students is a read-only property.

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It doesn't have a setter, only a getter.

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All this means is that
clients of this class can't

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overwrite the private
students field up here.

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However, they can still
alter the students list.

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Back here in main, let's add this
list of students to the SchoolRoll.

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I've cleared out everything
in main except for

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where we're creating the student list and
where we're printing it out.

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We'll create an instance of SchoolRoll and

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then add students to it using
the addStudents method.

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So say SchoolRoll schoolRoll

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= new SchoolRoll And

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then schoolRoll.AddStudents.

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And then we'll pass in the students list.

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Down here we'll want to change this to

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SchoolRoll.Students, so

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that we're printing out all of
the students in the school.

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Having this Students property is great

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because it gives us
access to the students.

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But because we've exposed this students
list using this property in the SchoolRoll

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class, we can do what ever we want to the
students list such as remove the students.

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So we can do something like
schoolRoll.Students.RemoveAt(0) or

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we could sort them.

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This could cause problems if the student
roll class was reliant on the students

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being in a certain order already.

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Or we could add more students and
completely bypass the AddStudent method.

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So we could do something like
schoolRoll.Students.AddRange(students):.

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This is dangerous because there might have
been some reason why we wanted clients of

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our SchoolRoll class.

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To use the AddStudents method instead
of adding them directly to the list

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like this.

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Perhaps the AddStudents method had checked
that duplicate students weren't being

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added to the rolls.

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By allowing this type of write
access to the student list directly

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via the student property.

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We've created an opportunity for clients
of the SchoolRoll class to introduce bugs.

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So what do we do?

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Once again,
interfaces can save the day here as well.

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If we only wanted clients of our
SchoolRoll class to be able to loop

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through the students, we could change the
type of our property here to IEnumerable.

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We can return the student list as
an IEnumerable because the list class is

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IEnumerable.

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Since IEnumerable doesn't
expose any methods that can

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alter the list in any way.

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This essentially makes the list
read only from the point of view of

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the client classes.

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Take a look at the other interfaces
that the List class implements.

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You'll notice a number of interfaces
that start with IReadOnly.

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IReadOnly collection allows clients to see
how many students are in the list using

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the count property and IReadOnly list
allows clients to index into the list.

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We could have used either of
these collection types as well.

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The purpose of read-only
collection interfaces

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is not to provide some level of
security against malicious coders,

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the underlying collection
here is still a list.

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If we really wanted
access to the list again,

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we just need to cast the object returned
by the student's property back to a list.

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This is just another form
of type encapsulation.

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By exposing the list this way,
we are making it clear to other developers

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how we expect the SchoolRoll
class to be used.

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You might say that this way
our code is self-documenting.

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We should always try to write
clear self-documenting code

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in order to help other coders
avoid accidentally misusing it.

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This can greatly reduce the chances for
bugs.