Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Preview
Start a free Courses trial
to watch this video
In this video, we'll see how to explicitly cast a base type to a subtype and we'll introduce our second challenge—creating a class in order to encapsulate our media type items array.
Instructions
2nd Challenge
-
Encapsulate the array of media type items inside of a class.
- Create a class named "MediaLibrary".
-
In the MediaLibrary class...
- Define a private array field of type MediaType named
_items. - Initialize the array via a constructor method that accepts an array of media type items.
- Add a method named
GetItemAt()that accepts an index value and returns the media type item at the provided index.
- Define a private array field of type MediaType named
-
In the Program.cs file
Main()method...- Instantiate an instance of the MediaLibrary class.
- Pass into the constructor the existing collection of items.
- Update the rest of the code to use the
GetItemAt()method to retrieve items from the media library.
Abstract Members
For more information about abstract classes and class members in C# see the Intermediate C# course in the Treehouse library.
Help
If you get stuck on any of the following topics or simply need a refresher, click on a topic in list below to view the associated video in the C# Objects course.
At the end of the previous video, we were
getting an error when attempting to call
0:00
the GetDisplayText method on an element
in our MediaType items array.
0:04
Type Treehouse.MediaLibrary.MediaType
does not contain a definition for
0:10
a GetDisplayText, and
no extension method GetDisplayText of type
0:15
Treehouse.MediaLibrary.MediaType could
be found.
0:19
Luckily, this error tells us
exactly what's happening.
0:23
Because our array items, RF type,
media type, and the GetDisplayText method
0:27
isn't defined on the MediaType class,
we can't call that method.
0:32
We can work around this limitation
using an approach similar
0:36
to what the DetectMediaType method uses.
0:40
Let's make a copy of that method.
0:43
First, I'll select all of this code for
the DetectMediaType method,
0:45
copy it to the clipboard, and
paste it right above that method.
0:49
Then I'll change the method
name to Display, and
0:57
remove the code contained within each of
the conditional statement code blocks.
1:00
So here, here, and here.
1:05
By using the C# is operator, we can
detect the MediaTypeItem subclass type.
1:11
Then we can use an explicit
cast to convert the item
1:16
from the MediaType type
to its subclass type.
1:20
So if (item is Album),
1:23
then var album = (Album) item.
1:28
This line of code explicitly casts
1:36
the MediaType item parameter
to the Album type.
1:39
You have to be careful when
using an explicit cast.
1:43
If the type can't be
successfully converted,
1:46
you'll get a run time exception.
1:49
We know that this explicit cast to
the Album subtype is safe to do, though.
1:51
Because we're using the is keyword to test
of the item, which is of the base type,
1:55
MediaType, is actually of
the more specific subtype, Album.
2:01
After casting the item to the Album type,
we can then successfully
2:07
call any method on that type,
including the GetDisplayText method.
2:12
And lets pass the return value from
the GetDisplayText method call into
2:19
a Console.WriteLine method call.
2:24
If you want, you could remove
the interim variable like this.
2:30
Here's the item parameter
being cast to Album.
2:40
Then we surround all of
that in parentheses in
2:44
order to call the GetDisplayText method.
2:47
Declaring a variable is useful when
you need to do more than one operation
2:50
with the object.
2:54
Otherwise I typically just do
the explicit cast in line.
2:55
Sometimes it's a little bit
more difficult to read, but
2:59
generally I like
the conciseness of the code.
3:02
Let's copy and
paste this line here and here.
3:05
And change Album to Book,
And Album here to Movie.
3:12
This completes our method implementation.
3:20
Now we can call our new display method.
3:23
Let's do that right after our
calls to DetectMediaType.
3:28
So Display, then, (items[0]).
3:31
I'll copy that to the clipboard and
paste four times.
3:38
Change this index to 1,
then 2, then 3, then 4.
3:44
Now, let's compile and run our program,
well, after saving the file.
3:50
View > Show Console,
3:54
And we get the expected output.
4:02
This solution works fine, but
unfortunately, it's not an ideal solution.
4:05
In a future course,
4:09
you'll learn how another feature of object
oriented programming, abstract classes and
4:11
methods, can provide an alternative
solution to this problem.
4:15
See the teacher's notes for
more information.
4:20
Now, let's look at another feature
of object oriented programming,
4:22
encapsulation.
4:26
Aside from our MediaType classes,
the implementation for
4:28
our media library lives entirely
here in the main method.
4:31
While this approach has
worked fine up to this point,
4:35
we want to make it as easy as possible
to write code for our media library.
4:37
Also, anyone using our media library
shouldn't have to know that we're using
4:42
an array for
our collection of MediaType items.
4:46
Let's add a media library
class to our program and
4:49
move our MediaType items
array into that class.
4:53
Doing that will hide, or encapsulate,
our media library implementation.
4:56
For your second challenge,
5:02
you'll encapsulate the array of
MediaType items inside of a class.
5:03
To do that,
you'll create a class named MediaLibrary.
5:07
Within the MediaLibrary class,
5:12
you'll define a private array field
of type MediaType named _items.
5:13
Then you'll initialize the array
via a constructor method
5:20
that accepts an array of media type items.
5:23
And, you'll add a method named GetItemAt
that accepts an index value and
5:26
returns the media type item
at the provided index.
5:32
In the Program.cs file Main method,
5:35
you'll instantiate an instance
of the MediaLibrary class,
5:38
passing into the constructor your
existing collection of items.
5:42
Then you'll update the rest of
the code to use the GetItemAt method
5:46
to retrieve items from the media library.
5:50
And that's your second challenge.
5:53
After the break,
I'll show you my solution.
5:55
You need to sign up for Treehouse in order to download course files.
Sign up