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In this video, we'll review the solution to the second challenge and introduce our third and final challenge—updating our MediaLibrary `GetItemAt()` method to gracefully handle bad array indexes.
Instructions
3rd Challenge
-
In the MediaLibrary class, update the
GetItemAt()method to gracefully handle a bad index.- Use the array's
Lengthproperty to determine if an index is available before attempting to access it. - Return
nullif the provided index isn't available in the array of items.
- Use the array's
In the Program.cs file, update the
Display()andDetectMediaType()methods to handle a nullitemparameter value.
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.
How did it go with the second challenge?
0:00
Let's walk through my solution.
0:02
To encapsulate the array of media
type items inside of a class,
0:04
I started with adding a file
named MediaLibrary.cs.
0:08
Then, I added the namespace,
Treehouse.MediaLibrary.
0:14
And stubbed out the class, MediaLibrary.
0:24
In the MediaLibrary class I defined
0:29
a private array field of type
0:34
MediaType named, _items;.
0:38
Remember the private keyword
makes this field only visible or
0:43
accessible to code within
the MediaLiibrary class.
0:47
The underscore here at the beginning
of the field name isn't required for
0:51
private fields, but
it's a common naming convention.
0:55
Following this convention will make it
easier to spot private fields in our code.
0:59
I then added a public constructor.
1:04
That accepts an array of MediaType items.
1:10
And then I used the items parameter to
initialize the _items private field.
1:14
I completed my MediaLibrary class
by adding a method named GetItemAt.
1:21
The method should accept
an index of type int.
1:28
So int index, and it should have
a return type, of MediaType.
1:31
Then, I just needed to return the item
from the underscore items' private field
1:41
at the provided index.
1:46
So return _items[index].
1:48
Moving on to the program.cs
file's main method.
1:55
I instantiated an instance
of the MediaLibrary class,
2:01
var mediaLibrary = new MediaLibrary.
2:08
And I passed my existing collection
2:13
of items into the constructor,
2:18
new MediaType[ ] and { }.
2:22
Then I cut and
pasted my items from here, into here.
2:30
Notice that I needed to add back the new
keyword, and the array type in order for
2:37
my program to successfully compile.
2:42
This is necessary because
the compiler can't infer what we're
2:44
trying to do here with our { } and
MediaTypes of class instances.
2:49
Adding new MediaType[ ] makes it clear
that we're initializing an array
2:54
of MediaType items.
2:59
Then, I updated the rest of my
code to use the GetItemAt method
3:01
to retrieve items from the MediaLibrary.
3:06
So, DetectMediaType,
so instead of items[0],
3:08
(mediaLibrary.GetItemAt).
3:14
We call it as a method because it is
a method and pass in 0, the index for
3:19
the element we want to retrieve.
3:23
I'll copy this code to the clipboard and
3:25
paste it inside of these
other method calls.
3:28
And change the indexes, 1, 2, 3, and 4.
3:37
Then do the same for
the Display method calls.
3:43
And lastly, I saved the Program.cs
file and compiled and ran my program.
3:53
And here, we can see the expected
output in the console.
4:01
The addition of a MediaLibrary
class fills a gap in our program
4:05
as we work towards a more
functional version.
4:09
Let's look at an issue with our
MediaLibrary GetItemAt method
4:12
implementation.
4:15
Let's make a call to the GetItemAt
method and pass in an index for
4:17
an element that doesn't exist in
our array of media type items.
4:21
I'll copy this line of code and
paste it right below the other and
4:25
pass in five for the index.
4:29
Remember an index of 5 would map
to the sixth element in our array.
4:33
Our array only contains five elements.
4:38
So if we compile and run our program,
we'll receive a runtime exception.
4:40
Exception: Index was outside
the bounds of the array.
4:47
And execution of our program stops
without running the rest of our code in
4:51
the main method.
4:55
Okay, so for our third and
last challenge in this practice session,
4:56
let's work to improve the situation.
4:59
In the MediaLibrary class,
5:05
update the GetItemAt() method to
gracefully handle a bad index.
5:07
Use the array's linked property to
determine if an index is available before
5:12
attempting to access it.
5:17
And return null if the provided index
isn't available in the array of items.
5:18
And in the Program.cs file,
update the Display and
5:24
DetectMediaType methods to handle when
the item parameter has a null value.
5:28
And that's it for the third challenge.
5:33
We'll see you in a bit.
5:35
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