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Handling Bad Array Indexes

5:36 with James Churchill

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.

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3rd Challenge

In the MediaLibrary class, update the GetItemAt() method to gracefully handle a bad index.
- Use the array's Length property to determine if an index is available before attempting to access it.
- Return null if the provided index isn't available in the array of items.
In the Program.cs file, update the Display() and DetectMediaType() methods to handle a null item parameter value.

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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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