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Practice Encapsulation and Arrays in C#

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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.

Instructions

3rd Challenge

  1. 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.

  2. 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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