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At the end of the previous video, we were
getting an error when attempting to call

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the GetDisplayText method on an element
in our MediaType items array.

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Type Treehouse.MediaLibrary.MediaType
does not contain a definition for

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a GetDisplayText, and
no extension method GetDisplayText of type

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Treehouse.MediaLibrary.MediaType could
be found.

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Luckily, this error tells us
exactly what's happening.

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Because our array items, RF type,
media type, and the GetDisplayText method

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isn't defined on the MediaType class,
we can't call that method.

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We can work around this limitation
using an approach similar

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to what the DetectMediaType method uses.

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Let's make a copy of that method.

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First, I'll select all of this code for
the DetectMediaType method,

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copy it to the clipboard, and
paste it right above that method.

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Then I'll change the method
name to Display, and

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remove the code contained within each of
the conditional statement code blocks.

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So here, here, and here.

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By using the C# is operator, we can
detect the MediaTypeItem subclass type.

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Then we can use an explicit
cast to convert the item

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from the MediaType type
to its subclass type.

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So if (item is Album),

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then var album = (Album) item.

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This line of code explicitly casts

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the MediaType item parameter
to the Album type.

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You have to be careful when
using an explicit cast.

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If the type can't be
successfully converted,

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you'll get a run time exception.

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We know that this explicit cast to
the Album subtype is safe to do, though.

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Because we're using the is keyword to test
of the item, which is of the base type,

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MediaType, is actually of
the more specific subtype, Album.

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After casting the item to the Album type,
we can then successfully

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call any method on that type,
including the GetDisplayText method.

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And lets pass the return value from
the GetDisplayText method call into

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a Console.WriteLine method call.

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If you want, you could remove
the interim variable like this.

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Here's the item parameter
being cast to Album.

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Then we surround all of
that in parentheses in

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order to call the GetDisplayText method.

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Declaring a variable is useful when
you need to do more than one operation

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with the object.

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Otherwise I typically just do
the explicit cast in line.

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Sometimes it's a little bit
more difficult to read, but

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generally I like
the conciseness of the code.

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Let's copy and
paste this line here and here.

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And change Album to Book,
And Album here to Movie.

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This completes our method implementation.

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Now we can call our new display method.

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Let's do that right after our
calls to DetectMediaType.

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So Display, then, (items[0]).

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I'll copy that to the clipboard and
paste four times.

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Change this index to 1,
then 2, then 3, then 4.

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Now, let's compile and run our program,
well, after saving the file.

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View > Show Console,

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And we get the expected output.

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This solution works fine, but
unfortunately, it's not an ideal solution.

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In a future course,

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you'll learn how another feature of object
oriented programming, abstract classes and

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methods, can provide an alternative
solution to this problem.

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See the teacher's notes for
more information.

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Now, let's look at another feature
of object oriented programming,

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encapsulation.

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Aside from our MediaType classes,
the implementation for

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our media library lives entirely
here in the main method.

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While this approach has
worked fine up to this point,

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we want to make it as easy as possible
to write code for our media library.

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Also, anyone using our media library
shouldn't have to know that we're using

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an array for
our collection of MediaType items.

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Let's add a media library
class to our program and

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move our MediaType items
array into that class.

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Doing that will hide, or encapsulate,
our media library implementation.

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For your second challenge,

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you'll encapsulate the array of
MediaType items inside of a class.

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To do that,
you'll create a class named MediaLibrary.

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Within the MediaLibrary class,

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you'll define a private array field
of type MediaType named _items.

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Then you'll initialize the array
via a constructor method

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that accepts an array of media type items.

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And, you'll add a method named GetItemAt
that accepts an index value and

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returns the media type item
at the provided index.

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In the Program.cs file Main method,

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you'll instantiate an instance
of the MediaLibrary class,

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passing into the constructor your
existing collection of items.

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Then you'll update the rest of
the code to use the GetItemAt method

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to retrieve items from the media library.

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And that's your second challenge.

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After the break,
I'll show you my solution.