How do it go with the third challenge? 0:00 Let's walk through my solution. 0:02 In the Program.cs file, I created a static method named detect media type and 0:04 to find the parameter of type media type named item. 0:10 I load the method right below the main method. 0:14 Static void DetectMediaType. 0:21 And for the parameter, it needs to be of type MediaType, and I'll name it item. 0:27 In the method body, I use the series of conditional statements and 0:34 the C# is operator to identify which particular media type subclass the item 0:38 parameter is an instance of. 0:43 So, if item, and then the is operator, 0:45 and then the type that we're testing for, Album. 0:49 Then a set of curly braces. 0:54 Then, else if item the is operator, then the Book type. 0:56 A set of curly braces, and 1:05 then another else if item is Movie. 1:08 And a final set of curly braces. 1:14 This series of conditionals will first check to see if the item parameter is 1:17 an Album. 1:21 And if that's not true, it'll check to see if the item is a Book, and 1:22 if that's not true, it'll check to see if the item is a Movie. 1:26 After determining the MediaType item subclass type, 1:30 I wrote a message to the console identifying that type. 1:33 So if the item is an Album, 1:37 I wrote Console.WriteLine item.Title, 1:40 +, and then a string literal is in album. 1:48 Copy that to the clipboard, then paste and paste. 1:54 All of this can stay the same here because item.Titles what 2:00 we need to output, but instead of is an album, say is a book. 2:05 Same thing down here for a movie. 2:11 We just need to change this to a movie. 2:15 And save the file. 2:20 Okay, so that completed my DetectMediaType method implementation, 2:22 then I needed to test my changes. 2:27 To do that, I passed each of my media type subclass instances into 2:33 a call to the detect media type method. 2:38 First, album1, so, DetectMediaType, 2:42 Then I pass album1 into the method call. 2:49 Let's copy that line of code and paste it four more times. 2:53 I'll pass album2 into the second call. 2:57 Album3 into the third, and then the fourth, 3:01 will get book and the fifth method call will get movie. 3:05 And lastly, I save the file, and I complied, and ran my program. 3:12 And here, we can see each MediaType item subtype written to the console. 3:23 Great work so far. 3:29 Now, let's take a look at a couple of shortcomings in our program. 3:30 First, let's see what happens if we pass null, or 3:35 an empty string as the argument for a media types title parameter. 3:38 Just after album3 here, I'll create a fourth album. 3:45 So var, album4 = new Album, and for the title, 3:48 I'll pass null instead of a value, then for 3:53 the artist name I'll say, Some Artist. 3:57 Then I'll create a fifth album, so album5, 4:04 = new Album, and this time, I'll pass an empty string. 4:10 And for the artist, I'll use Some Artist again. 4:16 Let's save the file, and compile, and run our program. 4:20 Looks like I forgot to call get display text on album4 and album5. 4:28 The compiler is telling us that we assigned a value to those variable but 4:34 we never used them. 4:38 So let's fix that problem, Right here. 4:40 Let's copy that line to the clipboard and paste it twice. 4:44 Then change album3 to album4, and album3 here to album5. 4:48 Save the file, And compile, and run the program. 4:53 And right here in the console, we can see that our program happily allows the null, 5:07 or empty string value in attempts to write the album's not 5:12 existent title to the console. 5:15 Let's look at another short coming. 5:17 In the detect MediaType method, let's comment out the last else if statement. 5:19 Then save, and compile, and run our changes. 5:29 We can see here that the three albums were identified. 5:37 And the book, but there's nothing displaying whatsoever for the movie. 5:41 This tells us, that the detect MediaType methods silently 5:45 fails to detect the media type for the movie subclass instance. 5:48 Ideally, the program would make it more apparent when it encounters either of 5:52 these situations, with a fourth and last challenge in this practice session. 5:56 Let's update our program to handle both of these unexpected situations in a more 6:01 robust manner. 6:06 In the MediaType.cs file, throw an exception in the MediaType class 6:11 constructor if the title parameter is set to null or an empty string. 6:16 Set the exceptions message to, A media type must have a title. 6:21 In the Program.cs file, throw an exception in the DetectMediaType helper 6:25 method when an unexpected media type is encountered. 6:30 Set the exception's message to Unexpected media subtype encountered. 6:34 Then, to finish this challenge up, 6:39 wrap the code in the Main method in a try/catch statement. 6:41 Define a parameter in the catch clause, in order to get a reference to the caught 6:45 exception, write the exception's Message property to the console. 6:49 And that's it for the fourth challenge. 6:53 See you in a bit. 6:55