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Throwing and Catching Exceptions

6:56 with James Churchill

In this video, we'll review the solution to the third challenge and introduce our fourth and final challenge—throwing and catching exceptions.

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Instructions

4th Challenge

In the MediaType.cs file...
- Throw an exception in the MediaType class constructor if the title parameter is set to null or an empty string.
- Set the exception's message to "A media type must have a title."
In the Program.cs file...
- Throw an exception in the DetectMediaType() helper method when an unexpected media subtype is encountered.
- Set the exception's message to "Unexpected media subtype encountered."
- Wrap the code in the Main() method in a try/catch statement.
- Define a parameter in the catch clause in order to get a reference to the caught exception.
- Write the exception's Message property to the console.

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 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

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