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Let's go back to main to test
out the GetLocationAt method.

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00:00:05,380 --> 00:00:10,579
Let's create a new path object here,

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00:00:10,579 --> 00:00:14,170
say Path path = new Path.

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00:00:14,170 --> 00:00:16,050
We can actually have the array created and

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00:00:16,050 --> 00:00:19,320
pass it into the constructor
in a single statement.

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00:00:19,320 --> 00:00:21,630
Like this, so we'll copy this.

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00:00:24,491 --> 00:00:25,510
Move it into here.

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Then we can say new array and

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00:00:33,920 --> 00:00:38,410
then, indent this like that.

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00:00:40,190 --> 00:00:41,920
Now the array is being created and

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immediately being passed into
the constructor of the path class.

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00:00:46,440 --> 00:00:47,170
See how that works?

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00:00:48,500 --> 00:00:51,550
Now, we can use our new
GetLocationAt method

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00:00:51,550 --> 00:00:53,700
to find the location
of a step on the path.

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00:00:55,220 --> 00:01:01,870
So, we'll call GetLocationAt and assign it
to a MapLocation variable called location.

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00:01:07,310 --> 00:01:12,480
Now let's print out the x and y values of
this step that we know is on the path.

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00:01:12,480 --> 00:01:15,400
So we'll say,

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Console.WriteLine(location.X + ","

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+ location.Y).

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Let's compile and
run this and see what we get.

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All right, we get the first
location in the path array.

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00:01:47,780 --> 00:01:51,890
Now, let's see what happens when we
pass in a step that isn't on the path.

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The path only has indexes 0 through 7.

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So if we put 8 here, this should be
one step past the end of the path.

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00:02:01,680 --> 00:02:02,760
Let's compile and run again.

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00:02:04,720 --> 00:02:07,660
We see unhandled exception
printed to the console.

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That doesn't tell us much.

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00:02:11,730 --> 00:02:14,610
Let's get some more information
about what happened.

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00:02:14,610 --> 00:02:18,480
We're catching every exception
that could possibly be thrown.

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00:02:18,480 --> 00:02:21,740
We catch it in one of
these three catch clauses.

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We saw unhandled exception
printed to the console.

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00:02:25,130 --> 00:02:28,880
So it looks like the one that
caught the exception is this final

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00:02:28,880 --> 00:02:29,980
catch all we have here.

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00:02:31,360 --> 00:02:34,230
Let's have this print out some
more detailed information

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00:02:34,230 --> 00:02:36,290
about the exception it caught.

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00:02:36,290 --> 00:02:39,740
First, we'll need to add
an exception variable here.

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Now we can just append this
variable to the end of our message.

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00:02:43,760 --> 00:02:48,040
Remember, the string concatenation
operator here attempts to convert anything

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that isn't a string to a string
before the concatenation.

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00:02:52,750 --> 00:02:56,940
It just so happens that the string
that exceptions are converted into

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have a lot of good information in them.

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00:02:59,560 --> 00:03:02,820
We'll learn exactly how this
works in a later course.

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00:03:02,820 --> 00:03:04,710
Now let's run our code
again to see what we get.

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00:03:08,310 --> 00:03:11,880
This is what we get when the exception
is converted to a string.

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00:03:11,880 --> 00:03:17,580
As you can see, the name of the exception
thrown is System.NullReferenceException.

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00:03:17,580 --> 00:03:22,620
This exception happens when we try
to use null like a normal object.

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00:03:22,620 --> 00:03:23,755
Look up here.

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00:03:23,755 --> 00:03:27,650
GetLocationAt is supposed to
return null if we ask for

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00:03:27,650 --> 00:03:29,920
a step that isn't on the path.

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00:03:29,920 --> 00:03:33,410
That means that the location
variable here is now null.

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00:03:35,390 --> 00:03:38,880
Now when we tried to get the X
field from the location object,

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it can't do what we asked,
because it isn't the location.

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00:03:42,800 --> 00:03:46,320
This is null, and
null is the absence of a location.

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00:03:47,450 --> 00:03:51,130
This causes the null reference
exception to be thrown.

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00:03:51,130 --> 00:03:53,250
This is a very common
exception to deal with.

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00:03:54,280 --> 00:03:57,620
So, what should we do
to make this code work?

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00:03:57,620 --> 00:04:01,380
One thing we could do is check
to see if location is null,

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00:04:01,380 --> 00:04:04,740
before we try to print out x and
y like this.

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00:04:04,740 --> 00:04:10,586
So we can say, if(location

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00:04:10,586 --> 00:04:15,960
!= null then print this to the console.

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00:04:15,960 --> 00:04:20,740
So if location is null,
nothing will get printed and

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00:04:20,740 --> 00:04:26,400
we won't be trying to access location.X or
location.Y.

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00:04:26,400 --> 00:04:29,890
This is a very common pattern
when dealing with null.

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00:04:29,890 --> 00:04:32,480
It's often called a null check.

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00:04:32,480 --> 00:04:36,060
There are some interesting links in the
teacher's notes if you want to dive deeper

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00:04:36,060 --> 00:04:37,570
into how to handle null values.

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00:04:39,110 --> 00:04:42,500
We'll learn other ways to deal
with null in future courses,

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including how to avoid null entirely.

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00:04:45,340 --> 00:04:49,760
A good practice when working with objects
is to always consider what would happen

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00:04:49,760 --> 00:04:52,490
if an object is null, and
take the proper precautions.

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00:04:53,520 --> 00:04:55,630
We've also just learned about arrays.

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00:04:55,630 --> 00:04:57,890
Arrays are pretty handy, are they not?

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00:04:57,890 --> 00:05:01,410
I mentioned before that arrays
are just one of the many ways

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to store a collection of objects.

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00:05:03,600 --> 00:05:05,960
We'll learn about those in other courses.

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00:05:05,960 --> 00:05:07,100
So in the future,

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00:05:07,100 --> 00:05:11,190
we might want to change our path class
to use one of those instead of an array.

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00:05:12,330 --> 00:05:17,040
We were smart and used encapsulation to
hide the fact that we're using an array.

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00:05:17,040 --> 00:05:20,130
We could change it from an array
to any other type of collection.

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00:05:20,130 --> 00:05:24,020
And it won't break any code that's using
the public parts of the path class,

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00:05:24,020 --> 00:05:25,550
because those parts won't change.

82
00:05:26,630 --> 00:05:28,680
Next we'll learn about properties.

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00:05:28,680 --> 00:05:32,340
They're an important part of C#
that helps with encapsulation.

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00:05:32,340 --> 00:05:32,840
I can't wait.