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You have completed C# Objects!
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Throw an exception to indicate that a method can't complete successfully.
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Let's get back to coding
up the MapLocation class.
0:00
We can make sure that users of this class
don't accidentally make a map location
0:03
that doesn't exist on the map.
0:07
The natural place to do this
validation is here in the constructor
0:09
because the constructor is called
when the object is created.
0:12
So we can actually stop
the object from being created
0:16
by causing this method to fail.
0:18
In order to do this validation,
0:21
the constructor will need to have
an instance of the map object.
0:22
So we'll add it to the list
of parameters here.
0:26
Now we can check this map location is
on the map by typing if map.OnMap and
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then pass it this, open and
closing curly brace.
0:38
Let me go over what we just did here.
0:44
We're using the maps
OnMap method to determine
0:46
if the MapLocation being
constructed is on the map.
0:49
OnMap returns true if
the Point is on the map.
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We only want to do something if
the Point is not on the map.
0:57
So we need to use the negation operator,
so that it reads, if not OnMap.
1:00
See how that works?
1:06
The word this is a special keyword
that refers to the current object.
1:08
You can use this from any method to get
the object that the method was called on.
1:13
In the case of a constructor method,
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this refers to the object
that's being constructed.
1:20
You need to be careful when using
this in a constructor though.
1:23
You see an object is not fully constructed
until the constructor has returned.
1:26
There might still be some fields that
haven't been fully initialized and
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using this too soon could
have unexpected results.
1:35
Just something to be aware of, the base
constructor is always called first.
1:40
So by now in the creation
of the map object,
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all of the fields of the object
have been initialized.
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So by the time the execution gets to here,
1:50
it's safe to pass this
to the OnMap method.
1:53
Now we need to decide what to do
if the point is not on the map?
1:57
Exceptions are used to tell
the calling code that a method was not
2:01
able to complete successfully.
2:05
In this case, the constructor is not
able to complete successfully and
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the MapLocation object cannot be created.
2:11
We've already learned
how to catch exceptions,
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now we need to throw an exception.
2:16
The most basic exception type
provided by the .NET Framework
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is just named exception.
2:22
It's in the system namespace.
2:25
Exception types are classes
just like any other class.
2:27
To throw an exception we need to create a
new exception instance and then throw it.
2:30
I will show how to do that here.
2:35
We instantiate exceptions the same way
we instantiate every other class because
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exceptions are really just classes.
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The only thing new here is that
we're using this throw keyword
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to throw the newly
created exception object.
2:54
Let's go see how this works in main.
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Let's delete the code
we don't need here and
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attempt to create a MapLocation
we know isn't on the map.
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Let's say 20, 20 which is way off our map.
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We also need to pass it the map object.
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Now we know this could
potentially throw an exception.
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We need to make a decision.
3:25
We can either try to handle the exception
here, or we can let the exception be
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propagated back to the method
that called the method we're in.
3:31
In our case,
3:35
we're in the main method which is
the first method called in our program.
3:36
So there's no other method that could
handle the exception appropriately.
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It's best to handle it here.
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Otherwise, the program will crash and
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the user will see a scary
looking error message.
3:47
To handle the exception,
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we need to wrap the code that can throw
the exception with the try catch.
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And the type of exception we need
to catch is system.exception.
4:00
We already have using system
here at the top of the file.
4:05
So we can just use the class name here.
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Here in the catch block,
we'll print to the screen what happened.
4:14
So we'll say console.rightline.
4:18
That map location is not on the map.
4:24
Now when we run this code we'll see
our message printed to the screen.
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We can ignore the warning about
the location variable not being used.
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Here's our error message.
4:42
See how well that worked.
4:44
There is more to learn about exceptions
and how to use them in the next video.
4:45
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