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Let's get back to coding
up the MapLocation class.

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We can make sure that users of this class
don't accidentally make a map location

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that doesn't exist on the map.

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The natural place to do this
validation is here in the constructor

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because the constructor is called
when the object is created.

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So we can actually stop
the object from being created

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by causing this method to fail.

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In order to do this validation,

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the constructor will need to have
an instance of the map object.

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So we'll add it to the list
of parameters here.

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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.

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Let me go over what we just did here.

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We're using the maps
OnMap method to determine

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if the MapLocation being
constructed is on the map.

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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.

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So we need to use the negation operator,
so that it reads, if not OnMap.

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See how that works?

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The word this is a special keyword
that refers to the current object.

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You can use this from any method to get
the object that the method was called on.

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In the case of a constructor method,

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this refers to the object
that's being constructed.

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You need to be careful when using
this in a constructor though.

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You see an object is not fully constructed
until the constructor has returned.

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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.

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Just something to be aware of, the base
constructor is always called first.

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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,

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it's safe to pass this
to the OnMap method.

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Now we need to decide what to do
if the point is not on the map?

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Exceptions are used to tell
the calling code that a method was not

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able to complete successfully.

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In this case, the constructor is not
able to complete successfully and

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the MapLocation object cannot be created.

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We've already learned
how to catch exceptions,

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now we need to throw an exception.

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The most basic exception type
provided by the .NET Framework

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is just named exception.

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It's in the system namespace.

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Exception types are classes
just like any other class.

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To throw an exception we need to create a
new exception instance and then throw it.

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I will show how to do that here.

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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.

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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.

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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.

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In our case,

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we're in the main method which is
the first method called in our program.

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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.

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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.

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We already have using system
here at the top of the file.

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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.

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So we'll say console.rightline.

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That map location is not on the map.

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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.

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See how well that worked.

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There is more to learn about exceptions
and how to use them in the next video.