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Object Equality
4:13What does it mean for two objects to be equal?
According to the rules of the game, towers
can't be placed directly on the path.
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We need a way to know if a map
location is on the path.
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Let's add another method to the Path
class that will return true,
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if the map location passed in,
is on the path.
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So we'll say public bool IsOnPath and
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it will take a MapLocation as a parameter.
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And this method, will loop through all
of the locations in the path array.
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So we'll say,
foreach(var pathLocation in _path,
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And if the location passed in
equals one of the path locations,
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We'll return true immediately.
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Otherwise, if we get through all
of the locations on the path and
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we get down here to the end of
the four H loop we'll return false.
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Because we didn't find a matching
location in the path array.
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To test this, let's create a map
location that is on the path.
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For now,
we'll just do this in the Game.cs file.
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So here we have our path.
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Under here,
let's create a new MapLocation.
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And we'll have it be right at
the beginning of the path, so
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right here at 0, 2.
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Just copy that down there.
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Now we can say if(path.IsOnPath and
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pass in that location we just created.
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Then on the console will print,
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location + " is on the path".
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And just so we don't continue down into
our program we'll return right here.
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Let's go ahead and compile and run this.
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Hm.
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We didn't see our message.
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We should have seen this
message printed to the console.
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And we shouldn't have seen any of this
stuff because we would have returned.
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We must have a bug in the IsOnPath method.
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Let's take a second look.
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On the surface it looks correct,
doesn't it?
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There's something very
subtle happening here.
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This double equals sign checks to see if
two variables refer to the same object.
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This is different than two locations
on the map being the same.
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Let's go back to Game.cs to see
the MapLocation objects that should have
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caused the IsOnPath method to return true.
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Here's the MapLocation that's
in the path array, and
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here's the MapLocation that
we're checking against.
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They may look the same, but these
are actually two very different objects.
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See, here we created a new MapLocation and
put it in this array, and
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down here we created a completely
different MapLocation.
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They just so happen to refer to
the same location on the map.
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It's like they're identical twins.
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They aren't the same person,
but they look identical.
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The equality operator by default,
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only checks to see if they
are the exact same object.
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Or in the case of twins,
the exact same person.
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On the computer, these two objects
sit in different locations in memory.
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The equality operator actually just calls
the static, system.object.ReferenceEqual
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method to determine if two variables
refer to the same location in memory.
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In which case, they are the same object.
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If two variables refer to
the same object in memory,
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we can say that they
have reference equality.
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That's all that double equals
equality operator checks for.
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