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A hash code is only a semi
unique identifier for an object.

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We've tried our best to make the hash
codes unique, but it is still possible for

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two different objects to
have the same hash code.

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Much like my name is Jeremy, but
there are also other people named Jeremy.

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In fact, it's a rather common name.

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If someone called out my
name in a large crowd,

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there'd be a good chance that more
than one person would respond.

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How could they know which person
is the Jeremy they're looking for?

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Well, they'd have to rely
on more than just our name.

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They'd probably need to get a look
at each of us in order to say

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which person they wanted.

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So whenever we override
the GetHashCode method

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we should also override the Equals method.

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The Equals method is called to
determine if two objects should be

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considered equivalent.

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If two objects map to the same hash code,

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the only way to tell them apart
is to check if they're equal.

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Let's override the Equals method here.

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So say public override returns a bool and

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it just takes a type of object.

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The first thing we need to do is to make
sure that this object passed in is of

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type student.

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So we'll attempt to cast it to
a student using the as operator so

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say Student call that
equals obj as Student.

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Now if obj isn't a student,
then this as expression will return null.

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So we can check for that.

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Say if that equals null.

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Then will return false.

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If it isn't null,
then it must have been a Student.

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So we can now check to see if each
of the properties is the same.

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So say, return this.Name == that.Name.

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And, this.GradeLevel

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== that.GradeLevel.

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Now the Equals method will return true
if all of the properties are the same.

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Now that we've overridden the GetHashCode
and Equals methods, let's compile and

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run this to see if we've
achieve the desired effect.

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So now both hashCodes are the same, and
there's only one Joe in the Student set.

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HashSets organize objects
internally by their hashCode.

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Once the hashCode has been computed,
the set can quickly determine whether or

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not the object with that
code is in the set.

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Adding an item to a hash set
isn't necessarily faster than

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adding it to a list.

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However, the hash set is
substantially faster when it comes to

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finding the item again.

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Sets are very fast at determining if
an item is already in the collection.

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We can do this by calling
the contains method.

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So here we could say something like,

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if students.Contains, Joe.

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Then, do something.

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The Contains method is defined
in the I Collection interface.

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So every collection
implements this method.

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Calling Contains on a list requires the
list to look for the item by looking at

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each item one by one from
the beginning of the list to the end.

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Calling Contains on a hash
set on the other hand

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just requires computing
the hashCode of the object.

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I've included more information about
how this works in the teacher's notes.

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Likewise calling Remove on a hash
set is also extremely fast.

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Remove requires first find the item and
then removing it.

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We just learned that hash sets
can quickly find items and

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since hash sets are unordered, there's
no shifting of items in the collection

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after the item has been removed.

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This makes hash sets remove method
substantially faster than lists.