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On the surface abstract base
classes appear to be more

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flexible than interfaces.

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They allow us to declare both public and

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protected members, that some classes must
have and they can also contain code.

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Interfaces on the other hand can only
declare the public members that subclasses

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must have and that's it.

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It's easy to see why we want to use
abstract base classes, but why interfaces?

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Let's explore why interfaces are so
useful in C#.

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You may think that interfaces
are more restrictive versions of

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abstract base classes, but in fact,
they serve different purposes.

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Abstract base classes are used to contain
code that should be shared by the concrete

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subclass implementations.

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The implementations of all the non
abstract members of the invader class

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are inherited by its subclasses.

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Each subclass doesn't have to
create their own implementation.

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This is an advantage to use
in an abstract base class.

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On the other hand C# only allows classes
to inherit directly from one other class.

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Being able to inherit directly from
multiple base classes is known as multiple

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inheritance.

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Multiple inheritance can cause a lot
of problems so it isn't allowed in C#.

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Think of the case when two base classes
both have methods that look identical.

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Which method implementation should
be inherited in the subclass?

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Instead of multiple inheritance C# allows
a class to Implement multiple interfaces.

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A class can implement as
many interfaces as needed.

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An interface does nothing about
the members' implementation.

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So, it doesn't matter if two interfaces
both have the same member with

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the same signature.

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The class only needs to provide
one implementation of the member

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to satisfy the interface.

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An interface can also
inherit other interfaces.

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Let's break up the Iinvader interface
into a couple different interfaces.

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We can break the location property out
Into another interface or just cut it from

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here, Go up here and we'll create
a new interface called IMappable.

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We'll move the property here.

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So now any class that implements
the IMappable interface

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must have a public map location property
that provides at the very least a getter.

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Just like how classes can
implement multiple interfaces,

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interfaces can also inherit
from multiple interfaces.

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Let's create an interface named IMovable.

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So say, interface I Move able.

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This one will require
a single method named,

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Move which will take from
the IInvader interface.

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There we go.

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To inherit from multiple interfaces,
we simply list them after the colon and

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separate them with commas.

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So we'll have the IInvader
interface inherit the IMappable and

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the IMovable interface.

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This is the same way that a class
can implement multiple interfaces.

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We just list them here after the colon,

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just like we do when we
inherit from another class.

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Now anything that implements the IInvader
interface must also implement

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the IMappable and IMovable interfaces.

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That means in addition to having to
have a HasScored, Health, IsNeutralized,

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IsActive and DecreaseHealth members,
they also have to have Move and Location.

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The usefulness of interfaces isn't obvious
right away, but they're important in good

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object oriented design and they make
extending and maintaining code easier.

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The point of an interface
is to only expose what is

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absolutely needed by the code
that is using the class.

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This gives us the greatest
flexibility when writing classes

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the implement the interface.

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We only need to match the interface.

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Say for example that we had a method
that only needed to call the Move method

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on one of its parameters.

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We could code it so that it only expected
a parameter of the type IMoveable.

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We could past that method
any object of type IMovable,

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IInvader is a type IMovable and
so is IInvader.

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And by extension so
are all of the other invader subclasses.

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In fact,

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we could even pass the method a class that
had nothing to do with being an invader.

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Say for example, we had a class named Bird
that implemented the IMovable interface,

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we could also pass it to the method
that expected in IMovable.

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Even though bird is
nothing like an invader,

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they both have a Move method because
they implement the IMovable interface.

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Can you see how using interfaces in
our code gives us the most freedom

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in how we wire up our objects?

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Combining new interface with an abstract
base class is a common pattern.

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It allows us to have
the flexibility of an interface and

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also provides a mechanism to
share code among subclasses.

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In the next video,

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we'll see how we can leverage the power
of the IInvader interface to create a new

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type of invader that doesn't inherit
from the invader base class at all