Polymorphism

Circle

Rectangle

Triangle

Draw

Bhushan Mulmule
bhushan.mulmule@gmail.com

www.dotnetvideotutorial.com

Ability of different objects to respond same message
in different ways.

Circle

Rectangle

Triangle

Draw

Inheritance, Upcasting and Late Binding
or

Abstract Class Implementation
or

Interface Implementation

Let us focus on first technique in this session

Polymorphism using Inheritance, Upcasting
and Late Binding


Upcasting is holding derived class object
in base class reference

If we have this class hierarchy

we generally create objects…


where references and objects are of same types

Shape s = new Shape();

s
shape

Circle c = new Circle();

c
Circle

Rectangle r = new Rectangle();

shape

Circle

r
Rectangle

Rectangle

As Circle and Rectangle are derived from Shape class.
You can also create object using upcasting…

where base class reference can hold
objects of derived class

Shape s = new Shape();

s
shape

shape

s = new Circle();
Circle

s = new Rectangle();

Rectangle


Now let us try to understand Late Binding

Binding is connecting method Invocation to method
implementation
By default .NET use Early Binding …

Early binding is also referred as Static Binding or Compile Time Binding

If we have Draw() method in base class and also
redefined it in derived classes


Redefining Draw() method
in Circle and Rectangle
Class hides the base class
implementation of Draw().
This is knows as
Shadowing.

Note: new keyword just confirms that you are
intentionally shadowing Draw() method in
derived class.

And we have created objects using upcasting
and invoked Draw() method

Then the output that we are expecting is…

Expected Output


But What’s this?
Output is surprising…

Actual Output


Why?


Because by default .NET use early binding.
Compiler simply checks type of reference and bind it
accordingly.
Compiler doesn't care about type of object. Actually there are no objects in
memory at compile time

As s is of Shape type so all the three calls will get binded to
Draw() of Shape class irrespective of Object it is pointing to

What's the solution?

Is it possible to bind method call depending on object type instead
of reference type?

And here is the magic keyword

virtual
Call to virtual method gets resolved at runtime depending on actual
object type.

The virtual keyword marks method for late
or dynamic binding.

Virtual method has to be overridden in a
derived class using override keyword and
can be modified

Now Let us run same client code


Will call Draw() of Shape

Will call Draw() of Circle

Will call Draw() of Rectangle

And yes this time output will be…

As expected.


Polymorphism is
ability of different objects

to respond same message
In different ways

For Video visit

Bhushan Mulmule
bhushan.mulmule@dotnetvideotutorial.com

Polymorphism

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