Upcasting and Downcasting

•  Upcasting: conversion from a derived class to a base class
•  All objects can be implicitly converted to a base class reference.  

      Shape shape = circle;   // Shape is base class and circle is derived class	

• Downcasting: conversion from a base class to a derived class 
• Downcasting requires a cast.

      Circle circle= circle;   // Shape is base class and circle is derived class	

• Casting can throw an exception if the conversion is not successful. We can use the as keyword to prevent this. If conversion is not successful, null is returned. 

      Circle circle = shape as Circle;
      if (circle != null)

• We can also use the is keyword

if (shape is Circle) {
   var circle = (Circle) shape; 
}