I have been recently studying upcasting and downcasting in c#. I understand that upcasting refers to conversion from a derived class to a base class. However, when i see a practical example of upcasting (like below), i get confused.
public class Shape
{
...
}
public class Circle : Shape
{
...
}
Circle circle = new Circle();
Shape shape = new Shape();
// Converting an object into its base class reference
shape = circle
If we are converting circle into its base class reference, shouldn't it be like
circle = shape
Sorry if it sounds too amateur. This is because I have always seen expressions in the following format:
int x = 3; // means assign 3 to variable x.
So I am just confused why circle is on the right hand side and not on the left hand side. Please advise. Consider me a beginner.
(Aside: This is called upcasting because, traditionally, class diagrams are drawn such that the base classes are shown physically above the derived classes.)
Now when you do:
shape = circle; // shape->(instance of Circle)
you are assigning a Circle reference to a Shape reference, so that after the assignment the reference shape will be referencing a Circle.
This is fine, because everything you can do with a Shape you can also do with a Circle.
If, however, you do:
circle = shape; // circle->(instance of Shape)
you are assigning a Shape reference to a Circle reference. You can't do this because (if this were possible) you would then be able to access Circle features that are not present in a Shape.
For example, imagine that Circle.Radius exists but Shape.Radius does not.
If you were allowed to point the circle reference at a Shape what would happen if you tried to access circle.Radius? The answer would be: undefined behaviour would happen, because Shape.Radius does not exist.