Why defer the decision?
The decision cannot be made at compile time because the compiler has no way of knowing (when the program is compiled) the actual type of the object whose reference will be stored in the reference variable.
For example, the type of the object might be the result of a choice made at runtime by a human user among several different possible choices.
Could be either type
For the situation described earlier, that object could just as easily be of type SuperClass as of type SubClass . In either case, it would be valid to assign the object's reference to the same superclassreference variable.
If the object were of the SuperClass type, then a call to the method named method on the reference would cause the version of the method defined in SuperClass , and not the version defined in SubClass , to be executed.
Preview
From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in C#:
- Method overloading
- Method overriding through class inheritance
- Method overriding through the C# interface inheritance
I covered method overloading as one form of polymorphism (compile-time polymorphism) in an earlier module.
I will begin the discussion of runtime polymorphism through method overriding and class inheritance in this module. I will cover interfaces in a futuremodule.
Discussion and sample code
Let's examine a sample program that illustrates runtime polymorphism using class inheritance and overridden methods. The name of theprogram is Polymorph03 . A complete listing of the program is provided in Listing 7 near the end of the module.
The class named A
I will discuss this program in fragments. Listing 1 shows the definition of a class named A , which extends the class named Object by default.
Listing 1 . Class A.
using System;
class A {public virtual void m() {
Console.WriteLine("m in class A");}//end method m()
}//end class A The class named A defines a simple method named m .
A virtual method
Note that the method named m is declared to be virtual . This means that it is allowable to override this method in a subclass.
Behavior of the virtual method
The behavior of the virtual method, as defined in the class named A , is to display a message indicating that it has been called, and that it isdefined in the class named A .
This message will allow us to determine which version of the method is executed in each case discussed later.
The class named B
Listing 2 shows the definition of a class named B that extends the class named A.
Listing 2 . Class B.
class B : A {
public override void m() {Console.WriteLine("m in class B");
}//end method m()}//end The class named B overrides (redefines) the method named m , which it inherits from the class named A .