Polymorphism

Polymorphism is one of the three major fundamental concepts in Object-Oriented Programming. The other 2 major OOP concepts are Inheritance and Encapsulation. Polymorphism is closely-related to inheritance.

Polymophism has 2 main conceptual components which are detailed in the MSDN reference sections below. Polymorphism is a run-time concept that controls whether a child-class method is executed, rather than the base-class method of the same name. In C#, it is necessary to declare a the base-class method as virtual if we intend to allow it to be overridden by the child class method, when a base-class reference, refers to a child-class object instance. If no override method is specified in a child class, then the base-class method is executed.

Polymorphism: MSDN Reference

"At run time, objects of a derived class may be treated as objects of a base class in places such as method parameters and collections or arrays. When this occurs, the object's declared type is no longer identical to its run-time type. Base classes may define and implement virtual methods, and derived classes can override them, which means they provide their own definition and implementation. At run-time, when client code calls the method, the CLR looks up the run-time type of the object, and invokes that override of the virtual method. Thus in your source code you can call a method on a base class, and cause a derived class's version of the method to be executed."

Virtual Methods - Base Class

When a base class method is marked as virtual, then this method can be overridden in the child class to allow the child class to provide custom implementation that is more specialized for a child class. Then, if the method is marked as override in the child class, then the child class method is executed at run-time if the object instance is of the child class type. If the method isn't overridden in the child class, then the base class method is executed. Example Code: NPC, Zombie

///NPC Base-Class
public virtual void doSomething(){
  Debug.Log("NPC Base Class DoSomething");
}

Override Methods - Child Class

When a child class wants a method to have a specialized implementation, then the child version of a method will be executed if the child method is declared as override.

///Zombie Child-class (extends NPC)
public override void doSomething(){
  Debug.Log("Zombie Child Class DoSomething");
}

///Other Class:
NPC myNPC = new Zombie();
myNPC.doSomething();   // console output:  "Zombie Child Class DoSomething"