A virtual method called a derivative rather than a base

Question

Can someone explain to me why the overridden method is called when I drop the class into the base one:

class Base { public virtual void VirtualMethod() { Console.WriteLine("Base virtual method"); } } sealed class Derived : Base { public override void VirtualMethod() { Console.WriteLine("Overriden method"); } } static void Main(String[] args) { Derived d = new Derived(); ((Base)d).VirtualMethod(); } 

I mean this code prints:

 Overriden method 

but not

 Base virtual method 

Its runtime or compilation time?

I know that I can call the base virtual method from a derivative by calling base.VirtualMethod() , but can I call it from the outside? (e.g. from Main or another class)

Answer 1

The implementation of the method is selected based on the runtime type of the object. This is a big part of it. Anyone can use:

 public void Foo(Base b) { b.VirtualMethod(); } 

... and you don’t need to know or care about what type of execution, because polymorphism will take care of that.

I know that I can call the base virtual method from a derivative by calling base.VirtualMethod (), but can I call it from the outside?

No (at least not without some horrible hacker to call a virtual method practically not), and that is an intentional part of encapsulation. The final implementation effectively replaced the original implementation for this object.

Answer 2

If you want to access the base implementation, you should not use overriding, you should use a new one. Override overrides any parent implementation, the new “hides” the parent implementation, so that you can access the implementation by casting as the parent object, then calling the method.

 internal class Program { private static void Main(string[] args) { Derived d = new Derived(); d.VirtualMethod(); ((Base) d).VirtualMethod(); Console.ReadLine(); } private class Base { public virtual void VirtualMethod() { Console.WriteLine("Base virtual method"); } } private sealed class Derived : Base { public new void VirtualMethod() { Console.WriteLine("Overriden method"); } } } 

This will output:

Method overcoming
Basic virtual method