Type of object after as cast?

Question

Given that code:

class B
{
  public virtual string Method()
  {
     return "base";
  }
}

class D : B
{
  public override string Method()
  {
     return "derived";
  }
}

D d = new D();
B b = d as B;
b.Method();

"derived" is the output.

But why exactly? I mean, b is a new object of type B, isnt it? Or is it the same object (in memory) as d? If so, what's the runtime type of b then, B or D?

Thanks

Answer 1

The rule is simple: b is a reference to an object of type D . You could say that the run-time type of b is D but that's not particularly helpful terminology.

Answer 2

You can do

B b = new D();
b.Method();

and you'd still get "derived". As @Bathsheba mentioned, what matters is the object type not the reference.

Imagine the typical OOP example where you have a base class Shape with derived classes Circle , Square , etc. with a virtual method Area .. if you have a method like this:

void ShowArea(Shape shape)
{
    Console.WriteLine(shape.Area());
}

The fact that the reference doesn't matter (but rather the actual object type) enables a method like the above to accept any type of Shape and still print the correct area

Answer 3

b is a new object of type B, isnt it?

No, b is an existing object of type B , which points to the same object you created just above it: d . The only difference is that you have cast the object as its parent type -- B , so b is treated as a B rather than the more-derived type D .

The reason you get the output "derived" is because the method is overridden in the derived class and that's how overriding works. Just because you declare a variable (ie b ) as its less-derived type B doesn't mean it isn't still actually the more-derived type D . That's the nature of polymorphism.

Answer 4

B b = d as B;

我相信这行代码仅将d转换为B类的类型，但该值仍保持为“派生”。