Question

I’ve been reading documentation about virtual methods :

In a virtual method invocation, the run-time type of the instance for
which that invocation takes place determines the actual method
implementation to invoke. In a non-virtual method invocation, the
compile-time type of the instance is the determining factor. In
precise terms, when a method named N is invoked with an argument list
A on an instance with a compile-time type C and a run-time type R
(where R is either C or a class derived from C), the invocation is
processed as follows… :
http://msdn.microsoft.com/en-us/library/aa645767(v=vs.71).aspx

However, I noticed something which is bold above. Lets say we have a code like this:

class Planet{
  public string Name;
  public float Size;
  public virtual void SpinPlanet(){
     Console.WriteLine("Hoooraaay!!!");
  }
}

class Earth : Planet{

}

And somewhere in my code I do:

Earth world = new Earth();
world.SpinPlanet();

In this case:

• N is SpinPlanet()
• C is Earth
• R is Planet

So how come R can be derived class of compile-time type C. Aren’t the base class types being resolved during run-time?

Answer 1

You are mistaken- the compile-time type (C) is Earth and the run-time type (R) is also Earth. The part of the specification that you point out is not really relevant here.

What is relevant is http://msdn.microsoft.com/en-us/library/aa691356(v=vs.71).aspx, specifically:

The set of candidate methods for the method invocation is constructed.
Starting with the set of methods associated with M, which were found
by a previous member lookup (Section 7.3), the set is reduced to those
methods that are applicable with respect to the argument list A.

The only candidate implementation of SpinPlanet just happens to be in the base class of Earth, not in the derived class.

The part of the spec that you refer to would apply if the code were:

Planet world = new Earth();
world.SpinPlanet();

(especially if Earth defined an override for SpinPlanet) because then the compile type (the type of the variable) would be Planet, but the runtime type would be Earth.