Overridden virtual method in C#

Question

In "The C# Programming Language" book Eric Lippert mentioned this:

A subtle point here is that an overridden virtual method is still considered to be a method of the class that introduced it, and not a method of the class that overrides it.

What is the significance of this statement? Why does it matter if the overridden virtual method is considered to be a method of the class that introduced it (or otherwise) since the overridden method will never be called unless you are dealing with the derived class?

Answer 1

It matters when you have a reference of one type pointing to an object of a different type.

Example:

public class BaseClass {
  public virtual int SomeVirtualMethod() { return 1; }
}

public class DerivedClass : BaseClass {
  public override int SomeVirtualMethod() { return 2; }
}

BaseClass ref = new DerivedClass();
int test = ref.SomeVirtualMethod(); // will be 2

Because the virtual method is a member of the base class, you can call the overriding method with a reference of the base class type. If it wasn't, you would need a reference of the derived type to call the overriding method.

When you are shadowing a method instead of overriding it, the shadowing method is a member of the derived class. Depending on the type of the reference you will be calling the original method or the shadowing method:

public class BaseClass {
  public int SomeMethod() { return 1; }
}

public class DerivedClass : BaseClass {
  public new int SomeMethod() { return 2; }
}

BaseClass ref = new DerivedClass();
int test = ref.SomeMethod(); // will be 1

DerivedClass ref2 = ref;
int test2 = ref2.SomeMethod(); // will be 2