派生类不调用基类方法

Question

class MyBaseClass
{
  virtual public void Print()
    {
      Console.WriteLine("This is the base class.");
    }
}

class MyDerivedClass : MyBaseClass
{
  override public void Print()
   {
     Console.WriteLine("This is the derived class.");
   }
 }

class Program
  {
    static void Main()
    {
      MyDerivedClass derived = new MyDerivedClass();
      MyBaseClass mybc = (MyBaseClass)derived;

      derived.Print();
      mybc.Print();

    }
   }

OUTPUT:

This is the derived class.
This is the derived class.

I do not understand why second call prints derived class's print() method because I cast mybc object to base class. I expect it to print base class print method instead. Am I missing something here?

Answer 1

The variable type and the instance type are two different types. Casting does not change the instance type.

When you declare a method to be virtual/abstract, you're saying that you want the instance type to determine behavior when called.

Also note that this assignment is valid - cast syntax is not needed to change variable type from subclass to baseclass. This kind of cast can be done implicitly.

MyBaseClass mybc = derived;

Answer 2

You have overridden it. It only calls the derived method. You have to explicitly call the base class' method :

override public void Print() {
     base.Print();
     Console.WriteLine("This is the derived class.");
}

Answer 3

The whole point of overriding virtual methods is that the version for the underlying (runtime) type of the object is called, rather than the one for the static (compile-time) type - even when you call it through a type declared as the base class.

So this is behaving exactly as it should.

If this was not the case, it would make a lot of the utility of a class hierarchy useless, because you couldn't change the behaviour of a class type passed to a method by passing it a customised derived class.