Questions about .Net inheritance

Question

public class Father
{
  // static string TYPE = "FATHER";
    public Father()
    {
        //Console.WriteLine("ctor");
    }
    public virtual void Print()
    {
        Console.WriteLine("I'm father");
    }
}

public class Son:Father
{
    public override void Print()
    {
        base.Print();
        Console.WriteLine("I'm son");
    }
}

As we konw, if we call Son.Print(),It'll print out "I'm father" and "I'm son".And Father.Print() is an instance method ,we need to create an instance first.So that's the question,who creates it?Obviously,not me... Or Son owns two Print methods in the methodtable.One of them can be accessed by Father,anthor can be accessed by itself? Which one is right?Or neither is right?Please tell me!Thanks!

Minor quibble, but Son : Father is a really bad example of inheritance in its OOP sense — Dan Bryant
– Dan Bryant, Commented Oct 6, 2012 at 2:59
If you access class Son as a Son type, you get both the class Father and class Son data and behaviors. — Mudassir Hasan
– Mudassir Hasan, Commented Oct 6, 2012 at 3:00
Indeed, the example itself is probably what's causing the confusion and not inheritance in general. Son is printing both statements because in this case a Son is a Father. While that holds true for a segment of the population, it's certainly not universally true as implied by this inheritance hierarchy. Son and Father should be interfaces, not inheriting classes. Each Person entity should implement the appropriate interfaces. This is a great example of how not to use inheritance. (Though I'm sure there's a good "inheritance" joke in here somewhere regarding a father and son...) — David
– David, Commented Oct 6, 2012 at 3:01
I always use Shape, Square, and Triangle as everyone can understand that squares are shapes and triangles are shapes but triangles and squares are different things. — Scott Chamberlain
– Scott Chamberlain, Commented Oct 6, 2012 at 3:01

Sergey Kalinichenko · Accepted Answer · 2012-10-06 02:54:22Z

4

Who creates it? Obviously not me

What makes you so sure? Of course you do:

Son s = new Son();

Or Son owns two Print methods in the methodtable.

No, it has just one Print method, but it has something else: it knows about its base class, Father, which has its own Print method. That's why Son has access to two Prints - its own and his Father's.

answered Oct 6, 2012 at 2:54

Sergey Kalinichenko

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10 Comments

Scott Chamberlain Over a year ago

Also to clarify. When you don't put anything after the constructor there is a implicit : base() added to the end of it. So Son is calling Father's parameter-less constructor, and Father is calling object's parameter-less constructor (because if you don't specify a parent class a implicit : object is added to your class definition).

Lyon Gu Over a year ago

Thank you! I know when we call Son's constructor ,it also calls its base's constructor.But who owns its base's reference or who stores the reference ,the keyword base(Sorry,It's wrong,but I don't know how to explain...)?

Sergey Kalinichenko Over a year ago

There is no reference to base, only knowledge about the base. That mysterious "knowledge" usually comes in the form of a reference to the virtual table of the base class, but the standard does not say that it must be that way.

Lyon Gu Over a year ago

I'm confused..."There is no reference to base",but how can we call its instance method?

Sergey Kalinichenko Over a year ago

@LyonGu You need two things to call an instance method - a reference to the object, and a pointer to the method. Because of the way the inheritance is implemented, you can pass a reference of a derived type to a method from its base class. That's why a reference to the base is not necessary: a reference to this is sufficient.

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