Inheritance - beginners question

Question

1.00/5 (2 votes)

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Hi,

Based on the classes below, what's the difference between

C#

ClassBase ObjectBase = new ClassDerived();
ObjectBase.Show();  // Output --- This is Derived Class.

and

C#

ClassDerived ObjectDerived = new ClassDerived();
ObjectDerived.Show();  // Output --- This is Derived Class.

C#

public class ClassBase
   {
     public virtual void Show()
        {
            MessageBox.Show("This is Base Class");
        }
    }

public class ClassDerived : ClassBase
   {
     public override sealed void Show()
        {
           MessageBox.Show("This is Derived Class");
        }
   }

Posted 18-Jun-13 3:03am

FTWO

Updated 18-Jun-13 9:22am

v6

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Ron Beyer · Accepted Answer · 2013-06-18T03:13:00

Solution 1

Nothing from the standpoint of functionality since both your classes have the same methods. The difference would be if the derived class had methods that the base class did not, you would not be able to access them in the first case without casting it.

C#

((DerivedClass)ObjectBase).SomeDerivedMethod()

Posted 18-Jun-13 3:13am

Ron Beyer

v2

Comments

FTWO 18-Jun-13 9:30am

I have the output in comments that show that I was able to access the derived method
in first example

ClassBase ObjectBase = new Derived();
ObjectBase.Show(); // Output --- This is Derived Class.

Sergey Alexandrovich Kryukov 18-Jun-13 9:57am

My 5, but OP is not getting anything. Also, this does not bring to understanding of the ideas of OOP and should not be used as a practical programming advice. For us, it should be obvious, but OP would need a note.
—SA

FTWO 18-Jun-13 10:42am

Thanks Sergey - Are you saying that first case may work, but should use second case ?

Sergey Alexandrovich Kryukov 18-Jun-13 11:59am

Which case? It's not about working, your sample makes no practical sense anyway. You see, you should not consider cases, you should understand the principles. Do you know how virtual method table works? Why do you need late binding, what's the use?
—SA

FTWO 18-Jun-13 12:17pm

It's a beginners question I suppose. "The use" is to help me understand it.
It's not code that is being used anywhere, it's an example I'm giving so I can understand the principles.
I saw the code here http://www.codeproject.com/Articles/602141/Polymorphism-in-NET
and was surprised that the class was being instantiated as
ClassBase ObjectBase = new Derived() ---I'm calling this case 1
and not
ClassDerived ObjectDerived = new Derived() -- I'm calling this case 2

So I'm asking the question why, so that I can learn from the answer - maybe that's the use.

Sergey Alexandrovich Kryukov 18-Jun-13 12:25pm

I don't know what is "beginners". What I mention is the basics of OOP, if you don't learn it, forget about OOP (and hence forget about .NET)
I makes no practical sense in the following sense: the issue exists only if you introduce some other method, not available in base, not only the different overridden method of some existing one in base.
Unless you do it, there is not difference what declaration to use for the compile-time type. (Do you understand compiler-time vs run-time types? This is where you are now...)
—SA

FTWO 18-Jun-13 14:29pm

just so there's no confusion can you tell me what you think I am asking

Sergey Alexandrovich Kryukov 18-Jun-13 16:08pm

Please, no, it may lead too far... It's time for you to read about OOP, pretty much from the very beginning, this is the point. This way, you questions may become more relevant...
—SA

FTWO 18-Jun-13 17:45pm

It may lead to far. That's funny.

I wanted to see if I was being clear in the question I was asking
ie why would you instantiate like this (using base class reference in derived class:
BaseClass objBaseClass= new DervivedClass()
and not like this
ClassDerived objDerivedClass = new Derived()
If objBaseClass.Show() and objDerivedClass.Show() would always run derived method.
In any case Sagar cleared things up somewhat for me.

Incidentally can you recommend good OOP literature, I have been reading about dynamic dispatch and virtual method tables but mainly from forums or Wikipedia, clearly I need a good recommendation.

Sergey Alexandrovich Kryukov 18-Jun-13 18:28pm

I say: there is no difference in the scope of your question. The difference takes place when you have essential difference in base and derived classes.
On left "=" there is a variable with declared compile-time type, but it on right, this is the same runtime class. Naturally, is the compile-time type is the base, the variable does not "see" new members of derived class. But you don't have those new members.

Actually, in practice, the idea of OOP is to be agnostic to derived classes, working only with the compile-time types...

—SA

Sergey Alexandrovich Kryukov 18-Jun-13 18:29pm

Your problem is: you are trying to jump over important concept with your question. You ask "why", but there is no why, because both ways don't make any sense in the scope of your code sample.
—SA

FTWO 19-Jun-13 4:37am

Ah right…
When instantiated like this (using base class reference in derived class)

ClassBase ObjectBase = new ClassDerived();

The only methods that the object can access in ClassDerived are methods that also exist in ClassBase (unless I’m casting it). Is that correct ?

Apologies for being slow on this !

Sagar Sumant · Accepted Answer · 2013-06-18T09:55:00

// If you want to know the difference run following snippet

C#

using System;

namespace ConsoleApplication1
{
    public class ClassBase
    {
        public virtual void Show()
        {
            Console.WriteLine("This is Base Class");
        }
    }

    public class ClassDerived : ClassBase
    {
        public override sealed void Show()
        {
            Console.WriteLine("This is Derived Class");
        }
    }
    class Program
    {
        // If you want to know the difference run following snippet
        static void Main()
        {
            ClassBase baseClass = new ClassBase();
            ClassBase derviedClass = new ClassDerived();//Note that i am using base class reference in derived class as well.

            Console.WriteLine("Case1: Child class is overriding the base class method.");
            //Case 1: child class is overriding virtual method from base class. 
            Console.WriteLine("Accessing base class object:");
            baseClass.Show();//"This is Base Class"
            Console.WriteLine("Accessing derived class object:");
            derviedClass.Show(); //"This is Derived Class"


            //Case2: Child class is not overriding base class method. Remove 'override' and 'sealed' qualifiers from 'Show' method of ClassDerived, then run this part.
            Console.WriteLine("Case2: Child class is not overriding the base class method.");
            Console.WriteLine("Accessing base class object:");
            baseClass.Show();//"This is Base Class"
            Console.WriteLine("Accessing derived class object:");
            derviedClass.Show(); //"This is Base Class"

            //In case2 if you want call show method from child class when you are using a reference of base class to child class object then you have to
            //do something like shown below.         
            Console.WriteLine("Accessing child class object (child class has not overridden base class method.)");
            ((ClassDerived)derviedClass).Show(); //"This is Derived Class"            

            Console.WriteLine("Accessing base and child class objects with 'new' qualifier used in child class method to hide base class method.:");
            //If you want to check real difference then one can use new operator.
            //Use 'new' instead of 'override' in above example and remove 'sealed' keyword.
            // Now if you run same two lines output is as follows
            Console.WriteLine("Accessing base class object:");
            baseClass.Show();//"This is Base Class"
            Console.WriteLine("Accessing child class object class object:");
            derviedClass.Show(); //"This is Base Class"

            Console.WriteLine("Accessing the child class object using typecasting to base and child class (when child class has not overridden the base class method)");
            //Also if you do something like this (assuming that child class is not overriding the method)
            ((ClassBase)new ClassDerived()).Show(); //"This is Base Class"
            //But if you have overriden method in child class then output would be as follows.
            ((ClassDerived)new ClassDerived()).Show(); //"This is Derived Class" 

            //To summarise we have following.
            //1. When a virtual method which is (not overriden by child class) but just implemented using child class with or without new keyword following scenarios happen
            //     i. If you are referring child class object using base class reference then base class' method will be invoked. eg. output - This is Base Class
            //     ii.If you are referring child class object using child class reference then child class method will be invoked.  eg. output - This is Derived Class
            //     iii. You are accessing child class object using child class reference but typecasting it to base class reference then it will invoke base class' method. eg. output - This is Base Class
            //2. When a virtual method is overriden in child class then in all the cases output is 
            //'This is Derived Class'. So when you override a method in child class then no matter how you try to access it in child class object(using base class reference or child class reference) only child class' method will be invoked and not from the base class.It is bit long explaination but i hope this helps in understanding runtime polymorphism through inhertance.
        }




        //I hope this clears the confusion...
    }
}