Instance of child class ?

Question

How to create the instance of child class in C#?

class Parent
{
 virtual void test()
 {
  Console.writeline("this is parent");
 }
}

class Child : Parent
{
 override void test()
 {
  Console.writeline("this is from child");
 }
}

public static void main()
{
 //Which one is right below?

 Child ch = new Child();

 Parent pa = new Parent();

 ch = new Parent();

 pa=new Child();

 ch.test();

 pa.test();
}

Answer 1

If you want an instance of Child, then new Child() is the right thing to do. However, as Child is a specialization of Parent, you may refer to it either through a Child or a Parent reference (ch and pa in your example).

So you have to decide if you want to access the instance as a Child or a Parent.

If you do

Child ch = new Child();

You have a reference, ch of the type Child pointing to an instance of Child.

If you do

Parent pa = new Child();

You have a reference, pa of the type Parent pointing to an instance of Child. I.e. you're taking advantage of the fact that inheritance builds a "is a" relationship between Parent and Child.

In other words the type Child is a specialization of Parent. Thus an instance of Child can be used anywhere an instance of Parent is needed.

Answer 2

child ch = new child();

Answer 3

In your code you have four instantiations, that all mean slightly different things:

// create a new instance of child, treat it as an instance of type child
Child ch = new Child();

// create a new instance of parent, treat it as an instance of parent
Parent pa = new Parent();

// this will not compile; you cannot store a less specialized type in a variable
// declared as a more specialized type
Child ch = new Parent();

// create a new instance of child, but treat it as its base type, parent
Parent pa = new Child();

Which one (of those three that works) that is correct depends on what you want to achieve.

Note that the following two cases both print "this is from child":

Child ch = new Child();
ch.test();  

Parent pa = new Child();
pa.test();

Answer 4

this tends to be more fundamental, than you thought! i recommend you reading a paper which explains you inheritance & polymorphism, eg msdn, msdn or codeproject

to me it's more about giving an explanation rather than a solution ...