object vs reference type

Consider we have follow scenario

Class base

{

public void print()

{

Console.writeline("Print from base class");

}

Class derived : base

{

public void print()

{

Console.writeline("Print from base class");

}

and in the main method if we write like this

main()

{

base b = new derived(); // My 1st question is what happens at this point in time

b.print();

} //Now then why do we need to write like this we could declared it as "base b= new base();" because anyway we can't call child class variables from b.

//also somebody pointed out that "base b = new derived();" in this case b become object of child class to a reference variable of base type.

//I want to know what does this mean.

Please suggest with your observations.

Thanks in advance

I'm going to start by making some alterations to your code so that it will compile without warnings and we can then run it to see what happens:

using System;

class Base

{

public void print()

{

Console.WriteLine("Print from base class");

}

class derived : Base

{

new public void print()

{

Console.WriteLine("Print from derived class");

}

class Program

{

static void Main()

{

Base b = new derived();

b.print();

Console.ReadKey();

}

The output is:

Print from base class

So what's happening here is that you're creating an object of the derived type and assigning a reference to it to a variable of type Base.

This is allowed in C# because, in a sense, a derived object IS a Base object and (apart from any constructors and destructor) inherits its members.

In particular, it inherits Base's print() method even though it's hidden by a method of the same name in the derived class. Notice that I've added 'new' to the declaration to avoid the compiler warning you'd otherwise get in this situation.

As we're using a variable of type Base here, it's Base's print() method that gets called.

If the code in Main() had been written like this then it would have been derived's print() method that was called instead:

derived d = new derived();

d.print();

As you correctly, point out when you assign a derived class reference to a Base class variable, you can only call the members of the Base class via this variable as it knows nothing about any members which have been added by the derived class itself.

Nevertheless, this sort of thing is often done in C# where you want to write generic code which will work for a family of classes using the 'lowest common denominator' i.e. the base class itself.

As well as hiding inherited members, you can also override them if you declare them to be 'virtual'. However, I won't go into all that here in case you haven't yet covered them in your studies.

1 Reply

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Vulpes12y ago
I'm going to start by making some alterations to your code so that it will compile without warnings and we can then run it to see what happens:

using System;

class Base
{
public void print()
{
Console.WriteLine("Print from base class");
}
}

class derived : Base
{
new public void print()
{
Console.WriteLine("Print from derived class");
}
}

class Program
{
static void Main()
{
Base b = new derived();
b.print();
Console.ReadKey();
}
}

The output is:

Print from base class

So what's happening here is that you're creating an object of the derived type and assigning a reference to it to a variable of type Base.

This is allowed in C# because, in a sense, a derived object IS a Base object and (apart from any constructors and destructor) inherits its members.

In particular, it inherits Base's print() method even though it's hidden by a method of the same name in the derived class. Notice that I've added 'new' to the declaration to avoid the compiler warning you'd otherwise get in this situation.

As we're using a variable of type Base here, it's Base's print() method that gets called.

If the code in Main() had been written like this then it would have been derived's print() method that was called instead:

derived d = new derived();
d.print();

As you correctly, point out when you assign a derived class reference to a Base class variable, you can only call the members of the Base class via this variable as it knows nothing about any members which have been added by the derived class itself.

Nevertheless, this sort of thing is often done in C# where you want to write generic code which will work for a family of classes using the 'lowest common denominator' i.e. the base class itself.

As well as hiding inherited members, you can also override them if you declare them to be 'virtual'. However, I won't go into all that here in case you haven't yet covered them in your studies.