C sharp reference variables..

mukul chakravarty

Suppose I have a Base class "Computers" and a derived class "Laptops" derived from the "Computers" class.

Now WHAT is the meaning of the following declaration ?? WHEN is it used ?? and HOW is it useful??

Computers C=new Laptops();

4 Replies

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Sunny Sharma13y ago
Accepted answer
Hi mukul,

This process is called object slicing. To be precise on that, when a Derived Class object is assigned to Base Class object, the content of derived class, only that belong to Base Class, is copied to Base Class object leaving the specific contents of Derived Class object. Simplifying that, the Base Class object can access only the Base Class members.

Now from the usage point of view, I would say, It's all up to you. For a simple demonstration, take an example of a "Vehicle" Base Class. Suppose the vehicle base class has only a basic property "POWER". Derived Classes could be Cycle, Car, Bus, etc. Derived classes may have specific properties depending on their type like seats, wheels etc. Now, no matter what kind of vehicle they are, they can just drive on the road. All are vehicles. So the base class is a general approach to work with "the group" of those objects. If it's needed we can also cast the vehicle as more specified version like Train otrain = (Train) Car. Suppose you need a new vehicle, given you have a Car with all the properties required for that new vehicle, you might simply prefer casting that Car to new vehicle instead of Instantiating a new vehicle and assigning all the properties.

Hope you get the idea behind the scene :)

Please don't forget to accept this as answer if it helps!
Thanks.
Vulpes13y ago
It's not really polymorphism (which means having many forms) because, unless the base class methods happen to be virtual or abstract, they are not 'specialized' by the derived classes - they're simply inherited.

Nor is it object slicing which means that when you assign a derived class object to a base class variable, the extra members in the derived class are 'sliced off'.

Although this can be a problem in C++, it's never a problem in C#. This is because the latter restricts inheritance to 'reference types' and, when you assign a derived class object to a base class variable, all you're doing is assigning a reference to that object to the variable - you're not copying the object itself or changing it in any way.

The compiler enforces the restriction that you can only access (non-virtual) base class members using a base class variable.

Technically, it's called an 'implicit reference type conversion' even though the object is not actually changed and is perhaps best thought of as a kind of non-parametric generic programming.
Jeramy Singleton13y ago
I could be wrong, but isn't this called Polymorphism?
Vulpes13y ago
The declaration means that an object of type Laptops is first created and a reference to that object is assigned to the variable C of type Computers.

This works because Laptops is derived from Computers and therefore contains all the members that Computers contains.

However, the variable C will only be able to access the members which are inherited in this way. Any members which were added by the Laptops class will not be accessible to C because the Computers class knows nothing about them.

This sort of code is useful where you want to be able to deal with not only Computers objects but objects of any derived classes as well using the same variable or parameter. In other words, it helps you to write generic code.

The drawback, of course, is that you can't access the members which are added by the derived classes themselves using such a variable.