C sharp reference variables..

Question

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();

Sunny Sharma · 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.

Vulpes · Answer

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 Singleton · Answer

I could be wrong but isn't this called Polymorphism?

Vulpes · Answer

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.