In our earlier Article, we have discussed about benefits and usage of virtual modifier in C#. We learnt that virtual keyword enables to override the members in the derived classes.
In this Article, we will discuss about override and new modifiers. These modifiers are used during the inheritance in derived the class which will inherit from it’s base class.
The override Modifier
This modifier is used to override the behavior of the inherited member from it’s base class. That means, using this we can redefine the member in the derived class. The base class definition will be override in the derived class.
Keep in mind that, we can not override all the members from the base class. The member from the base class must be defined with virtual, abstract or override modifiers in order to override them in the virtual class.
C# doesn’t allow to override a static member and also doesn’t allow to override non-virtual members.
private members are local to the class where the private members are defined. These will not inherit to the derived classes. Hence can not be overridden.
The new Modifier
To hide the base class members in the derived classes, we use new modifier. By default, C# compiler hides the base class members in the derived classes. You must have to declare the inherited member with this modifier; if you are not intended to override it. Otherwise, the compiler will throw the Warning message, during the compilation.
Warning 1 ‘Modifiers.AnotherDerivedClass.Display(string)’ hides inherited member ‘Modifiers.BaseClass.Display(string)’. Use the new keyword if hiding was intended.
Let’s take a simple example; where BaseClass has two methods; named Display; one will takes String as an argument another will take an int as an argument. The BaseClass has derived class; DerivedClass inherited these base class methods.
Override the Display method using override modifier in DerivedClass. And define another Display method to hide it’s base class version in DerivedClass using new modifier. That means, now we have 2 Display methods in DerivedClass; one is defined using override modifier and another one is defined using new modifier.
Keep that in mind that, we can not use both new and override modifiers on the same member. If you attempt to use, the compiler will throw the below Error;
Error 1 A member ‘Modifiers.DerivedClass.Display(string)’ marked as override cannot be marked as new or virtual
Let’s create an instance of the derived class of type BaseClass. And call the derived class’s Display method using BaseClass‘s object. The code looks like below:
BaseClass b = new BaseClass();
BaseClass d = new DerivedClass();
b.Display("Hello!");
d.Display("Hi!");
b.Display(100);
d.Display(200);
When calling the Display method using BaseClass‘s object; if the method is overridden, it will executes the overridden method. If the method is hidden in derived class, it executes the BaseClass‘s version of the method.
Putting together, here is the working example.
The result looks like below.
Base Class :Hello! Derived Class :Hi! Base Class :100 Base Class :200
We will discuss more topics as we go.
(Raju)