Abstract Class
These classes are either partially implemented, or not at all implemented.
- This class can contain declarations and also implementations.
- An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
- A non-abstract class derived from an Abstract class must include implementations for all inherited abstract methods.
- Abstract class can contain abstract methods, abstract property as well as other members (just like normal class).
Interface
Interface is a pure abstract entity:
- An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. It contains only the declaration.
- We cannot make instance of an Interface. We must inherit to use Interface.
- A class which implements the interface should implement all the members of an Interface.
- Members of an Interface are always public.
- The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.
Few Advantages
An advantage of Abstract class over Interface :
If you want to have additional functionalities for the interface, you must implement the new ones in all of the classes that implement the changed interface. The code will not work until you make the change. But if you want to have additional functions to the Abstract class, you can have default implementation. Your code still might work without many changes.
An advantage of Interface over Abstract class :
A class can inherit multiple interfaces but can inherit only one class.
Both Together
- When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.
- When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.
- There are some similarities and differences between an interface and an abstract class that I have arranged in a table for easier comparison:
Feature | Interface | Abstract class |
Multiple inheritance | A class may inherit several interfaces. | A class may inherit only one abstract class. |
Default implementation | An interface cannot provide any code, just the signature. | An abstract class can provide complete, default code and/or just the details that have to be overridden. |
Access Modifiers | An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public | An abstract class can contain access modifiers for the subs, functions, properties |
Core VS Peripheral | Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface. | An abstract class defines the core identity of a class and there it is used for objects of the same type. |
Homogeneity | If various implementations only share method signatures then it is better to use Interfaces. | If various implementations are of the same kind and use common behavior or status then abstract class is better to use. |
Speed | Requires more time to find the actual method in the corresponding classes. | Fast |
Adding functionality (Versioning) | If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method. | If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly. |
Fields and Constants | No fields can be defined in interfaces | An abstract class can have fields and constants defined |
Conclusion
- Abstract class gives an Identity to the derived object.
- Interface extends the ability of an object.
- Interfaces give polymorphic behavior.
- Abstract class also gives polymorphic behavior. But it is more meaningful to say that Abstract class simplifies versioning, as the base class is flexible and inheriting class can create many versions of it by additional functions and also by implementing Interfaces.
- Interfaces and Abstract Classes both promote re usability.
