Abstract class: An abstract class cannot be instantiated. The purpose of an abstract
class is to provide a common definition of a base class that multiple
derived classes can share. For example, a class library may define an
abstract class that is used as a parameter to many of its functions, and
require programmers using that library to provide their own
implementation of the class by creating a derived class.
Interface : An interface contains only the signatures of methods, properties, events or indexers. A class or struct that implements the interface must implement the members of the interface that are specified in the interface definition. In the following example, class ImplementationClass must implement a method named SampleMethod that has no parameters and returns void.
Interface : An interface contains only the signatures of methods, properties, events or indexers. A class or struct that implements the interface must implement the members of the interface that are specified in the interface definition. In the following example, class ImplementationClass must implement a method named SampleMethod that has no parameters and returns void.
| 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 Modfiers | 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 behaviour 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 constrants defined |