this article says Codeproject Abstract Class -can have method Declaration and method definition.
But this article says programcall Abstract - can have only method Declaration.
I got confused. Can anybody clear me what is the exact difference of both ?
See this article:
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.
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
As far as I know, abstract classes can already implement some methods or variable's values, as long as it does not implement them all (which would make it a normal class). An interface cannot do this; it can only provide method and variable stubs.
An interface is more of a contract - it details what methods or properties will be found on an object that implements it.
An abstract class is a base object - it can contain methods, variables and behaviour - however you cannot create a concrete implementation of it - you must inherit from it and extend its behaviour.
