Abstract vs interface

Question

Abstract vs interface

Which should be used and when?
Just saw that if all abstract methods, then it becomes interface.
Still not got any good example that can justify the usage of abstract, interface and which is better.

Niradhip Chakraborty · Answer

Interfaces are closely related to abstract classes that have all members abstract. a. For an abstract class, at least one method of the class must be an abstract method that means it may have concrete methods. b. For an interface, all the methods must be abstract c. Class that implements an interface much provide concrete implementation of all the methods definition in an interface or else must be declare an abstract class ïƒ¼ In C#, multiple inheritance is possible only through implementation of multiple interfaces. Abstract class can only be derived once. d. An interface defines a contract and can only contains four entities viz methods, properties, events and indexes. An interface thus cannot contain constants, fields, operators, constructors, destructors, static constructors, or types. e. Also an interface cannot contain static members of any kind. The modifiers abstract, public, protected, internal, private, virtual, override is disallowed, as they make no sense in this context. f. Class members that implement the interface members must be publicly accessible.

Roei Bar · Answer

additional to what Kirtan said, the idea of Abstract is also to make you create default Operations on the basis, like for example, you give a public that is the Method that can be overriden, and it runs a private to create the right string to access the db or other services... or so on

Kirtan Patel · Answer

Interface provides method signature and not the body. Its up to the developers to provide body for these methods once they implement it. Abstract provides both methods with and without body. Adding new methods in abstract class in middle of development life cycle won't break existing sub classes unless these methods are declared as must override. If there are frequent addition of new methods properties and so on. one should use abstract.. Adding new methods to the interface will force users to go through each and every sub class that implemented that interface and call these methods. So its always preferred to use abstract over interface because of this limitation. Whereas on the other hand interface can fill gap of multiple inheritance. One can implement from more than one interface. Not same with Abstract. One can inherit from only one abstract class. Happy Coding :)