what is Abstract class and Interface and where exactly should use?

Kiran Nath

15y3kVisual Basic .NET

Abstract class and Interface

4 Replies

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Datta Kharad15y ago
Hi

Abstract Class:-
1) An abstract method is created by specifying the abstract type modifier.
2) An abstract method contains no body.
3) An abstract method is not implemented by the base class.
4) An abstract method is automatically virtual.
5) A derived class must override it.
6) Abstract class can have modifiers for methods,properties etc.,
7) An abstract class can implement a property.
8) The abstract modifier cannot be applied to static methods.
9) Properties can also be abstract.
10) A class containing abstract methods must be declared as abstract with the abstract specifier.
11) There can be no objects of an abstract class.
12) If a derived class doesn't implement all of the abstract methods in the base class, then the derived class must also be specified as abstract.
13) An abstract class can inherit from a class and one or more interfaces.
14) An abstract class can implement code with non-Abstract methods.
15) Abstract class can have constant and fields.
16) An abstract class can have constructors or destructor's.
17) An abstract class cannot be inherited from by structures.
18) An abstract class cannot support multiple inheritance.
19) 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.

Interface:-
1) Interfaces cannot be instantiated directly.
2) Interfaces can contain events, method, indexer and properties.
3) An Interface can contain property definitions.
4) Interfaces contain no implementation of methods.
5) Classes and Structs can implement more than one interface.
6) An Interface can be inherited from by structures.
7) An interface itself can inherit from multiple interfaces (Interface can support multiple
inheritance).
8) An abstract class can implement a property.
9) 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.

Please mark as Accepted answer if your query resolved.
Jaganathan Bantheswaran15y ago
Hi,

Refer this - http://www.c-sharpcorner.com/Forums/Thread/110261/interfaces-vs-abstract-classes.aspx
Suthish Nair15y ago
Kiran, check Our recommended articles section for articles.
Vulpes15y ago
There are some circumstances where you can use one but not the other:

1. A class can only inherit from one class (abstract or otherwise) but can implement multiple interfaces.

2. An interface cannot include static members (including constants, operators and nested types) but an abstract class can include all of these.

3. An interface cannot contain any 'state' (i.e. fields) but an abstract class can.

4. All interface members are implicitly public and need to be implemented as such (unless they are implemented 'explicitly' when they are only public when invoked via an interface reference). However, an abstract class can include non-public members as well.

5. Interfaces never contain code - only the definitions of members - and so cannot have a default implementation. In constrast an abstract class can provide a default implementation of some or all of its methods or properties by declaring them virtual rather than abstract. A descendant class can then override them if appropriate.

It's possible using extension methods to work around #3 and #5 to some extent.

If these rules do not prevent you from using either, then I'd use an abstract class if it and its derived classes naturally share an implementation but I'd use an interface if the classes which implement it have independent implementations.