Saturday, August 27, 2011

Difference between abstract classes and interfaces

What is the difference between an interface and an abstract class ?


Abstract Class

Interfaces are implemented  i.e.
instances of interfaces cannot be created by derived classes.

Classes are inherited   i.e.
derived classes do create instances of abstract classes

You cannot use access specifiers with members of an interface. All interface
members are implicitly public and cannot have any other access specifier.

Members of abstract class can have all access specifiers except private i.e.
they can be internal, public, protected and protected internal but not private.

All the members of an interface are implicitly abstract (although you cannot
explicitly use abstract keyword because it not allowed to use access specifiers
in interface).

An abstract class can have non-abstract members.

Interface cannot have static members.

Abstract classes can have non-abstract static members.

A class can inherit one or more interfaces.

A class can inherit from only one abstract (or non-abstract ) class.

Modifying an interface results in breaking the existing clients. This is because
clients must implement all interface members, and interface members are only
declarations , no code or values are allowed.

Not all modifications in an abstract class break existing clients. E.g. adding a
new non-abstract method to the base abstract class will not break existing

Interfaces CANNOT have constructors and destructors.

Constructors and destructors ARE ALLOWED in abstract classes.
The abstract class constructors cannot be abstract or private.

When will an abstract class become equivalent to an interface ?
To equivalent to an interface , the abstract class must meet following conditions :
•    All of its members must be abstract.
•    It should not have members other than methods,properties,events and indexers.
•    Access modifiers to all of its members must be public.

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