Interface and Abstract Class

Hi,

If I am asked to make a choice between an abstract class and the interface, which one should i opt for. what are the things which i have to consider before choosing one among them.

Thanks in advance
Rajeeva
rajeeva_nagarajAsked:
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Meir RivkinConnect With a Mentor Full stack Software EngineerCommented:
the most substantial difference between abstract and interface is that interface is declarative only while abstract can contain implementation.
this is a good post to start with:
http://www.codeproject.com/Articles/11155/Abstract-Class-versus-Interface

in general, u choose abstract when u have shared functionality among your concrete objects.
for instance, let's say you have interface class Shape and concrete classes Triangle, Square etc, which derive from Shape base class.
now, and u want to add SaveToFile functionality.
instead of having the same implantation to all concrete classes, you could change the Shape class to abstract and implement SaveToFile there.
in relate to my previous example, when passing the shape objects as interface and not as abstract class, u can hide shared functionality which are internal.
for instance, the SaveToFile can be declared in the IShape interface.
assume u want to have common function to calculate area of shape but not to expose it in the interface.
the Shape abstract class is the place to add this function.
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