abstact vs interface

Question

in interview i faced one question why abstract when interface ?

Afzaal Ahmad Zeeshan · Answer

Simple rule of thumb: Abstract classes must be implemented and they do not provide any working of the functions before they can be used in projects otherwise their subclasses are also abstract. Note that an abstract class is still a class but it cannot be instantiated (you cannot do new Class()). Whereas an interface defines the basic structure of a service for example in ASP.NET we have IService and that is used by EmailService SmsService and they have one function in common. They will define their own behavior either send using SMTP or use an SMS gateway. In my own opinion you must not use abstract classes unless you are defining a software API such as a library where you require the users to  always  define the behavior of the objects. If that is not required such as your own projects interfaces are the best choices. For more on this please read this thread: http://stackoverflow.com/questions/761194/interface-vs-abstract-class-general-oo

prabhu p · Answer

thanks Prabhu