When should I choose an abstract class over an interface?

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I have done lots of search regarding this answer and I've come up with conclusion mentioned below.

I want to know whether it is a fairly enough and a justified reason for choosing one over the other?

What I have tried:

Use abstract class when we have some default functionality or methods whose definition will remain same for all
the sub-classes. So by using abstract class, we do not have to write same code again and again, even in future if
we need to change the functionality we just need to make changes at a single place.

Posted 7-May-18 0:23am

knackCoder

Updated 7-May-18 0:54am

CPallini

v2

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Maciej Los · Accepted Answer · 2018-05-07T00:49:00

Solution 1

Please, chech this: oop - Interface vs Abstract Class (general OO)[^]

Posted 7-May-18 0:49am

Maciej Los

Comments

CPallini 7-May-18 6:53am

5.

Maciej Los 7-May-18 7:51am

Thank you, Carlo.

Ehsan Sajjad · Accepted Answer · 2018-05-07T00:56:00

Solution 2

The simplest answer is that when we have some default behaviors that will be same for every derived class which would be inheriting from the base type then abstract class is suitable as we can define method with implementation that will be re-used by child classes too. While for interface we only define the signatures which means that it only tell what kind of behaviors it expects to be provided when some type is implementing it.

Note that in newer version of C# we would be able to define methods with default implementation in interfaces too.

You can also refer to the following threads for more clarity:

Abstract Class versus Interface

Interfaces vs. abstract classes

Posted 7-May-18 0:56am

Ehsan Sajjad

Updated 7-May-18 0:59am

v4

Comments

knackCoder 7-May-18 7:12am

From your answer and mine doubt above, I'm understanding that "providing default functionality in base class" is something which determines whether we should go for Interface or a Abstract class; right?

Ehsan Sajjad 7-May-18 8:14am

in most of the cases yes, otherwise it does not makes sense to have abstract class, but mind that there are no hard and fast rules, but in general practice interfaces is preferred in situation where there is no default behavior needed but just contract.

BillWoodruff 7-May-18 10:52am

+5 good answer, and I bet the "Gang of Four" would have nightmares about the idea of any "implementation" in Interfaces: this idea is a proposed feature of C# 8, and we don't know whether it will be implemented.

See: https://dotnetcoretutorials.com/2018/03/25/proposed-default-interface-methods-in-c-8/

Ehsan Sajjad 7-May-18 11:19am

Thanks @Bill and yes it is not yet out as part of C# 8