Abstract Class v/s Interface What is the diffrence and When to use which

This post will explain what is abstract class and interface and when to consider abstract class over interface and vice versa.  There are numerous posts on this topic already on other sites but thought sharing the information here will be helpful for the visitors like you who are visiting to read some advanced interview questions and will have some brush-up to this basic but important topic.

Abstract Class

In simple words, Abstract class is a class which cannot be instantiated i.e. you cannot create an object of a class when you declare it as an abstract class. Using abstract class you can define some common methods in it and on top you can provide abstract methods which will get overridden by inherited members. This behavior helps you to provide base functionality with a choice of having independent implementation for some functionality.

In above image you can see there is GetPersonalInfo() method is common method for Employee class so it will be available in SoftwareEngineer class and ProjectManager class too as a base functionality but CalculatePerformance() method is defined as abstract which means all the classes inherting from Employee class must provide implementation for this method. With this you have forced inherited members to provide an implementation for CalculatePerformance() method but with a option to have independent logic for each class.

Interface

An interface contains only the signatures of methods, properties, events or indexers. A class or struct that implements the interface must implement the members of the interface that are specified in the interface definition. Interface helps us to define a contract i.e. when some class implements a interface it says to outer world that it is capable to provide a functionality defined in interface contract.

You can see in above image Interface forces inherited classes to implement all the methods defined in the interface as a contract. So there is not a base functionality available as like abstract class.

Difference between Abstract class and Interface

• The main difference between abstract class and interface is in abstract class you have a choice of providing base methods with implementation along with abstract methods but with interface all methods are abstract (only signature is there).
• Another difference you can achieve multiple inheritance with interface which is not possible with abstract class i.e. you can inherit from only one abstract class at a time.

When to use Abstract class?

• If you are expecting a multiple versions of your component then consider using abstract class as if abstract class updated all inheriting members automatically gets updated i.e. you can easily add non-abstract methods to abstract class without breaking any code but if you had implemented interface then such addition will break the code as all inheriting member need to implement the new methods.
• If you are designing large functional units which are closely related in hierarchy then consider using abstract classes over interface.
• If you want some base functionality commonly implemented then you can abstract class.
• If you want to decouple the contract from implementation then use abstract class as they are more maintainable.

When to use Interface?

• If you want to provide polymorphic behavior to Value type then you need to use Interface as Value Types must and can inherit from value types only.
• If the functionality you are developing will be useful across a wide range of disparate objects, use an interface.
• If you are designing little, fragment of functionality use interface.           
• If you want to achieve an effect similar to that of multiple inheritance, consider using interface.

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