When is it appropriate to use abstracts as opposed to interfaces to define base class behavior? What is the most common, practical usage for each? For instance, if I were to write a database applications that required to read a wide variety of database formats, would I define by database classes as abstracts or interfaces?

What is an Abstract Class?

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

What is an Interface?

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn't support multiple inheritance, interfaces are used to implement multiple inheritance.

When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.

Both together

When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.

Note that:
An Abstract class without any implementation just looks like an Interface; however there are lot of differences than similarities between an Abstract class and an Interface. Let's explain both concepts and compare their similarities and differences.

For more info:

commented: Good explanation! +7

If you can define your idea as what it does without the direct need on how it has to be done, use an interface. If you need to have some implementation(how do I do it), use an abstract class.

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The best way, in my opinion, to figure out what is most appropriate is to start writing an abstract base class. If, upon finishing the class, you find that the only things in this class are abstract functions, then the class is, at least conceptually, an interface. Essentially, if the base class requires actual data and implementation, use an abstract base class. However, if it only requires a list of functions to be implemented by a derived class, use an interface.