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September 5, 2014

Abstract Class vs Interface

Filed under: C#,OOP @ 5:10 pm

Since publishing my course on Object-Oriented Programming Fundamentals in C# for Pluralsight, I often receive questions from viewers. One of the common questions is on the difference between an Abstract Class and an Interface.

Abstract Class

An abstract class is a class that cannot be instantiated, meaning you can’t use the new keyword to create one. Some key qualities of an abstract class:

  • It is intended to be used as a base class for other classes that inherit from it.
  • It can provide implementation, meaning that it can contain the code for an operation.

For example, you could define an abstract product base class:

public abstract class ProductBase

{

   public bool Save {

       // Code here to save the product

   }

}

The Save method here would contain the code to save a product.

You could then create specialized product classes: BookProduct, GroceryProduct, and OfficeProduct. Each of these specialized classes inherit from the ProductBase class. The specialized product classes then don’t need to contain the logic for the save, because it is provided in the base class.

Say you then define another abstract class for logging.

public abstract class Logging

{

   public void Log{

       // Code here to log information

   }

}

You need the logging functionality in the BookProduct class. Since C# does not allow multiple inheritance, the BookProduct class cannot inherit from *both* the ProductBase class and the Logging class. You instead have to create a hierarchy:

BookProduct class inherits from ProductBase class which inherits from the Logging class.

GroceryProduct class also inherits from ProductBase class which now inherits from the Logging class. So even if the GroceryProduct class does not want or need logging, it still has all of the logging features because Logging is part of its class hierarchy.

This can lead to deep inheritance chains that make the code more complex and difficult to maintain/extend.

Interface

An interface is a set of properties and methods. You can think of an interface as a contract whereby if a class implements the interface, it promises to implement *all* of the properties and methods in the interface. Some key qualities of an interface:

  • It is intended to be implemented by other classes. Each class that implements the interface promises to provide code for each property and method in the interface.
  • An interface cannot contain any implementation, meaning that it does not contain any code, just declarations.

Using the same example as above, you could define IProduct:

   public interface IProduct{

       bool Save();

   }

This interface cannot contain any code for the save. So each product class (BookProduct, GroceryProduct, and OfficeProduct) would need to implement this interface and provide its own code for the save feature.

Say you then define an interface for logging.

   public interface ILoggable{

       void Log();

   }

Any of the product classes that need logging can implement this interface. You can think of an interface as defining a role that a class can take on. For example, the BookProduct class could take on a logging role, email role, and printing role by implementing a logging interface, email interface, and printing interface.

By using an interface:

  • Only the specialized product classes that need logging can implement the ILoggable interface.
  • Each product class that does implement the ILoggable interface needs to provide its own implementation.
  • Other classes (completely unrelated to products) can use the ILoggable interface. The class does not need to be defined within the same hierarchy.

Summary

You can think of an abstract class as a bucket of functionality shared by all of your relatives. And an interface as a role that anyone in the community can take on.

Use an abstract class if:

  • You are building a base class.
  • You want to provide code that the specialized child classes could use (or override)
  • You don’t need the functionality in the abstract class for any classes other than those within the class hierarchy.

Use an interface if:

  • You are defining a “role” that a class could take on.
  • You may want to use the interface on unrelated classes.
  • Each class implementing the interface will provide an implementation.

Thoughts? Questions? Please use the space below to submit your comments.

Enjoy!

PS circle

Check out my Pluralsight courses!

3 Comments

  1.   Jon — September 19, 2014 @ 11:41 am    Reply

    OOP has been murky to me for a very long time, but I am slowly starting to pull back the curtain thanks to succinct and descriptive resources like this. Thank you.

  2.   Subin — October 14, 2014 @ 6:52 am    Reply

    Nicely Explained!!!

  3.   SteveD — November 26, 2014 @ 12:33 am    Reply

    Nice, clear and succinct explanation. Thanks.

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