Abstract Classes and Interfaces

In object-oriented programming, two important concepts are abstract classes and interfaces. These concepts provide a way to define common behaviors or contracts that can be implemented by multiple classes.

Abstract Classes

An abstract class is a class that cannot be instantiated. It is designed to serve as a base or blueprint for concrete classes. Abstract classes can contain both regular methods with implementations and abstract methods without any implementation.

To declare an abstract class in C#, you use the abstract keyword before the class definition. For example:

public abstract class Shape
    public abstract double CalculateArea();
    public void Display()
        Console.WriteLine("Displaying shape...");

In the above example, the Shape class is declared as abstract, and it contains an abstract method CalculateArea() and a regular method Display(). Concrete classes derived from Shape will be required to implement the CalculateArea() method, but they can inherit the Display() method.


An interface, like an abstract class, cannot be instantiated. It represents a contract that defines a set of methods, properties, or events without any implementation details. Interfaces provide a way to achieve multiple inheritances and enable classes to be more flexible by implementing multiple interfaces.

To declare an interface in C#, you use the interface keyword. Here's an example:

public interface IResizable
    void Resize(int width, int height);

public interface IRotatable
    void Rotate(float angle);

In the above example, the IResizable interface declares a method Resize(), and the IRotatable interface declares a method Rotate(). Any class implementing these interfaces will have to provide an implementation for the declared methods.


While both abstract classes and interfaces serve as contracts, they have some differences:

  • An abstract class can have constructors, fields, properties, and methods with or without implementations. Interfaces can only declare methods, properties, events, and indexers.
  • A class can inherit only one abstract class, but it can implement multiple interfaces.
  • Abstract classes are more suitable when creating a family of closely related classes, while interfaces are more useful when designing different unrelated classes with a common behavior.
  • Abstract classes can provide default implementations, but interfaces cannot.

Both abstract classes and interfaces are important tools in C# programming and have their own use cases. Choosing between the two depends on the specific requirements and architecture of your application.


Abstract classes and interfaces are crucial components of object-oriented programming in C#. They allow for the definition of common behaviors and contracts that can be implemented by multiple classes. Understanding the differences and use cases of these concepts will help you design more flexible and maintainable code.

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