Difference Between Abstract Class and Interface in C#

In object-oriented programming (OOP), two essential concepts often come up: abstract classes and interfaces. Both play pivotal roles in shaping the architecture of software applications. However, they serve distinct purposes and have unique characteristics. In this article, I’ll go deep into the “difference between abstract class and interface in C#” and provide real-life examples to clarify their usage.

What is an Abstract Class?

An abstract class in C# is a class that cannot be instantiated on its own. It acts as a base class for other classes, allowing you to define methods, properties, and fields that must be implemented by derived classes. The keyword abstract is used to define such a class.

public abstract class Animal
{
    public abstract void Speak();
}

In the example above, the Animal class is abstract and has an abstract method Speak(). Any class that inherits from Animal must provide an implementation for this method.

What is an Interface?

An interface in C# is a contract that defines a set of methods, properties, and events without any implementation. Classes and structs that implement an interface must provide an implementation for all its members. Interfaces are defined using the interface keyword.

public interface IFlyable
{
    void Fly();
}

Here, the IFlyable interface has a method Fly(). Any class or struct that implements this interface must provide an implementation for the Fly() method.

Key Differences Between Abstract Class and Interface

1. Nature: An abstract class is still a class, albeit one that cannot be instantiated. An interface, on the other hand, is a completely separate entity that represents a contract.
2. Inheritance: In C#, a class can inherit from only one abstract class but can implement multiple interfaces. This is because C# doesn’t support multiple inheritance for classes.
3. Access Modifiers: Abstract classes can have access modifiers like public, protected, and private for their members. Interfaces cannot have access modifiers; all their members are implicitly public.
4. Fields & Implementation: Abstract classes can have fields and can provide a default implementation for some members. Interfaces cannot have fields and cannot provide any implementation.
5. Constructors: Abstract classes can have constructors, while interfaces cannot.

Real-life Example

Imagine you’re building a game with various characters. Some characters can fly, while others can swim.

Using an abstract class:

public abstract class Character
{
    public abstract void Move();
}

public class Bird : Character
{
    public override void Move()
    {
        // Implementation for flying
    }
}

Using interfaces:

public interface IMovable
{
    void Move();
}

public class Fish : IMovable
{
    public void Move()
    {
        // Implementation for swimming
    }
}

In the examples above, using an abstract class makes sense when there’s a general concept of a “Character” that has a movement method. However, when specific actions like flying or swimming are needed, interfaces like IMovable offer more flexibility.

Conclusion

Understanding the difference between abstract class and interface in C# is crucial for designing robust and flexible software architectures. While both serve the purpose of enforcing certain behaviors in derived classes, they do so in distinct ways. Abstract classes offer a foundation with optional default implementations, while interfaces provide pure contracts that can be implemented by multiple classes. By leveraging both appropriately, developers can ensure code reusability, clarity, and maintainability.