Abstract Class vs Interface in C#: Difference and Comparison

In C#, an abstract class can provide a partial implementation of methods, fields, and properties, while an interface defines a contract of methods and properties that implementing classes must provide without any implementation details.

Key Takeaways

1. Abstract class is a class that cannot be instantiated and can have both abstract and non-abstract methods.
2. Interface is a blueprint for a class and can have only abstract methods.
3. A class can inherit only one abstract class but can implement multiple interfaces.

Abstract Class vs Interface in C#

In speed, Abstract class is quicker compared to Interface in C#. in the former, you can define fields, whereas in Interface you can’t. A single abstract class can extend only one interface, while one Interface can extend multiple interfaces. The former contains Data Members, while the latter doesn’t.

Multiple inheritances are not possible in an abstract class, whereas an interface can make multiple inheritances as possible.

Comparison Table

Feature	Abstract Class	Interface
Definition	A class with abstract methods that provide partial implementation and force subclasses to implement the remaining methods	A contract with only method and property declarations but no implementation
Purpose	Used to define a common base for subclasses and enforce specific functionality	Used to define a contract that different classes can implement, promoting code reuse and flexibility
Implementation	Can have fields, properties, constructors, abstract methods, and concrete methods	Can only have properties, methods, and events
Inheritance	A class can only inherit from one abstract class	A class can implement multiple interfaces
Access Modifiers	Abstract methods can have any access modifier (public, protected, private)	Methods and properties in interfaces are implicitly public
Instantiability	Abstract classes cannot be directly instantiated	Interfaces cannot be directly instantiated
Abstraction Level	Can provide a more complete implementation than an interface	Provides a less specific contract than an abstract class
Example	Animal (abstract class) with abstract methods Move() and Eat()	IShape (interface) with methods CalculateArea() and CalculatePerimeter()

What is Abstract Class in C#?

An abstract class in C# is a partially implemented class that serves as a blueprint for subclasses to inherit from. It defines a common base for related classes and enforces specific functionality through abstract methods, while allowing subclasses to provide their own implementations for other functionalities.

Here’s a breakdown of its key characteristics:

Definition:

• A class designed to be inherited from by other classes.
• Contains at least one abstract method with no implementation.
• Provides a common base and partially implemented functionality for subclasses.

Purpose:

• Enforce specific functionalities and behaviors in subclasses.
• Promote code reuse by providing a common base for related classes.
• Encourage code organization and structure.
• Improve code readability and maintainability.

Key Features:

• Can have fields, properties, constructors, concrete methods (with implementation), and abstract methods (without implementation).
• Abstract methods define the functionality that subclasses must implement.
• Subclasses can inherit from only one abstract class.
• Abstract classes cannot be directly instantiated.

Benefits:

• Promotes code reuse and reduces code duplication.
• Enforces consistent behavior and functionality across related classes.
• Improves code organization and maintainability.
• Provides a clear separation between abstract and concrete functionality.

Examples:

• Animal (abstract class) with abstract methods Move() and Eat().
• Shape (abstract class) with abstract methods CalculateArea() and CalculatePerimeter().

When to use an abstract class:

• When you must define a common base for a group of related classes.
• When you want to enforce specific functionality in subclasses.
• When you want to encourage code reuse and reduce code duplication.
• When you want to separate the abstract concept from its concrete implementations.

What is Interface in C#?

In C#, an interface is a powerful tool for promoting code reuse and abstraction. It defines a contract that specifies the expected behavior and functionalities of a class, but does not provide any implementation details.

Here’s a breakdown of its key features:

Definition:

• A blueprint that defines a set of methods, properties, and events without providing their implementation.
• Acts as a contract that classes can implement to guarantee specific functionalities.
• Promotes loose coupling between classes, enabling them to collaborate without relying on their internal implementations.

Purpose:

• Promote code reuse and improve maintainability by defining a common set of functionalities that different classes can implement.
• Decouple the implementation details of different classes, making them more independent and flexible.
• Provide a clear and concise specification of the expected behavior of a class.
• Enable different classes to implement the same functionality in different ways.

Key Features:

• Can only have methods, properties, and events.
• Methods and properties have no implementation details, forcing classes that implement the interface to provide their own implementations.
• Events allow classes to communicate with each other.
• Interfaces are implicitly public, meaning their members are accessible to other classes.
• A class can implement multiple interfaces.
• Interfaces cannot be directly instantiated.

Benefits:

• Promotes code reuse and reduces code duplication.
• Improves code flexibility and maintainability.
• Encourages loose coupling and better separation of concerns.
• Makes code more readable and understandable.

Examples:

• IShape (interface) with methods CalculateArea() and CalculatePerimeter().
• IComparable (interface) with method CompareTo().
• ICloneable (interface) with method Clone().

When to use an interface:

• When you want to define a set of functionalities that different classes can implement in different ways.
• When you want to promote loose coupling and flexibility between classes.
• When you want to encourage code reuse and reduce code duplication.
• When you want to create a public contract that specifies the expected behavior of a class.

Main Differences Between Abstract Class and Interface in C#

1. Implementation:
   • Abstract Class: An abstract class can partially or completely implement methods, fields, and properties.
   • Interface: An interface defines a contract of methods and properties that implementing classes must provide but does not contain any implementation details.
2. Inheritance:
   • Abstract Class: A class can inherit from only one abstract class using single inheritance in C#.
   • Interface: A class can implement multiple interfaces, allowing for multiple interface inheritance.
3. Constructors:
   • Abstract Class: An abstract class can have constructors, which can be used to initialize the state of the class.
   • Interface: Interfaces cannot have constructors because they do not define state or instance-specific behavior.
4. Access Modifiers:
   • Abstract Class: Abstract classes can have access modifiers for methods, fields, and properties, allowing for more control over visibility and accessibility.
   • Interface: All members of an interface are implicitly public and cannot have access modifiers.
5. Fields:
   • Abstract Class: Abstract classes can have fields, which can store and manage data within the class.
   • Interface: Interfaces cannot have fields; they only define method and property signatures.
6. Multiple Inheritance:
   • Abstract Class: While a class can inherit from a single abstract class, it can also implement multiple interfaces, allowing for a limited form of multiple inheritance.
   • Interface: Interfaces are used to achieve multiple inheritance by enabling a class to implement multiple interfaces.
7. Use Cases:
   • Abstract Class: Abstract classes are suitable when you want to provide a common base with shared implementation for related classes, and when you expect classes to have some common behavior.
   • Interface: Interfaces define a contract for classes that may not have a common base but need to adhere to a specific set of behaviors or functionality.

References

1. https://csharp.net-tutorials.com/classes/abstract-classes/
2. https://dl.acm.org/doi/abs/10.1145/512035.512041