Abstract Class VS. Interface in C#

By: Rajesh P.S.

Abstract Class and Interface are both important concepts in the .NET Framework, but they have distinct characteristics and usage scenarios. Let's investigate into their differences in more detail:

Abstract Class

An abstract class is a class that cannot be instantiated and serves as a blueprint for deriving subclasses. It may contain abstract methods (without implementation) and/or concrete methods (with implementation).

Interface

An interface is a contract that defines a set of members (methods, properties, events) that a class must implement. It provides a way to achieve multiple inheritance-like behavior in .NET.

Instantiation

Abstract Class: Abstract classes cannot be directly instantiated using the new keyword. They are meant to be subclassed and serve as a base for derived classes.
Interface: Interfaces cannot be instantiated on their own. They define a set of members that implementing classes must provide.

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Usage

Abstract Class

Abstract classes are used when you want to provide a common base implementation for a group of related classes. They are suitable for creating class hierarchies where some common behavior and characteristics are shared among the derived classes.

public abstract class Shape { public abstract double GetArea(); } public class Circle : Shape { public override double GetArea() { // Implementation for calculating the area of a circle } }

Interface

Interfaces are used when you want to define a contract that multiple unrelated classes can adhere to. They provide a way to achieve polymorphism by allowing different classes to be treated interchangeably based on their shared interface.

public interface ILoggable { void Log(); } public class FileLogger : ILoggable { public void Log() { // Implementation for logging to a file } } public class DatabaseLogger : ILoggable { public void Log() { // Implementation for logging to a database } }

Inheritance

Abstract Class: A class can inherit from only one abstract class using the : baseClass syntax. Inheritance from an abstract class allows the subclass to inherit the properties, methods, and implementation of the abstract class.
Interface: A class can implement multiple interfaces using the : interface1, interface2 syntax. Implementing an interface means the class must provide an implementation for all the members defined in the interface.

Members

Abstract Class: Abstract classes can have fields, properties, constructors, and both abstract and non-abstract methods. They can also provide default implementations for methods.
Interface: Interfaces can define properties, methods, events, and indexers. All members declared in an interface are implicitly public and must be implemented by the implementing class.

Flexibility

Abstract Class: Abstract classes can have both abstract and non-abstract methods. This allows them to provide default implementations for common behavior while also allowing derived classes to override and extend certain methods as needed.
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Interface: Interfaces only define member signatures without any implementation details. Implementing classes have the flexibility to define their own unique implementations for each member. It's important to note that while a class can inherit from an abstract class and implement multiple interfaces, it cannot inherit from multiple abstract classes.

Conclusion

Abstract classes are used to create class hierarchies and provide common functionality among related classes, while interfaces define contracts that unrelated classes can adhere to and enable polymorphism. Understanding their distinctions is crucial when designing class structures and determining the appropriate approach for achieving code reusability and abstraction in the .NET Framework.

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