Difference Between Abstract Class and Interface in C#

Have you ever wondered about the distinctions between abstract classes and interfaces in C#? Abstract classes offer a partially built structure and can hold state, but you can only inherit from one. Interfaces don't store state and allow a class to adopt multiple sets of behaviours, enhancing flexibility without the limits of single inheritance. Let's understand more!

Abstract classes and interfaces in C# are constructs used for abstraction and polymorphism, but they serve different purposes and are used in different scenarios. In this blog, we will understand the differences between abstract class and interface in C#!

Table of Content

• Difference Between Abstract Class and Interface in C#
• What is Abstract Class in C#?
• What is Interface in C#?

Difference Between Abstract Class and Interface in C#

Below is a table of differences between abstract class and interface in C#

What is Abstract Class in C#?

An abstract class in C# is a special type of class that cannot be instantiated on its own and is designed to be a base class. It provides a common definition of a base class that multiple derived classes can share.

Key Points About Abstract Class in C#

• Cannot Be Instantiated: An abstract class cannot be instantiated directly, meaning you cannot create an object of an abstract class using the new keyword. It's intended to be a base class for other classes.
• Inheritance: Abstract classes are used as a base class for other classes. Classes that derive from an abstract class must implement all abstract methods and properties of the abstract class unless they are also abstract classes.
• Abstract Members: Abstract classes can contain abstract methods and properties. An abstract method or property is declared with the abstract keyword and does not have a body. Instead, it’s meant to be overridden in a derived non-abstract class.
• Non-Abstract Members: In addition to abstract methods and properties, an abstract class can also contain regular methods with full implementations, properties, fields, and events. This allows you to define default behaviour that can be shared among multiple derived classes.
• Purpose: The primary use of an abstract class is to define a common interface for its derivatives. It can enforce a certain structure while also providing shared functionality that derived classes can use or override.
• Polymorphism: Abstract classes support polymorphism. A method defined in an abstract class can be overridden by a derived class. This enables you to call methods of derived classes through a reference of the abstract class type.

What is Interface in C#?

An interface in C# is a reference type that defines a set of function signatures (methods, properties, events, indexers) to enforce a particular contract of behaviour onto implementing classes. Interfaces are a fundamental aspect of C# that enables flexible and loosely-coupled designs. 

Key Points About Interface in C#

• Contract Definition: An interface defines a contract that can be implemented by any class or struct. The contract is a set of public members, like methods, properties, events, or indexers, without any implementation details.
• Implementation: When a class or struct implements an interface, it must provide concrete implementations of all the interface's members. An interface itself contains no implementation for any member.
• Multiple Inheritance: While C# does not support multiple inheritance for classes, it allows a class to implement multiple interfaces. This provides a way to use multiple inheritance-like features.
• Polymorphism: Interfaces enable polymorphic behavior. A class that implements an interface can be accessed using a reference of the interface type, allowing different classes to be used interchangeably if they implement the same interface.
• Abstraction and Loose Coupling: Interfaces are used to define a high level of abstraction and to create loosely-coupled systems. Implementations can be easily switched without affecting consumers that rely on the interface.
• Default Implementation (C# 8.0 and later): Starting with C# 8.0, interfaces can provide default implementations for members. This allows you to add new methods to interfaces with less risk of breaking existing implementations.
• No Constructor or State: Interfaces cannot contain constructors, fields, or any implementation detail. They define only the signature of functionality.
• Public Members Only: All members of an interface are public by default and cannot include any access modifiers.
• No Static Members: Prior to C# 8.0, interfaces could not contain static members. With C# 8.0, static members are allowed in interfaces.

Thus, the differences between abstract classes and interfaces in C# primarily revolve around the way they are used to design software and the type of relationship they establish with the classes that use them. 

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