Interface vs abstract class decision in C Sharp (C#) - Quick Revision & Key Differences

Recall & Review

beginner

What is an interface in C#?

An interface is a contract that defines a set of methods and properties without implementation. Classes or structs that implement the interface must provide the implementation.

beginner

What is an abstract class in C#?

An abstract class can have both implemented and unimplemented methods. It cannot be instantiated directly and is meant to be a base class for other classes.

intermediate

When should you choose an interface over an abstract class?

Choose an interface when you want to define a contract for unrelated classes or when multiple inheritance of behavior is needed, since a class can implement multiple interfaces.

intermediate

When is an abstract class a better choice than an interface?

Use an abstract class when you want to share common code among closely related classes and provide some default behavior along with abstract methods.

beginner

Can a class implement multiple interfaces or inherit multiple abstract classes in C#?

A class can implement multiple interfaces but can inherit only one abstract class.

Which of the following can a C# class inherit from multiple times?

A Concrete classes

B Abstract classes

C Interfaces

D Structs

In C#, a class can implement multiple interfaces but can inherit only one abstract or concrete class.

What is a key difference between an interface and an abstract class?

A Abstract classes can have method implementations, interfaces cannot

B Interfaces can have method implementations, abstract classes cannot

C Interfaces can be instantiated, abstract classes cannot

D Abstract classes cannot have fields, interfaces can

Abstract classes can have implemented methods and fields, while interfaces cannot have fields and traditionally had no implementations (though default interface methods exist in newer C# versions).

When would you prefer an abstract class over an interface?

A When you want to share common code among related classes

B When you want multiple inheritance of behavior

C When you want to define a contract without implementation

D When you want to implement unrelated behaviors

Abstract classes allow sharing common code and default behavior among related classes.

Can interfaces contain fields in C#?

A Yes, interfaces can have fields

B Only private fields are allowed in interfaces

C Only static fields are allowed in interfaces

D No, interfaces cannot have fields

Interfaces cannot contain fields; they only declare methods, properties, events, or indexers.

Which statement is true about abstract classes?

A They can be instantiated directly

B They can contain both abstract and concrete methods

C They must implement all interface methods

D They cannot have constructors

Abstract classes can have both abstract methods (without implementation) and concrete methods (with implementation).

Explain the main differences between interfaces and abstract classes in C# and when to use each.

Think about code sharing and inheritance rules.

Describe a real-life scenario where choosing an interface is better than an abstract class.

Consider when different objects share capabilities but not implementation.