Difference Between Abstract Class and Interface in C#

When designing applications or starting a programming career, it is necessary to know the difference between Abstract Class and Interface in C#. Both abstract class and interface are used to accomplish abstraction where one can understand the abstract methods.

Though they both can’t be used at the same time. Abstract classes and interfaces may seem similar in a few ways but there are key differences that help to determine which is the best choice for someone to achieve a certain goal related to programming.

One should not ever intend to use an Abstract class directly. This class must have one abstract method at the very least.

And it will be marked by the keyword or modifier abstract in the definition of class. On the other hand, an interface only allows someone to define functionality.

It cannot be implemented. Although a class can develop only one abstract class, it can also take advantage of various interfaces.

Abstract Class vs Interface in C#

The main difference between Abstract Class and Interface in C# is that the “Main” function of abstract class is to explain the real identity of a class whereas the main function of an interface is to explain a class’s outer possibilities. Full abstraction is not provided by an abstract class whereas an interface can provide full abstraction.

Abstract Class vs Interface in C

Multiple inheritance is not possible in an abstract class whereas an interface can make multiple inheritance as possible.

Comparison Table Between Abstract Class and Interface in C#

Parameter of ComparisonAbstract ClassInterface in C#
FunctionalityAdding a new method to an abstract class will give the option of providing default implementation and thus all the current code will work properlyAdding a new method to an interface will allow to track down all the implementations of the interface and define implementation for the recent method
Various inheritanceOnly one abstract class might be inherited by a classSeveral interfaces might be inherited by a class
What they contain?Abstract class contains Data Member and ConstructorsAn interface in C# doesn’t contain Data Member and Constructors
ProvideAn abstract class can provide full, default code and/or the details that should be overrideAn interface in C# is not capable of providing any code, just the signature
Rate of motionAn abstract class is fast when it comes to speedAn interface in C# needs more time to get the real method in the corresponding classes

What is Abstract Class in C#?

An abstract class is something that can be intended only to be a base class of other classes. They are classes that have one or more abstract methods.

An abstract method is known as a method that is announced, but it doesn’t contain any implementation. To create a class, we need to use an ‘abstract’ modifier.

This modifier indicates that there might be few missing implementations that will require to put into action in the class taken from it. Both abstract and non-abstract members can be contained by an abstract class.

This abstract class should have one abstract method at the very least which has already been discussed above. Without the method, we cannot declare that class as an “abstract” one.

Sometimes abstract classes might not be instantiated, and they need subclasses to provide implementations for the abstract methods.

If we are creating an abstract class then we are creating a base class that might contain one or more completed methods but we will at least find one or more methods that are left uncompleted and declared abstract.

If all the methods of an abstract class stay uncompleted then it will just like be an interface.

To deliver a definition of a base class for how a set of derived classes will work and then let the programmers fill the implementation in the derived classes is the actual purpose of an abstract class. We can also say that the purpose of an abstract class is to provide a base class definition.

abstract class

What is Interface in C#?

On the other hand, an interface is not a class that has only an indication of functionality. We can call it a pattern with no implementation.

Theoretically, it is only a definition of methods that has the declaration of members. We can think of it as an empty shell that does not contain the implementation of its members.

The interface is like an abstract base class that will only have abstract members such as methods, indexers, events, properties, etc. Instantiating it straightly is not possible and its members can be implemented by any class.

Also, various interfaces can be implemented by a class, nonetheless, a class can only obtain a single class. In other words, we can say that an interface has only the declaration of members and it doesn’t contain the implementation of these members.

A class implements the interface, and it must deliver the definition of methods stated in the interface. Meaning an interface brings a standard structure that the derived class will require to follow.


Main Differences Between Abstract Class and Interface in C#

  1. An abstract class is known as a certain kind of class which may contain an explanation with no implementation. On the other hand, an interface is just a pattern that is not capable of doing anything. So technically it is just a shell, which is empty.
  2. A class can use one abstract class only, therefore more than one inheritance is not supported. On the other hand, an inheritance can support various inheritance, meaning a class can inherit any number of inheritances.
  3. An abstract class can contain a few access modifiers such as subs, functions, properties, etc. While an interface is not allowed to contain access modifiers and all methods have to be inevitably defined as public.
  4. An abstract class tends to act as a base class for all other classes so it is capable of declaring or using any variable while an interface is not allowed to declare any.
  5. An abstract class tends to use itself to define the main identity of a class and can be used for objects of a similar type of date. And an interface is used to define the peripheral ability of a class.

Frequently Asked Questions (FAQ) About Abstract Class and Interface in C#

What is the use of abstract class in C#?

C# is a general-purpose object-oriented programming language. It is from Microsoft.

The main aim of C# is to use the computing power of C++ with the programming ease of Virtual Basic.

It is mainly based on C++ and also contains similar features as Java. An abstract class is a way to achieve abstraction in C# and it cannot be directly instantiated.

The function of the abstract class is to create some functionality or create a common behavior that subclasses can inherit, implement or override.

It basically provides a blueprint to the derived classes and gives instructions on what to implement by derived classes when an abstract class is inherited by them.

What is interface in C# and why it is used?

An interface in C# can be defined as a type definition like a class. But it only provides declarations and is inherited by classes and structs.

It is used with a class to define a contract (an agreement on what is provided by the class to an application). It only declares the methods and different properties and a class will dictate what the method will actually do.

It cannot be instantiated as an object and is only used as a collection of methods and property declarations. It defines what operations can be or will be performed by a class.

What is polymorphism in C#?

Polymorphism is usually expressed as one interface and multiple functions. Polymorphism in C# means providing the ability to take multiple forms.

After encapsulation and inheritance, it is another pillar concept of object-oriented programming. C# provides the ability to classes of implementing different methods which are usually called with the same name.

There are two types of polymorphisms in C#:

Compile Time Polymorphism – It means defining multiple methods with different parameters but with the same name. It enables us to use the same method name to perform various tasks only by passing different parameters.

Run Time Polymorphism – It means creating a similar function in the derived class to override a base class method. In this method, we can create a method with the same name and parameters to perform different tasks.


To understand the difference between the two is quite a difficult task. We can combine all the information we can find and still it won’t be enough.

Theoretically speaking, both of them are the most important terms in programming and are pretty similar, yet in terms of functionality, they differ a lot.

While an abstract class is a certain type of class which will work as a base for other classes, an interface will just be an empty shell with only member declarations.

Key Differences Between Abstract Class and Interface in C# (PDF Format)

Download the key differences in .PDF format, to read or print them later:

Difference Between Abstract Class and Interface in C#


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