Abstract Class vs Interface in C#: Difference and Comparison

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

Though they both can’t be used simultaneously, abstract classes and interfaces may seem similar in a few ways. Still, key differences help determine the best choice for someone to achieve a particular 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 class definition. 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 use various interfaces.

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

Parameter of Comparison	Abstract Class	Interface in C#
Functionality	Adding a new method to an abstract class will give the option of providing default implementation, and thus all the current code will work properly.	Adding a new method to an interface will allow tracking down all the interface implementations and defining implementation for the recent method.
Various inheritance	A class might inherit only one abstract class	A class might inherit several interfaces
What do they contain?	Abstract class contains Data Member and Constructors	An interface in C# doesn’t contain Data Members and Constructors
Provide	An abstract class can provide complete default code and the details that should be override	An interface in C# is not capable of providing any code, just the signature
Rate of motion	An abstract class is fast when it comes to speed	An interface in C# needs more time to get the real method in the corresponding classes.

What is Abstract Class in C#?

An abstract class can be intended only as 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 doesn’t contain any implementation. We need to use an ‘abstract’ modifier to create a class.

This modifier indicates that there might be a few missing implementations that will require to be put into action in the class taken from it. An abstract class can contain both abstract and non-abstract members.

This abstract class should have one abstract method, 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 be just like an interface.

The actual purpose of an abstract class is 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. We can also say that the purpose of an abstract class is to provide a base class definition.

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 with 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 with only 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 doesn’t contain the implementation of these members.

A class implements the interface and must deliver the definition of methods stated in the interface. An interface brings a standard structure the derived class will require to follow.

Main Differences Between Abstract Class and Interface in C#

1. An abstract class is a certain kind of class that may contain an explanation with no implementation. On the other hand, an interface is just a pattern that cannot do 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 inheritances, 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 cannot contain access modifiers, all methods must inevitably be defined as public.
4. An abstract class tends to act as a base class for all other classes, so it can declare or use any variable, while an interface is not allowed to declare any.
5. An abstract class tends to use itself to define the primary 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.

References

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