Introduction to Abstract Classes in C#

Introduction to Abstract Classes in C#

• Abstract Classes in C# are created by using Abstract Keyword, and are used to create abstract members, which have their declaration but no implementation (as in C# Interfaces) but unlike Interfaces, Abstract Classes can also have complete methods, properties etc.
• Abstract Methods in C# are also created by using Abstract Keyword, these methods are only declared in Abstract classes and are then implemented in one of its derived classes & the implementation of the Method must have override Keyword in it.

• You must be finding these definitions a bit difficult but no need to worry as we will discuss them one by one in detail.
• The Abstract Modifier / Keyword is used to indicate an incomplete implementation that's why we can't instantiate such objects. We can use it with classes, methods, properties, events and Indexers.
• Moreover, the class inherited from Abstract Class must have the complete implementation of all the members of Abstract Class, along with override keyword in their declaration. (We will discuss it shortly)
• It's not compulsory for the Abstract Class to must have some Abstract members, instead we can add only normal members as well.
• Let's have a look at the syntax of Abstract Class having 1 Abstract Method & 1 normal Method, shown in below figure:

• In the above figure, we have created a public class with abstract keyword/modifier in it named StudentsAbstract, so it's an abstract class.
• As StudentsAbstract is an Abstract Class, so we can create both Abstract (Incomplete) members and normal (complete) members in it.
• So, I have created two methods, printMsg() is an abstract Method as it has Abstract Keyword in its declaration, that's why we haven't placed the implementation of this method.
• While printMsg2() is a normal method so it's fully implemented in Abstract Class.
• These Abstract Classes are normally used as a Parent Class in Inheritance and it's kind of their main role. So, let's have a look at Inheritance in Abstract Classes:

Inheritance in C# Abstract Classes

• As I mentioned earlier, Inheritance is the main role of C# Abstract Classes and they normally play the role of Parent Class and we inherit different classes & structs from them.
• We can't use the sealed keyword with abstract class, as then we can't inherit any other class. ( We have studied sealed keyword in C# Classes Lecture )
• Abstract Class can inherit from other Abstract Classes & C# Interfaces. ( Interfaces can't inherit from Abstract Classes )
• The class inherited from Abstract Class, must have the implementation of all abstract members of its Parent Class, otherwise, we will get a compiler error.

• In the above figure, we can see that I have added a new class StudentsClass derived from our Abstract Class StudentsAbstract.
• Now, this new derived class must have the implementation of its Parent's Abstract Members, as I did for printMsg() Method in above figure.
• Moreover, we need to use override keyword in our derived class method's declaration.
• So, now we have created an Abstract Class and have also added an Abstract Method in it, after that we have derived a new class and provided implementation of our Abstract Method.
• Let's now invoke this Abstract Method, declared in Abstract Class & Implemented in Derived Class, shown in below figure:

• In the Main function, I have simply created a reference variable of Child Class and then invoked both of these methods.
• You must have noticed that Abstract Classes are quite similar to Interfaces but they have few differences as well, let's have a quick look at them:

Abstract Classes Vs. Interfaces

• Abstract Classes & Interfaces have a lot in common as both are used for creating incomplete members i.e. declaration only, yet they have some differences as well.
• We can't create implementation of any member in Interfaces but that's not the case in Abstract Classes, we have the option to create Full members in Abstract Classes.
• Members in Interfaces can't use access modifier and they are all public by default, but in Abstract Classes we can provide access modifier and can make them private etc.
• We can't declare C# Fields in Interfaces but that's possible in Abstract Classes.