In this section of the course, we have covered several key concepts in object-oriented programming (OOP), including classes, objects, and inheritance. However, one powerful concept that we did not explicitly cover in the video lectures is Abstract Classes and Methods. To ensure a focused learning experience, we have provided this written explanation so you can study this concept in detail at your own pace.
This article will introduce Abstract Classes and Methods, explain their purpose, draw an analogy for better understanding, and walk through their implementation with clear examples. By the end, you’ll see where and why to use them in real-world applications.
An abstract class is a class that cannot be instantiated directly. Instead, it serves as a blueprint for other classes. It is meant to be inherited by other classes that provide specific implementations for its abstract methods.
An abstract method is a method that is declared but not defined in the abstract class. It must be overridden in a derived class.
This allows developers to enforce a structure and behavior across multiple related classes while allowing for flexibility in implementation.
Think of an abstract class as an architectural blueprint for a house. The blueprint itself cannot be lived in, but it provides a general structure that builders must follow.
The blueprint might specify that the house must have a kitchen, bedroom, and bathroom (abstract methods).
However, the blueprint does not dictate exactly how each room should look—that is left to the builder (the derived class) to decide.
Similarly, an abstract class defines a general structure for other classes to follow, while the actual implementation details are provided by the subclasses.
To create an abstract class, use the abstract keyword before the class keyword.
To declare an abstract method, use the abstract keyword within the abstract class, but do not provide a body (no curly braces {} or method logic).
// Abstract class - cannot be instantiated
abstract class Animal
{
// Abstract method - must be implemented in derived classes
public abstract void MakeSound();
// Non-abstract method - can have a default implementation
public void Sleep()
{
Console.WriteLine("Sleeping...");
}
}
// Concrete class that inherits from Animal
class Dog : Animal
{
// Providing implementation for abstract method
public override void MakeSound()
{
Console.WriteLine("Woof! Woof!");
}
}
class Program
{
static void Main()
{
// Animal animal = new Animal(); // ERROR: Cannot instantiate abstract class
Dog myDog = new Dog();
myDog.MakeSound(); // Output: Woof! Woof!
myDog.Sleep(); // Output: Sleeping...
}
}
Creating an Abstract Class (Animal)
Declared with abstract class Animal.
Contains an abstract method MakeSound(), which has no body.
Includes a regular method Sleep(), which has a body.
Creating a Derived Class (Dog)
Inherits from Animal using class Dog : Animal.
Implements the MakeSound() method with actual functionality (Woof! Woof!).
Instantiating and Using the Class
Animal cannot be instantiated directly.
Dog can be instantiated, and it provides its own implementation for MakeSound().
Sleep() remains unchanged and is inherited directly.
Abstract classes and interfaces are both used to define a contract for other classes, but they have key differences:
FeatureAbstract ClassInterfaceCan have method implementations?✅ Yes (regular methods allowed)❌ No (before C# 8.0)Can have instance fields (variables)?✅ Yes❌ NoCan have constructors?✅ Yes❌ NoCan a class inherit multiple?❌ No, only one abstract class✅ Yes, multiple interfaces allowedIs it used for a “blueprint”?✅ Yes, provides partial implementation✅ Yes, but only method declarations
✅ Use an abstract class when:
You need to provide default behavior for some methods but require specific implementation for others.
You want to share common fields and methods among all subclasses.
You need to enforce a structure in a set of related classes.
Example: A Vehicle class that provides a common framework for cars, motorcycles, and trucks.
abstract class Vehicle
{
public int Speed { get; set; }
public abstract void Move();
}
class Car : Vehicle
{
public override void Move()
{
Console.WriteLine("The car is driving.");
}
}
✅ Use an interface when:
You need to define only method signatures without any implementation.
You need multiple inheritance (since a class can inherit multiple interfaces).
You want to enforce a contract across completely unrelated classes.
✔️ Use abstract classes for closely related objects (e.g., Animal, Vehicle).
✔️ Provide common functionality in the abstract class to avoid code duplication.
✔️ Use abstract methods only when absolutely necessary, as they require subclasses to implement them.
❌ Trying to instantiate an abstract class → new AbstractClass() will result in an error.
❌ Forgetting to override an abstract method → A subclass must implement all abstract methods.
❌ Using an abstract class when an interface is more appropriate → If no method implementations are needed, an interface is a better choice.
Abstract classes and methods are essential tools in object-oriented programming. They allow developers to create a blueprint for related classes, enforcing a structure while still allowing flexibility in implementation.
If you have any questions, feel free to ask in the Q&A.
Happy coding! 🚀