Both abstract classes and interfaces are used to achieve abstraction in C#.
However, many beginners get confused about when to use which one.

In this lesson, you’ll learn:

• What is an abstract class?
• What is an interface?
• Key differences (easy table)
• When to use abstract class vs interface
• Practical C# examples

Let’s make it simple and clear.

🌟 What is an Abstract Class?

An abstract class is a partially defined class that may contain both implemented and unimplemented methods.

✔ Key points:

• Cannot be instantiated
• Can contain abstract methods (no body)
• Can contain normal methods (with body)
• Can have fields, constructors, and properties
• Child classes must implement abstract methods

🧑‍💻 Example:

public abstract class Animal
{
    public abstract void MakeSound();   // abstract method

    public void Sleep()                 // normal method
    {
        Console.WriteLine("Sleeping...");
    }
}

Child class MUST implement MakeSound().

🌟 What is an Interface?

An interface defines a contract with method signatures only.
Classes or structs that implement an interface MUST provide the method implementation.

✔ Key points:

• 100% abstraction (before C# 8)
• Cannot have fields
• Cannot have constructors
• A class can implement multiple interfaces
• Provides flexibility and loose coupling

🧑‍💻 Example:

public interface IAnimal
{
    void MakeSound();   // no body
}

🧱 Abstract Class Example (Full Implementation)

public abstract class Vehicle
{
    public abstract void Start();  // must be overridden

    public void Stop()
    {
        Console.WriteLine("Vehicle stopped");
    }
}

public class Car : Vehicle
{
    public override void Start()
    {
        Console.WriteLine("Car starting...");
    }
}

Usage:

Vehicle v = new Car();
v.Start();
v.Stop();

🧱 Interface Example (Full Implementation)

public interface IDrive
{
    void Start();
}

public class Bike : IDrive
{
    public void Start()
    {
        Console.WriteLine("Bike starting...");
    }
}

Usage:

IDrive d = new Bike();
d.Start();

🔍 Abstract Class vs Interface — Easy Comparison Table

🎯 When Should You Use an Abstract Class?

Use abstract class when:

✔ You want shared code + abstract methods
✔ Your classes share a strong relationship
✔ You want to define a base class with common logic
✔ You need constructors
✔ You want fields or protected members

Example:

All vehicles have wheels, fuel, engine → use abstract class.

🎯 When Should You Use an Interface?

Use interface when:

✔ You want completely unrelated classes to follow the same contract
✔ You need multiple inheritance
✔ You want to achieve loose coupling
✔ You want to define only behavior (no state)

Example:

• Save() method for saving data
• Start() method for any device
• Log() method in different classes

🌈 Real-World Analogy

✔ Abstract Class → “Family Blueprint”

A shared structure among related members.
Example: Human → Man, Woman, Child.

✔ Interface → “Job Role Contract”

Any person can become a teacher, driver, or player by following the role requirements.

🧠 Practical Example — Payment System

Interface:

public interface IPayment
{
    void Pay(double amount);
}

Abstract class:

public abstract class PaymentBase
{
    public void Validate()
    {
        Console.WriteLine("Validating payment...");
    }

    public abstract void Pay(double amount);
}

Derived class:

public class UpiPayment : PaymentBase, IPayment
{
    public override void Pay(double amount)
    {
        Console.WriteLine($"Paid {amount} via UPI");
    }
}

📝 Mini Exercise

Create:

1️⃣ An abstract class Shape with method Draw() and normal method Info()
2️⃣ An interface IColor with method SetColor()
3️⃣ A class Circle that inherits Shape and implements IColor

Print output.

🔍 FAQs

Q1: Can a class implement multiple interfaces?

Yes.

class Car : IEngine, IBrake { }

Q2: Can a class inherit from multiple abstract classes?

No. C# does not support multiple inheritance.

Q3: Which is faster — interface or abstract class?

Abstract classes are slightly faster, but difference is small.

Q4: Can interfaces contain private members?

No. All interface members are public.

Q5: Can abstract classes contain fields?

Yes.

🎉 Conclusion

You now understand:

✔ What abstract classes are
✔ What interfaces are
✔ Key differences
✔ When to use which
✔ Detailed C# examples
✔ Best practices and analogies

Abstract classes create strong hierarchies, while interfaces provide flexibility and multiple inheritance.

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