Understanding Interfaces in C#: A Complete Guide for Beginners and Professionals DoonProgramming

If you are learning OOP in C# or preparing for C# interview questions, one concept you must understand is the Interface in C#. Don’t worry if it sounds complicated – we’ll break it down step by step, in a simple way, just like a teacher explaining to beginners.

🌟What is an Interface in C#?

Think of an interface like a contract.

Imagine your school teacher saying:

“Every student must bring a pen, notebook, and bag.”

The teacher does not care about the color of the pen or the size of the bag – only that you must have them.

In the same way, an interface in C# defines what a class must do, not how it does it.

👉 In simple words:

Interface = List of rules/methods.
Class = Actual implementation of those rules.

📝 Syntax of Interface in C#

// Defining an Interface
public interface IAnimal
{
    void Speak();   // method without body
    void Eat();
}

// Implementing an Interface in a Class
public class Dog : IAnimal
{
    public void Speak()
    {
        Console.WriteLine("Dog says: Woof!");
    }

    public void Eat()
    {
        Console.WriteLine("Dog is eating.");
    }
}

✔ Here, IAnimal is an interface.
✔ The Dog class must implement all methods (Speak() and Eat()) – otherwise, it will give an error.

🎯 Key Features of Interface in C#

No implementation inside an interface – only method signatures (rules).
Multiple inheritance supported – A class can implement more than one interface.
Cannot create object of interface – because it only defines rules.
Improves flexibility and reusability – different classes can share the same contract.

🚀 Why Use Interface in C#?

To achieve abstraction in OOP in C# (show only essential details).
To support multiple inheritance (something normal classes in C# cannot do).
To make code flexible, testable, and loosely coupled.

🔑 Example with Multiple Interfaces

public interface IAnimal
{
    void Speak();
}

public interface IMovable
{
    void Move();
}

public class Bird : IAnimal, IMovable
{
    public void Speak()
    {
        Console.WriteLine("Bird says: Chirp!");
    }

    public void Move()
    {
        Console.WriteLine("Bird is flying.");
    }
}

✔ The Bird class implements both IAnimal and IMovable.
✔ This is called multiple inheritance in C# using interfaces.

✅ Quick Summary

Interface in C# is like a contract (rules).
Classes that implement the interface must follow all the rules.
Interfaces help achieve abstraction, multiple inheritance, and flexibility in OOP in C#.
Very important concept for C# tutorials and C# interview questions.

🎤 Common C# Interface Interview Questions

1. What is the difference between Abstract Class and Interface in C#?

Answer:

Abstract Class can have both implemented methods (with body) and abstract methods (without body).
Interface can only have method signatures (no body).
A class can inherit only one abstract class, but it can implement multiple interfaces.
Use abstract class when objects share common behavior.
Use interface when you only need rules (without caring about implementation).

2. Can we create an object of an Interface in C#? Why or Why not?