Interfaces, Default Members & Dependency Inversion Masterclass

⚡ C# 12 & .NET 8 🟢 Chapter 18 of 35 📂 Phase 6: Methods & OOP 📅 2026 Edition

📌 Covered in this chapter: Interface Contracts · Multiple Interfaces · Default Interface Members · Explicit Implementation · Dependency Inversion · Interface vs Abstract Class

Welcome to Phase 6 (Chapter 18): C# Interfaces, Default Members & Dependency Inversion Masterclass! An Interface defines a contract that any implementing class or struct must fulfill. Interfaces enable multiple inheritance of behavior, decoupling, unit test mocking, and dependency inversion in enterprise application design.

1Interface Definition & Implementation

C# — Interface Contract Example ▶ Run in Compiler

interface IPayment
{
    void Pay(decimal amount);
}

class CardPayment : IPayment
{
    public void Pay(decimal amount)
    {
        Console.WriteLine($"Paid: {amount:C} via Credit Card");
    }
}

class UpiPayment : IPayment
{
    public void Pay(decimal amount)
    {
        Console.WriteLine($"Paid: {amount:C} via UPI Transfer");
    }
}

IPayment payment = new CardPayment();
payment.Pay(100.50m);

payment = new UpiPayment();
payment.Pay(250.00m);

2Multiple Interfaces & Dependency Inversion

C# — Multiple Interfaces ▶ Run in Compiler

interface IPrintable { void Print(); }
interface IStorable  { void Save(); }

class Document : IPrintable, IStorable // Multiple interface implementation
{
    public void Print() => Console.WriteLine("Printing Document...");
    public void Save()  => Console.WriteLine("Saving Document to Disk...");
}

Document doc = new();
doc.Print();
doc.Save();

3Technical FAQs

Q1: Why use Interfaces instead of Abstract Classes?

A class can inherit from only ONE abstract class (single class inheritance), but can implement MULTIPLE interfaces (multiple implementation).

Q2: What is Dependency Inversion?

Dependency Inversion is a software design principle where high-level modules depend on abstractions (Interfaces) rather than concrete implementations, promoting loose coupling and testability.