Introduction:
Interfaces are one of the most powerful building blocks in Object-Oriented Programming (OOP) and Clean Architecture. Whether you work in ASP.NET Core APIs, Microservices, Desktop (WPF/WinForms) applications, MAUI mobile apps, or enterprise cloud systems, you will interact with interfaces daily — often without even realizing it.
This article explains:
What an interface is
Why interfaces are important
How they are used in real projects
Why Microsoft added default interface implementation in C# 8, even though abstract classes already existed
And how this feature solves real enterprise-scale problems
What Is an Interface?
An interface defines a contract — a set of methods, properties, events, or indexers that a class must implement.
Key Idea:
Interface = Contract
Class = Implementation
An interface defines what a class should do — but not how the work will be done.
The implementation details are always provided by the class that implements the interface.
Why Do We Need Interfaces?
Interfaces solve two major software engineering challenges:
Dependency Management
Without interfaces:
With interfaces:
Code depends on abstractions, not concrete classes
The system becomes scalable, testable, extendable, and loosely coupled
This is the foundation of Dependency Injection (DI) and the Dependency Inversion Principle (SOLID – D).
Example:
public interface ILoggerService
{
void Log(string message);
void Error(string error);
}
Implementation
public class FileLogger : ILoggerService
{
public void Log(string message)
{
Console.WriteLine($"[File Log] {message}");
}
public void Error(string error)
{
Console.WriteLine($"[File Error] {error}");
}
}
Usage in ASP.NET Core DI:
services.AddScoped<ILoggerService, FileLogger>();
public class HomeController : Controller
{
private readonly ILoggerService _logger;
public HomeController(ILoggerService logger)
{
_logger = logger;
}
public IActionResult Index()
{
_logger.Log("Home Page Loaded");
return View();
}
}
✔ The controller does not care which logger is being used
✔ Tomorrow you can replace FileLogger → DatabaseLogger → CloudLogger without changing any controller code.
The Real Problem Before C# 8
Abstract classes already offered:
✔ shared logic
✔ inheritance
✔ structure
…but they failed in one critical scenario:
Updating a shared interface in live enterprise systems broke every application that implemented it
Example:
public interface IPaymentService
{
bool Pay();
}
// One year later…
bool Refund(); // <-- Adding this breaks all implementing classes
If that interface was part of:
A NuGet package
A shared SDK
A microservice contract
…then every consumer application using that interface would fail to compile.
Imagine 500+ production systems across multiple cities depending on the same interface.
Updating all of them overnight = impossible, expensive, high-risk
Microsoft Was Responding to Real-World Industry Needs:
✔ .NET had become the foundation of global enterprise systems
✔ Microservices increasingly shared interface-based contracts
✔ NuGet libraries needed safe evolution
✔ Companies needed gradual migration, not forced rewrites
✔ Backward compatibility became critical
In short — software had to grow without breaking the world around it.
C# 8 Solution: Default Interface Implementation
C# 8 introduced a new capability — interfaces can now contain optional method bodies.
public interface IPaymentService
{
bool Pay();
public bool Refund(decimal amount) // Default implementation
{
return false; // Safe fallback
}
}
What Does This Change Enable?
Benefit Explanation
Existing apps continue working No compile-time errors
New apps may override the method Custom logic only if needed
NuGet packages evolve safely Add new API surface without breaking clients
Migration becomes optional Gradual modernization
Conclusion:
Microsoft introduced default interface implementation because enterprise-scale software needed a way to evolve shared interfaces without breaking dependent applications.
Abstract classes could not solve:
backward compatibility
interface contract evolution
multi-inheritance behaviour
safety across distributed systems
Default interface methods make .NET:
In simple terms — default interface implementation allows software to grow safely, instead of breaking everything when a new feature is needed.