TL;DR

Prefer interfaces over abstract classes in C#.
Interfaces give you flexibility, loose coupling, easier testing, and better team collaboration. Abstract classes are useful in rare cases, shared state or strict algorithm templates, but they often lead to rigid inheritance chains.
Start with interfaces. Reach for abstract classes only when absolutely necessary.

If you’ve been writing C# for a while, you’ve probably faced this choice: should I use an interface or an abstract class? After years of building production systems and working with teams, I’ve learned that interfaces win 95% of the time. They create more flexible, testable code that doesn’t lock your team into rigid inheritance hierarchies.

This isn’t about academic purity, it’s about writing code that actually works in the real world. Code that’s easy to test, extend, and maintain when your requirements change (and they always do).

Why Prefer Interfaces Over Abstract Classes in C#

Let’s start with the practical reasons why interfaces make your life easier as a developer.

Multiple Inheritance Support

C# doesn’t support multiple class inheritance, but it supports multiple interface implementation. This matters more than you might think:

// This works - implementing multiple interfaces
public class EmailService : INotificationService, ILoggable, IDisposable
{
    public void SendNotification(string message) { /* implementation */ }
    public void Log(string message) { /* implementation */ }
    public void Dispose() { /* cleanup */ }
}

// This doesn't work - multiple inheritance from abstract classes
public class EmailService : NotificationBase, LoggableBase // Compile error!
{
}

Team Workflow Benefits

When you use interfaces, team members can work independently on different implementations without stepping on each other’s toes. One developer can build the production implementation while another creates a mock for testing.

Loose Coupling

Interfaces force you to think in terms of contracts, not implementations. This leads to cleaner architecture where your classes depend on abstractions, not concrete types.

// Tight coupling - depends on concrete class
public class OrderService
{
    private readonly EmailNotificationService _emailService; // Bad
    
    public OrderService(EmailNotificationService emailService)
    {
        _emailService = emailService;
    }
}

// Loose coupling - depends on interface
public class OrderService
{
    private readonly INotificationService _notificationService; // Good
    
    public OrderService(INotificationService notificationService)
    {
        _notificationService = notificationService;
    }
}

Flexibility and Testability: The Developer’s Perspective

Easier Mocking

Interfaces play beautifully with mocking frameworks. Here’s why this matters for your daily workflow:

// Testing with interfaces is straightforward
[Test]
public void ProcessOrder_ShouldSendNotification()
{
    // Arrange
    var mockNotificationService = new Mock<INotificationService>();
    var orderService = new OrderService(mockNotificationService.Object);
    
    // Act
    orderService.ProcessOrder(new Order { Id = 1 });
    
    // Assert
    mockNotificationService.Verify(x => x.SendNotification(It.IsAny<string>()), Times.Once);
}

Dependency Injection Container Friendliness

Modern .NET applications rely heavily on dependency injection. Interfaces work seamlessly with DI containers:

// In Program.cs (ASP.NET Core 8)
builder.Services.AddScoped<INotificationService, EmailNotificationService>();
builder.Services.AddScoped<IPaymentProcessor, StripePaymentProcessor>();

You can easily swap implementations without changing any consuming code. Need to switch from email to SMS notifications? Just change the DI registration.

When Abstract Classes Make Sense (The Rare Exceptions)

Abstract classes aren’t inherently bad, but they should be your last resort, not your first choice. In my experience, less than 5% of scenarios actually require abstract classes.

Exception 1: Complex Shared State That Can’t Be Composed

Only consider abstract classes when you have genuinely shared internal state that’s impossible to externalize:

public abstract class StatefulConnection
{
    private ConnectionState _state;
    private readonly object _stateLock = new();
    
    // Complex shared state management that's hard to externalize
    protected bool TryChangeState(ConnectionState newState)
    {
        lock (_stateLock)
        {
            //.....
            return false;
        }
    }
    
    public abstract Task ConnectAsync();
}

Better alternative: Even here, consider a state machine service injected via interface.

Exception 2: Strict Algorithm Templates

Template Method pattern where the algorithm must be enforced:

public abstract class DataProcessor
{
    public void Process(string data) // Non-virtual - algorithm is fixed
    {
        Validate(data);
        Transform(data);
        Save(data);
    }
    
    protected abstract void Validate(string data);
    protected abstract void Transform(string data);
    protected abstract void Save(string data);
}

Reality check: Most “template methods” can be better implemented with composition and interfaces.

The Bottom Line

Before reaching for an abstract class, ask yourself:

  • Can I use composition instead?
  • Can I inject dependencies via interfaces?
  • Am I just being lazy and want to share some code?

In rare cases where you truly need shared internal state or a strict template algorithm, abstract classes are acceptable. But for most real-world systems, composition and interfaces are safer, more flexible choices that won’t lock you into inheritance hierarchies.

Note on Template Method Pattern: If you’re implementing a strict algorithm with variable steps, the Template Method Pattern may require an abstract class. But even this can often be replaced with strategy interfaces for better flexibility and testability. For a deeper comparison, see Template Method vs Strategy Pattern.

Real-World Example: Building a Flexible Logging Pipeline

Let’s compare interface-based composition to abstract class inheritance with a logging system.

// Core logging interface
public interface ILogger
{
    Task LogAsync(LogLevel level, string message, Exception? exception = null);
}

// Formatting interface
public interface ILogFormatter
{
    string Format(LogLevel level, string message, Exception? exception);
}

// Output interface
public interface ILogWriter
{
    Task WriteAsync(string formattedMessage);
}

// Composition-based logger
public class Logger : ILogger
{
    private readonly ILogFormatter _formatter;
    private readonly ILogWriter _writer;
    
    public Logger(ILogFormatter formatter, ILogWriter writer)
    {
        _formatter = formatter;
        _writer = writer;
    }
    
    public async Task LogAsync(LogLevel level, string message, Exception? exception = null)
    {
        var formatted = _formatter.Format(level, message, exception);
        await _writer.WriteAsync(formatted);
    }
}

// Multiple implementations
public class JsonLogFormatter : ILogFormatter
{
    public string Format(LogLevel level, string message, Exception? exception)
    {
        return JsonSerializer.Serialize(new { level, message, exception?.Message });
    }
}

public class ConsoleLogWriter : ILogWriter
{
    public Task WriteAsync(string formattedMessage)
    {
        Console.WriteLine(formattedMessage);
        return Task.CompletedTask;
    }
}

Abstract Class Approach: Why This is Less Flexible

public abstract class LoggerBase
{
    protected abstract string Format(LogLevel level, string message, Exception? exception);
    protected abstract Task WriteAsync(string formattedMessage);
    
    public async Task LogAsync(LogLevel level, string message, Exception? exception = null)
    {
        var formatted = Format(level, message, exception);
        await WriteAsync(formatted);
    }
}

public class JsonConsoleLogger : LoggerBase
{
    protected override string Format(LogLevel level, string message, Exception? exception)
    {
        return JsonSerializer.Serialize(new { level, message, exception?.Message });
    }
    
    protected override Task WriteAsync(string formattedMessage)
    {
        Console.WriteLine(formattedMessage);
        return Task.CompletedTask;
    }
}

Notice how you’re locked into a single inheritance chain. You can’t swap formatters or writers independently, and you can’t easily test components in isolation.

The interface approach wins because:

  • You can mix and match formatters and writers
  • Each component is independently testable
  • You can add new capabilities without inheritance chains
  • Multiple teams can work on different components simultaneously

Testing and Mocking: Interfaces Make It Simple

Here’s how interfaces simplify your testing workflow:

public class OrderService
{
    private readonly IPaymentProcessor _paymentProcessor;
    private readonly INotificationService _notificationService;
    
    public OrderService(IPaymentProcessor paymentProcessor, 
                       INotificationService notificationService)
    {
        _paymentProcessor = paymentProcessor;
        _notificationService = notificationService;
    }
    
    public async Task<bool> ProcessOrderAsync(Order order)
    {
        var paymentResult = await _paymentProcessor.ProcessPaymentAsync(order.Amount);
        
        if (paymentResult.IsSuccess)
        {
            await _notificationService.SendNotificationAsync($"Order {order.Id} processed successfully");
            return true;
        }
        
        return false;
    }
}

// Testing becomes straightforward
[Test]
public async Task ProcessOrder_PaymentSucceeds_SendsNotification()
{
    // Arrange
    var mockPaymentProcessor = new Mock<IPaymentProcessor>();
    var mockNotificationService = new Mock<INotificationService>();
    
    mockPaymentProcessor
        .Setup(x => x.ProcessPaymentAsync(It.IsAny<decimal>()))
        .ReturnsAsync(new PaymentResult { IsSuccess = true });
    
    var orderService = new OrderService(mockPaymentProcessor.Object, 
                                       mockNotificationService.Object);
    
    // Act
    var result = await orderService.ProcessOrderAsync(new Order { Id = 1, Amount = 100 });
    
    // Assert
    Assert.True(result);
    mockNotificationService.Verify(x => x.SendNotificationAsync(It.IsAny<string>()), Times.Once);
}

Common Pitfalls - Overusing Abstract Classes

// Don't do this - rigid inheritance hierarchy
public abstract class NotificationBase { }
public abstract class EmailNotificationBase : NotificationBase { }
public abstract class TransactionalEmailBase : EmailNotificationBase { }
public class OrderConfirmationEmail : TransactionalEmailBase { }

This creates a brittle design where changes ripple through multiple levels.

Tips to avoid overusing abstract classes:

  • Use interfaces for contracts, not behavior
  • Favor composition over inheritance
  • Keep abstract classes for shared state or strict algorithms only

Breaking Changes in Abstract Classes

When you modify an abstract class, you potentially break all inheriting classes. With interfaces, you can use interface versioning or create new interfaces.

Performance Considerations: Do Interfaces Really Perform Better?

Let’s address the performance question quickly: in 99% of real-world scenarios, the performance difference between interfaces and abstract classes is negligible.

Modern .NET JIT optimizations mean that interface method calls are often inlined just like virtual method calls. The flexibility benefits far outweigh any microscopic performance differences.

If you’re building a high-frequency trading system or a game engine, profile first. For typical business applications, choose interfaces for architectural benefits, not performance.

Interface-Driven Clean Architecture

Interfaces align perfectly with Clean Architecture principles:

// Application layer - depends on interfaces
public interface IOrderRepository
{
    Task<Order> GetByIdAsync(int id);
    Task SaveAsync(Order order);
}

public class OrderService
{
    private readonly IOrderRepository _repository;
    
    public OrderService(IOrderRepository repository)
    {
        _repository = repository; // Depends on abstraction
    }
}

// Infrastructure layer - implements interfaces
public class SqlOrderRepository : IOrderRepository
{
    private readonly DbContext _context;
    
    public SqlOrderRepository(DbContext context)
    {
        _context = context;
    }
    
    public async Task<Order> GetByIdAsync(int id)
    {
        return await _context.Orders.FindAsync(id);
    }
    
    public async Task SaveAsync(Order order)
    {
        _context.Orders.Update(order);
        await _context.SaveChangesAsync();
    }
}

This follows the Dependency Inversion Principle: high-level modules (OrderService) don’t depend on low-level modules (SqlOrderRepository). Both depend on abstractions (IOrderRepository).

Key Takeaways: Default to Interfaces

  • Flexibility: Interfaces support multiple inheritance and composition over inheritance
  • Testability: Easy to mock and test in isolation
  • Team Collaboration: Multiple developers can work on different implementations simultaneously
  • Dependency Injection: Work seamlessly with DI containers
  • Loose Coupling: Depend on contracts, not implementations
  • Extensibility: Add new implementations without changing existing code

Use abstract classes only when you need shared state or the Template Method pattern, and even then, consider if composition might work better.

Senior Developer’s Perspective: Why I Default to Interfaces

After building systems for enterprise clients and working with teams of varying skill levels, I’ve learned that interfaces lead to better outcomes. They force you to think about contracts upfront, make testing easier, and prevent the inheritance tangles that plague many codebases.

When a junior developer asks me “interface or abstract class?”, I tell them: “Start with an interface. If you find yourself duplicating code across multiple implementations, then consider composition or, rarely, an abstract class.”

The best codebases I’ve worked on use interfaces extensively and abstract classes sparingly. They’re easier to understand, test, and extend. That’s the kind of code that survives and thrives in production.

Remember: good architecture isn’t about using every feature of the language, it’s about choosing the right tool for the job. Most of the time, that tool is an interface.

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