Why Composition over Inheritance Is More Than Just a Design Principle

In many codebases, inheritance is still the default tool for reusing behavior. But in real-world, evolving systems, inheritance often brings more rigidity than flexibility. Composition offers a significantly more adaptable and maintainable alternative. Where Inheritance Falls Short Inheritance implies an “is-a” relationship between objects. While this seems clean in theory, it quickly leads to practical problems: Rigid hierarchies: Behavior is locked into base classes and difficult to adjust without side effects. Tight coupling: Subclasses depend heavily on base classes. A small change in the base can ripple through the system. Low reusability: Behavior is tied to a particular class hierarchy and hard to extract or share. Poor maintainability: Deep inheritance trees obscure behavior and make debugging harder. Composition as a Practical Solution With composition, behavior is not inherited but injected at runtime. This approach brings several concrete benefits: Modularity: Components are clearly separated and can be reused independently. Loose coupling: Classes depend on abstractions (e.g., interfaces), not implementations. Flexibility: Behaviors can be combined or swapped dynamically. Testability: Components are easily mocked or isolated in unit tests. Example: Multi-Channel Notification Service in .NET Instead of building an inheritance chain like EmailNotificationService, SmsNotificationService, and MultiChannelNotificationService, we model each channel as a behavior and inject them via composition: public interface INotificationChannel { void Send(string message); } public class EmailChannel : INotificationChannel { public void Send(string message) => Console.WriteLine($"Sending Email: {message}"); } public class SmsChannel : INotificationChannel { public void Send(string message) => Console.WriteLine($"Sending SMS: {message}"); } public class NotificationService { private readonly IEnumerable _channels; public NotificationService(IEnumerable channels) { _channels = channels; } public void Notify(string message) { foreach (var channel in _channels) channel.Send(message); } } // Usage var service = new NotificationService(new[] { new EmailChannel(), new SmsChannel() }); service.Notify("System update available."); This setup allows you to mix and match any number of channels without subclassing, without fragile overrides, and without bloating your class hierarchy. Conclusion Composition isn’t just a theoretical ideal — it’s a practical, robust tool. Especially in modern .NET applications, where interfaces, dependency injection, and modular design are the norm, composition often leads to simpler, more maintainable code. Inheritance still has its use cases. But when designing for flexibility, reusability, and testability, think in components — not class trees.

Jun 21, 2025 - 13:20

Why Composition over Inheritance Is More Than Just a Design Principle

In many codebases, inheritance is still the default tool for reusing behavior. But in real-world, evolving systems, inheritance often brings more rigidity than flexibility. Composition offers a significantly more adaptable and maintainable alternative.

Where Inheritance Falls Short

Inheritance implies an “is-a” relationship between objects. While this seems clean in theory, it quickly leads to practical problems:

Rigid hierarchies: Behavior is locked into base classes and difficult to adjust without side effects.
Tight coupling: Subclasses depend heavily on base classes. A small change in the base can ripple through the system.
Low reusability: Behavior is tied to a particular class hierarchy and hard to extract or share.
Poor maintainability: Deep inheritance trees obscure behavior and make debugging harder.

Composition as a Practical Solution

With composition, behavior is not inherited but injected at runtime. This approach brings several concrete benefits:

Modularity: Components are clearly separated and can be reused independently.
Loose coupling: Classes depend on abstractions (e.g., interfaces), not implementations.
Flexibility: Behaviors can be combined or swapped dynamically.
Testability: Components are easily mocked or isolated in unit tests.

Example: Multi-Channel Notification Service in .NET

Instead of building an inheritance chain like EmailNotificationService, SmsNotificationService, and MultiChannelNotificationService, we model each channel as a behavior and inject them via composition:


public interface INotificationChannel
{
    void Send(string message);
}

public class EmailChannel : INotificationChannel
{
    public void Send(string message) =>
        Console.WriteLine($"Sending Email: {message}");
}

public class SmsChannel : INotificationChannel
{
    public void Send(string message) =>
        Console.WriteLine($"Sending SMS: {message}");
}

public class NotificationService
{
    private readonly IEnumerable<INotificationChannel> _channels;

    public NotificationService(IEnumerable<INotificationChannel> channels)
    {
        _channels = channels;
    }

    public void Notify(string message)
    {
        foreach (var channel in _channels)
            channel.Send(message);
    }
}

// Usage
var service = new NotificationService(new[]
{
    new EmailChannel(),
    new SmsChannel()
});

service.Notify("System update available.");

This setup allows you to mix and match any number of channels without subclassing, without fragile overrides, and without bloating your class hierarchy.

Conclusion

Composition isn’t just a theoretical ideal — it’s a practical, robust tool. Especially in modern .NET applications, where interfaces, dependency injection, and modular design are the norm, composition often leads to simpler, more maintainable code.

Inheritance still has its use cases. But when designing for flexibility, reusability, and testability, think in components — not class trees.