Interface Segregation Principle (ISP) in C# — SOLID Made Simple DoonProgramming

What is Interface Segregation Principle (ISP)?

The Interface Segregation Principle (ISP) is the “I” in SOLID principles. It states:

👉 Clients should not be forced to implement interfaces they don’t use.

In simple terms: instead of designing one large fat interface in C#, split it into smaller, role-specific interfaces. This makes your code cleaner, testable, maintainable, and flexible.

🚫 Bad Example — Fat Interface in C#

// Fat interface with too many responsibilities
public interface IAllInOneDevice
{
    void Print(string content);
    void Scan(string content);
    void Fax(string content);
}

// Cheap printer cannot scan or fax, but must implement them
public class CheapPrinter : IAllInOneDevice
{
    public void Print(string content) 
    {
        Console.WriteLine("Printing: " + content);
    }

    public void Scan(string content) 
    {
        throw new NotSupportedException("No scanner available");
    }

    public void Fax(string content) 
    {
        throw new NotSupportedException("No fax available");
    }
}

🔴 Problem: The CheapPrinter is forced to implement methods it doesn’t support. This breaks clean architecture in C# and violates SOLID principles.

✅ Good Example — Interface Segregation in C#

// Smaller, role-specific interfaces
public interface IPrinter
{
    void Print(string content);
}

public interface IScanner
{
    void Scan(string content);
}

public interface IFax
{
    void Fax(string content);
}

// Simple printer implements only what it needs
public class SimplePrinter : IPrinter
{
    public void Print(string content)
    {
        Console.WriteLine("Printing: " + content);
    }
}

// Multi-function device implements multiple roles
public class MultiFunctionPrinter : IPrinter, IScanner, IFax
{
    public void Print(string content) => Console.WriteLine("Printing: " + content);
    public void Scan(string content)  => Console.WriteLine("Scanning: " + content);
    public void Fax(string content)   => Console.WriteLine("Faxing: " + content);
}

✔️ Each class implements only what it actually uses. This is clean code in C# following the SOLID ISP principle.

Another Classic Example — Workers in C#

// ❌ Bad: one fat interface
public interface IWorker
{
    void Work();
    void Eat();
}

public class Robot : IWorker
{
    public void Work() => Console.WriteLine("Robot working...");
    public void Eat() => throw new NotSupportedException("Robots don't eat!");
}

// ✅ Good: split interfaces
public interface IWorkable
{
    void Work();
}

public interface IEatable
{
    void Eat();
}

public class Human : IWorkable, IEatable
{
    public void Work() => Console.WriteLine("Human working...");
    public void Eat() => Console.WriteLine("Human eating...");
}

public class RobotFixed : IWorkable
{
    public void Work() => Console.WriteLine("Robot working...");
}

Why Use ISP in C#? (SEO-rich benefits)

Cleaner code in C# — No unnecessary methods in interfaces.
Better maintainability — Easy to update or extend interfaces without breaking all implementations.
Unit testing made simple — Small interfaces are easier to mock.
Loose coupling in .NET applications — Promotes dependency injection and modular architecture.
Aligns with SOLID principles in C# — Especially useful in enterprise projects and ASP.NET Core clean architecture.

Quick ISP Checklist for C# Developers

✅ Does any class throw NotSupportedException for unused methods? → Split the interface.
✅ Are your interfaces role-based (e.g., ILogger, IRepository, IAuthenticator) instead of god interfaces?
✅ Do your unit tests mock small interfaces, not huge ones?
✅ Would multiple microservices or layers depend only on what they need?

The Interface Segregation Principle in C# (ISP) helps you design small, role-specific interfaces instead of fat ones, ensuring clean code, maintainability, and testability while staying true to the SOLID principles in C#.