Interface Segregation Principle in C# Explained with Examples

Interface Segregation Principle (ISP) is one of the most important, yet commonly violated, SOLID principles in real-world C# and .NET applications. In particular, it focuses on reducing unnecessary dependencies between components.

It clearly states that:

Clients should not be forced to depend on interfaces they do not use.

In practice, ISP encourages developers to create small and focused interfaces instead of large, “fat” ones. As a result, this approach produces cleaner code, reduces breaking changes, and improves maintainability. Moreover, these benefits become especially valuable in enterprise-scale applications.

What Is Interface Segregation Principle?

The Interface Segregation Principle says that no class should be forced to implement methods it doesn’t need.

In simple terms:

One big interface ❌
Multiple small interfaces ✅

ISP is closely related to:

Single Responsibility Principle (SRP)
Liskov Substitution Principle (LSP)

Together, they ensure safe abstraction and clean architecture.

Real-World Example (Easy to Understand)

Imagine an office printer:

Some printers:

Print only
Some printers:
Print + Scan
Some printers:
Print + Scan + Fax

As a result, if you create one large interface with all features, every printer must implement everything—even when it doesn’t support those features.

That’s exactly where ISP gets violated.

ISP Violation Example (Bad Design)

public interface IPrinter
{
    void Print();
    void Scan();
    void Fax();
}

public class SimplePrinter : IPrinter
{
    public void Print()
    {
        Console.WriteLine("Printing...");
    }

    public void Scan()
    {
        throw new NotImplementedException();
    }

    public void Fax()
    {
        throw new NotImplementedException();
    }
}

❌ What’s wrong here?

SimplePrinter is forced to implement unused methods
Runtime exceptions are introduced
Violates Interface Segregation Principle
Harder to maintain and test

ISP Followed (Correct Design)

public interface IPrinter
{
    void Print();
}

public interface IScanner
{
    void Scan();
}

public interface IFax
{
    void Fax();
}

public class SimplePrinter : IPrinter
{
    public void Print()
    {
        Console.WriteLine("Printing...");
    }
}

public class AdvancedPrinter : IPrinter, IScanner, IFax
{
    public void Print()
    {
        Console.WriteLine("Printing...");
    }

    public void Scan()
    {
        Console.WriteLine("Scanning...");
    }

    public void Fax()
    {
        Console.WriteLine("Faxing...");
    }
}

✅ Benefits

Classes depend only on what they use
No unused code
Easier maintenance
Better testability

ISP in Enterprise .NET Applications

In real projects, ISP helps:

Avoid breaking changes in APIs
Keep microservices loosely coupled
Improve dependency injection usage
Reduce regression bugs

This is why ISP is heavily used with:

Clean Architecture
Hexagonal Architecture
Microservices design

ISP vs SRP (Common Confusion)

SRP	ISP
Focuses on class responsibility	Focuses on interface responsibility
One reason to change	One reason to implement
Applies to classes	Applies to interfaces

👉 Both aim for focused, maintainable design

Common Interview Question

Q: How do you identify ISP violation?

Answer:
If a class:

Implements methods it doesn’t need
Throws NotImplementedException
Depends on large interfaces

➡️ ISP is violated.

When NOT to Over-Apply ISP

Very small applications
Rapid prototypes
Over-segmentation can reduce readability

✔ Apply ISP where change is expected, not everywhere.

Key Takeaways

Prefer small, specific interfaces
Avoid fat interfaces
ISP improves flexibility and testability
Critical for senior-level .NET interviews

In the next post, we will explore the Dependency Inversion Principle (DIP) with real-world C# examples and refactoring techniques.

Read our detailed guides on Single Responsibility Principle, Open/Closed Principle, and Liskov Substitution Principle to fully understand SOLID principles in C#.

Explore more important concepts here…

Interface Segregation Principle (ISP) in C# – SOLID Principles Explained