Interface Segregation Principle (ISP)

Definition

Clients should not be forced to depend on interfaces they do not use.
👉 Prefer small focused interfaces instead of one large interface.

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Purpose

• Reduce unnecessary implementation
• Keep interfaces clean
• Improve flexibility
• Avoid unused methods
• Build modular systems

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Bad Example ❌

Large Interface

public interface IWorker
{
    void Work();
    void Eat();
}

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Human Worker

public class Human : IWorker
{
    public void Work()
    {
        Console.WriteLine("Human Working");
    }

    public void Eat()
    {
        Console.WriteLine("Human Eating");
    }
}

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Robot Worker

public class Robot : IWorker
{
    public void Work()
    {
        Console.WriteLine("Robot Working");
    }

    public void Eat()
    {
        throw new Exception(
            "Robot doesn't eat"
        );
    }
}

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Problem

Robot forced to implement:

Eat()

even though it doesn't need it.
👉 Violates ISP.

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Good Example ✅

Work Interface

public interface IWorkable
{
    void Work();
}

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Eat Interface

public interface IEatable
{
    void Eat();
}

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Human Class

public class Human :
    IWorkable,
    IEatable
{
    public void Work()
    {
        Console.WriteLine("Human Working");
    }

    public void Eat()
    {
        Console.WriteLine("Human Eating");
    }
}

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Robot Class

public class Robot : IWorkable
{
    public void Work()
    {
        Console.WriteLine("Robot Working");
    }
}

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Key Concept

Instead of:

One huge interface

We do:

Multiple focused interfaces

👉 Clients implement only what they need.

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Real-Time Scenario

Notification System:

• IEmailSender
• ISmsSender
• IPushNotificationSender

👉 Services implement only required functionality.

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Advantages

• ✔ Cleaner interfaces
• ✔ Reduced unnecessary code
• ✔ Better flexibility
• ✔ Easier maintenance
• ✔ Better modularity

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Disadvantages

• ✖ More interfaces/classes
• ✖ Slight design complexity increase

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When to Use

Use ISP when:

• Interfaces become too large
• Classes implement unused methods
• Different clients need different functionality
• System requires modular design

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Real Project Mapping (.NET + Angular)

Feature	ISP Usage
Repository interfaces	ISP
Notification services	ISP
Payment providers	ISP
File storage services	ISP
API contracts	ISP

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ASP.NET Core Real Example

Repository Interfaces

IReadRepository
IWriteRepository

👉 Separate read/write responsibilities.

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Advanced Example – Cloud Storage

IFileUpload
IFileDownload
IFileDelete

👉 Services implement only required operations.

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ISP vs SRP

ISP	SRP
Focus on interfaces	Focus on classes
Small interfaces	Single responsibility
Prevent unused methods	Prevent mixed responsibilities

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Common ISP Violation Signs

• Interfaces with too many methods
• Empty method implementations
• throw new Exception() in methods
• Classes implementing unrelated functionality

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Pro Tip

ISP works best with:

• Clean Architecture
• Microservices
• Repository Pattern
• CQRS
• Dependency Injection

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Summary

ISP helps you:

• Create small focused interfaces
• Reduce unnecessary dependencies
• Build modular systems

👉 Perfect for:

• Enterprise applications
• APIs
• Microservices
• Service-oriented architectures