Interface Segregation Principle (ISP)

Interface Segregation Principle (ISP)

The Interface Segregation Principle states: Do not force a class to implement interfaces it does not use. Instead of having one large, general-purpose interface, it is better to have smaller, specific interfaces that are client-focused.

 

Real-Time Example: Multifunction Printer

Imagine designing an interface for printers in an office automation system:

Violates ISP (Bad Design):

public interface IPrinter

{

    void Print(Document doc);

    void Scan(Document doc);

    void Fax(Document doc);

}

Now imagine a basic printer that can only print — not scan or fax. Still, it must implement the whole interface:

public class BasicPrinter : IPrinter

{

    public void Print(Document doc)

    {

        Console.WriteLine("Printing document...");

    }

 

    public void Scan(Document doc)

    {

        throw new NotImplementedException(); // Not supported

    }

 

    public void Fax(Document doc)

    {

        throw new NotImplementedException(); // Not supported

    }

}

This violates ISP because BasicPrinter is forced to implement methods it doesn’t need or support.

 

Follows ISP (Good Design):

Split the large interface into smaller, more specific ones:

public interface IPrinter

{

    void Print(Document doc);

}

 

public interface IScanner

{

    void Scan(Document doc);

}

 

public interface IFax

{

    void Fax(Document doc);

}

Now different classes can implement only what they need:

public class BasicPrinter : IPrinter

{

    public void Print(Document doc)

    {

        Console.WriteLine("Basic printing...");

    }

}

 

public class MultiFunctionPrinter : IPrinter, IScanner, IFax

{

    public void Print(Document doc)

    {

        Console.WriteLine("Printing...");

    }

 

    public void Scan(Document doc)

    {

        Console.WriteLine("Scanning...");

    }

 

    public void Fax(Document doc)

    {

        Console.WriteLine("Faxing...");

    }

}

Now, each class only depends on the interfaces relevant to it, making the design cleaner and more maintainable.

 

Why Interface Segregation Principle Matters

  • Encourages modular, flexible interfaces
  • Avoids unnecessary code and exceptions
  • Makes unit testing easier (mock only the required interfaces)
  • Reduces tight coupling and code bloat

This principle is particularly helpful in large-scale applications and systems with diverse or evolving functionalities. It enables each class to grow independently based on its own needs.

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