Mar 12, 25 | 4 min read

Interface Segregation: Don't Force Your Minions to Do Useless Work

Imagine having an army of minions at your disposal. You can spin up as many as you want—no limits! The more minions you have, the crazier the dev you are. But here's the catch: you wouldn't make all of them do every single task, right? Some minions are great at cooking, some at serving, and some at handling cash. You wouldn’t force your chef minion to process payments, would you?

That’s exactly what the Interface Segregation Principle (ISP) is all about.

It states:

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

In simpler words: A class shouldn’t be forced to implement things it doesn’t need. Period.

Imagine we have a IRestaurantEmployee interface that includes methods for both chefs, waiters, and cashiers:

// ❌ This interface forces unnecessary methods on employees
public interface IRestaurantEmployee
{
    void CookFood();
    void ServeFood();
    void TakeOrders();
    void ProcessPayment();
}

// 👨‍🍳 Chef should only cook food, but is forced to implement other methods
public class Chef : IRestaurantEmployee
{
    public void CookFood() => Console.WriteLine("👨‍🍳 Cooking food...");

    public void ServeFood() => throw new NotImplementedException("❌ Chef doesn't serve food!");

    public void TakeOrders() => throw new NotImplementedException("❌ Chef doesn't take orders!");

    public void ProcessPayment() => throw new NotImplementedException("❌ Chef doesn't process payments!");
}

Issues in this Design:

Applying ISP Correctly

Now, let’s break down the interface into smaller, more specific interfaces:

// 🔹 Only relevant methods for each role
public interface ICook
{
    void CookFood();
}

public interface IWaiter
{
    void ServeFood();
    void TakeOrders();
}

public interface ICashier
{
    void ProcessPayment();
}

// 👨‍🍳 Chef now implements only what is needed
public class Chef : ICook
{
    public void CookFood() => Console.WriteLine("👨‍🍳 Cooking food...");
}

// 🏃‍♂️ Waiter only serves food and takes orders
public class Waiter : IWaiter
{
    public void ServeFood() => Console.WriteLine("🍽 Serving food...");
    public void TakeOrders() => Console.WriteLine("📝 Taking orders...");
}

// 💰 Cashier only handles payments
public class Cashier : ICashier
{
    public void ProcessPayment() => Console.WriteLine("💵 Processing payment...");
}

Why is This a Better Design?

Key Takeaways