Mastering the Open-Closed Principle in Real Projects

The Open-Closed Principle (OCP) is the second letter in the SOLID acronym. It encourages developers to design software that is open to extension but closed for modification. In other words, when business requirements evolve, your code should allow you to add new behavior without rewriting existing logic.

🛠 Why the Open-Closed Principle Matters

Software is built to solve business problems. As business goals change, so do the software requirements. If a system must be updated every time a minor rule changes, it leads to:

Increased risk of introducing bugs
Harder maintenance
Fragile code that breaks easily under pressure

By following the OCP, you reduce risk by isolating changes in new components instead of modifying old ones.

🎥 Case Study: Streaming Platform Accounts

Imagine you're building a platform where users can sign up and watch movies. Initially, all users have the same account type. The logic is simple: for every user, a PlatformAccount is generated.

public class PlatformAccount {
    public void Generate(User user) {
        // create Customer from User and save to DB
    }
}

This works fine until the business decides to introduce premium accounts. Now, the logic must change based on the account type.

❌ OCP Violation: Modify Existing Class

A naive solution would be:

public void Generate(User user) {
    if (user.IsPremium) {
        GeneratePremiumAccount(user);
    } else {
        GenerateStandardAccount(user);
    }
}

This breaks the OCP because:

You modified an existing method
You added branching logic that can grow uncontrollably
Future account types like Gold or Silver would bloat the class even more

✅ OCP-Compliant Design: Use Abstractions

A better solution is to create separate implementations for each account type, all using a common interface.

public interface IPlatformAccount {
    void Generate(User user);
}

Now you can create:

StandardAccount that implements IPlatformAccount
PremiumAccount that also implements IPlatformAccount

Each account type has its own logic without interfering with others. No need to touch existing code when adding new types. This makes the system flexible and future-proof.

👨‍💻 Code Example

public class StandardAccount : IPlatformAccount {
    public void Generate(User user) {
        // logic for standard user
    }
}

public class PremiumAccount : IPlatformAccount {
    public void Generate(User user) {
        // logic for premium user
    }
}

When generating accounts, you assign the appropriate account strategy:

User user = new User(new StandardAccount());
User premiumUser = new PremiumUser(new PremiumAccount());

user.PlatformAccount.Generate(user);
premiumUser.PlatformAccount.Generate(premiumUser);

🔄 Why This Matters in Production

In real systems:

Business rules change often
Features get added fast
Production stability is crucial

By designing for extension, your code becomes:

Easier to test
Easier to maintain
Resistant to regression bugs

You avoid modifying classes that already work, which protects existing features while enabling growth.

✅ Summary

The Open-Closed Principle empowers you to:

Extend functionality without rewriting working code
Keep your system modular and scalable
Handle business changes with minimal risk

OCP is essential for long-living software and becomes more valuable as your system grows. Make it part of your design mindset from day one.