Open/Closed Principle in C#: Making Your Code Flexible Without Breaking Stuff

Ever built a feature that worked great—until someone added a small change and it all fell apart? Welcome to the pain of code that violates the Open/Closed Principle. Software entities should be open for extension, but closed for modification. It’s the “O” in SOLID, and it’s all about writing code that can grow new behaviors without you having to rewrite what already works. Let’s unpack that—with real C# code, not vague theory. Our Old Friend: The InvoiceProcessor We’ll pick up from where we left off in the SRP post. Here's a class that's responsible for calculating invoice totals: public class InvoiceCalculator { public decimal CalculateTotal(Invoice invoice) { decimal total = 0; foreach (var item in invoice.LineItems) { total += item.Price * item.Quantity; } return total; } } All good. But now your product manager wants to add discounts. Tomorrow, someone else might ask for tax rules. Next week? Promotional pricing based on customer loyalty. Do we keep modifying this method every time?

Mar 23, 2025 - 11:23

Open/Closed Principle in C#: Making Your Code Flexible Without Breaking Stuff

Ever built a feature that worked great—until someone added a small change and it all fell apart?

Welcome to the pain of code that violates the Open/Closed Principle.

Software entities should be open for extension, but closed for modification.

It’s the “O” in SOLID, and it’s all about writing code that can grow new behaviors without you having to rewrite what already works.

Let’s unpack that—with real C# code, not vague theory.

Our Old Friend: The InvoiceProcessor

We’ll pick up from where we left off in the SRP post. Here's a class that's responsible for calculating invoice totals:

public class InvoiceCalculator
{
    public decimal CalculateTotal(Invoice invoice)
    {
        decimal total = 0;
        foreach (var item in invoice.LineItems)
        {
            total += item.Price * item.Quantity;
        }
        return total;
    }
}

All good. But now your product manager wants to add discounts. Tomorrow, someone else might ask for tax rules. Next week? Promotional pricing based on customer loyalty.

Do we keep modifying this method every time?