OrderFlow Ten Design Patterns, One Real C# App

Ten design patterns. One real C# application. Not a single IShape in sight.

Most pattern books teach with throwaway examples — a Circle, a Square, a Visitor that visits nothing. You nod along, then forget why you’d ever reach for the thing.

OrderFlow does the opposite. Every pattern in this book is built into one cohesive application: a multi-restaurant food-delivery and kitchen-operations platform, written in C# and .NET 10 with Blazor Server. Clone the companion repository, run the app, click a real feature, set a breakpoint, and step through the exact code printed on the page.

The patterns aren’t the hard part. Knowing when each one earns its keep — and when it’s just ceremony — is the hard part. That’s what this book is about.

What’s inside

The ten classic Gang of Four patterns, each powering a real feature:

• Builder — assembling an order step by step
• Factory Method — choosing a customer’s notification channel
• Singleton — one shared kitchen queue per restaurant
• Adapter — one payment interface over Stripe-, Square-, and PayPal-style SDKs
• Decorator — stackable toppings and add-ons
• Facade — one “Place Order” button hiding four subsystems
• Composite — a menu that nests categories, items, and combo meals
• Observer — live order status pushed to three screens at once
• State — a strict order lifecycle that won’t let you skip a step
• Strategy — swappable delivery-pricing rules

Plus two bonus chapters: Command (undo and an audit trail) and Chain of Responsibility (a checkout gate pipeline).

Every chapter, the same eight sections

The Problem · The Naive Approach · The Pattern (with class and sequence diagrams) · The Implementation · The Payoff: Now It’s Testable · Seeing It Run · Debug Walkthrough · When Not to Use This

That last section is not an afterthought. Every pattern has a cost — another interface, another indirection, another file for the next developer to open — and this book is honest about when the naive code was fine all along.

Who it’s for

Intermediate C# developers who can read a class and a switch expression but want to stop guessing about design. If you’ve ever sat in a code review unsure whether to say “let’s make this a Strategy,” this book is for you.

What you’ll need

The .NET 10 SDK and an editor that speaks C#. That’s the only install — the app, the database, and the test suite all run from the dotnet CLI. The code in the book is the code in the repo, reproduced verbatim. Walk it, run it, break it, fix it.

One app. Ten patterns. Every topping on for a reason.