After going through all the classic design patterns (creational, structural, and behavioral), the inevitable question arises:
“Ok… but which one do I use here?”
This article is the decision guide:
-
when a pattern is worth it
-
when it’s overengineering
-
what fits in modern .NET applications (Web API, Blazor, MAUI, microservices)
🎯 Why you should NOT “collect” patterns
A design pattern is not:
❌ a rule
❌ an obligation
❌ an architect’s medal
It is:
✅ a reusable solution to a known problem
✅ a common language among developers
✅ a way to reduce long-term complexity
👉 If you don’t have a clear problem, don’t introduce a pattern.
🧱 Quick recap – patterns and their purpose
🔨 Creational – how you create objects
|
Pattern |
When to use it |
|---|---|
|
Factory / Abstract Factory |
when instantiation depends on context |
|
Builder |
when the object is complex |
|
Singleton |
when you have a single instance (⚠️ use with care!) |
➡️ Very common in .NET + DI container
🧩 Structural – how you connect objects
|
Pattern |
When to use it |
|---|---|
|
Adapter |
integrating external libraries |
|
Decorator |
adding behavior without inheritance |
|
Facade |
simplifying a subsystem |
|
Proxy |
lazy-loading, caching, security |
➡️ Common in APIs, infrastructure, middleware
🔄 Behavioral – how objects collaborate
|
Pattern |
When to use it |
|---|---|
|
Observer |
events, notifications |
|
Strategy |
interchangeable algorithms |
|
State |
state-dependent behavior |
|
Template Method |
fixed steps + extensibility |
|
Visitor |
new operations without modifying structure |
|
Memento |
undo / rollback |
|
Command |
decoupling between request and execution |
➡️ Dominates business logic
🧠 How to choose the right pattern – practical guide
1️⃣ Do you have a REAL problem or just assume one?
Ask yourself:
-
Will this behavior change?
-
Are there multiple variants?
-
Is the code repeated?
❗ If the answer is “no”, don’t use a pattern.
2️⃣ Does the algorithm change or just the data?
|
Situation |
Pattern |
|---|---|
|
Multiple calculation methods |
Strategy |
|
Common steps + extensions |
Template Method |
|
Triggered actions |
Command |
3️⃣ Does behavior depend on state?
➡️ State Pattern
No infinite if/else, but separate objects.
4️⃣ Do you need extensibility without “breaking” existing code?
➡️ Visitor Pattern
Excellent for:
-
reports
-
export
-
multiple validations
5️⃣ Do you need decoupling?
|
Scenario |
Pattern |
|---|---|
|
events |
Observer |
|
complex subsystems |
Facade |
|
external integration |
Adapter |
⚖️ Pattern vs Overengineering
❌ Clear signs of overengineering
-
you have 5 interfaces for one class
-
you use Strategy with only one implementation
-
Factory that creates… only one class
-
“because that’s clean architecture”
✅ Good signs
-
easier to test code
-
local changes, not cascading
-
clarity for other developers
🏗️ Patterns + modern .NET architectures
🧱 Clean Architecture / DDD
-
Strategy → Domain logic
-
Factory → Aggregate creation
-
Command → Application layer
-
Observer → Domain Events
🌐 ASP.NET Web API
-
Facade → Service layer
-
Decorator → logging / caching
-
Adapter → external service integration
🖥️ Blazor / MAUI
-
State → UI state
-
Observer → UI notifications
-
Command → user actions
🧠 The golden rule
Patterns are tools, not goals.
Use them:
-
when the problem arises
-
when the code starts to “hurt”
-
when you want to communicate intent clearly
🧩 Mini-checklist before choosing a pattern
✔ Will this behavior change?
✔ Do I have multiple variants?
✔ Do I want decoupling?
✔ Does it increase readability?
✔ Does it help testing?
If you have at least 3 ✔, the pattern is worth it.
🏁 Conclusion
Design patterns do not automatically make you a better architect.
Good decisions do.
If you have gone through the entire series, you now have:
-
a common vocabulary
-
concrete examples in C#
-
real criteria for choosing
👉 Next level: apply them judiciously in your real projects.
