ASP.NET Core Patterns That Actually Scale in Enterprise

After working on ASP.NET Core C# systems that need to hold up under real enterprise load (not toy apps, not tutorials), a few patterns emerge as consistently reliable. These are not new ideas. But they are the ones that actually matter in production.

Keep Controllers Thin

Controllers should do one thing: receive a request, hand it off, and return a response. Any controller with business logic in it is a liability. It becomes hard to test, hard to change, and it accumulates complexity over time.

The moment you find yourself writing conditional logic, database calls, or string manipulation inside a controller action, move it out. A controller that cannot be read in 30 seconds is already too fat.

Use the Mediator Pattern for Complex Request Flows

For anything beyond simple CRUD, MediatR (or a similar mediator) is worth the dependency. It decouples request handling from the HTTP layer, makes unit testing straightforward, and forces a clear boundary between receiving a request and processing it.

Each command or query becomes its own class with its own handler. When requirements change, and they always do, you change one handler rather than a controller that has grown to 400 lines.

Be Deliberate About Dependency Lifetimes

Scoped, singleton, transient. Most developers know the definitions but do not think carefully about the implications. Injecting a scoped service into a singleton is one of the more common sources of subtle, hard-to-reproduce bugs in .NET applications.

Document the intended lifetime of every service your team registers. If you are reviewing a PR and you see a service registered without thought, ask the question.

Centralise Error Handling, Do Not Scatter It

Exception filters or global middleware should handle errors in one place. Wrapping every action or service method in try-catch blocks is repetitive and makes it easy to swallow exceptions silently.

A single error handling middleware that logs the exception, maps it to the appropriate HTTP status code, and returns a consistent error response shape is all you need. Everything else is noise.

Razor Templating Still Has a Place

On projects where a full JavaScript frontend is not justified, Razor is a solid, underrated choice. We have worked on ASP.NET Core codebases using Razor for server-rendered views and the developer experience is productive once you stop fighting it.

The mistake teams make with Razor is treating it like a frontend framework. It is not. It is a templating engine. Keep logic out of views, use view models, and it works well.

Write Tests Around Behaviour, Not Implementation

Unit tests that test the internals of a class break whenever the internals change, which is precisely when you need tests to be reliable. Test what the system does, not how it does it.

For API endpoints this means integration tests that exercise the full request pipeline: middleware, routing, controller, service, and response against a test database. These tests are slower but they catch the things that matter.

The Pattern That Matters Most

None of the above matters without discipline in a team context. The best architecture breaks down if it is not consistently applied. Code reviews, shared conventions, and the occasional refactor are what keep a large codebase from becoming a liability.

The pattern that matters most is the one your whole team understands and follows.