🧰 Core Tech Refresher

ASP.NET Core Essentials

Middleware pipeline, Minimal APIs vs Controllers, DI lifetimes.

Middleware pipeline — an ordered chain of delegates; each can act before/after calling next(). Order matters: e.g. UseExceptionHandler before UseRouting, UseAuthentication before UseAuthorization, UseAuthorization before endpoint execution.

Minimal APIs vs Controllers — both are valid in .NET 8+; Minimal APIs reduce ceremony for small services/endpoints, Controllers still make sense for larger surface areas with shared conventions (model binding, filters, [ApiController] validation). Be ready to say you'd pick based on team size and endpoint count, not because one is "outdated."

DI service lifetimes — the single most commonly-probed ASP.NET Core question: - Transient: new instance every time it's requested. - Scoped: one instance per HTTP request (or per scope you create manually). - Singleton: one instance for the app's lifetime.

The classic trap: injecting a Scoped (e.g. DbContext) into a Singleton — the singleton captures the first request's scoped instance forever, causing subtle bugs or an exception if you have ValidateScopes enabled (on by default in Development). Fix: inject IServiceScopeFactory into the singleton and create a scope per use.

Flashcards (4)

What breaks if you inject a Scoped service into a Singleton?

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The Singleton captures the Scoped instance from whichever request first resolved it and reuses it forever — e.g. a DbContext shared across all requests. ASP.NET Core throws in Development by default (scope validation).

How do you correctly use a Scoped/DbContext-dependent service from within a Singleton?

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Inject IServiceScopeFactory, call CreateScope() when you need the work done, resolve the scoped service from that scope, and dispose the scope afterwards.

Name two things that must happen in a specific middleware order and why.

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UseAuthentication before UseAuthorization (must know WHO before deciding WHAT they can do); UseExceptionHandler/UseHsts early so they wrap everything downstream.

When would you choose Controllers over Minimal APIs for a new ASP.NET Core 8 service?

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Larger endpoint surface needing shared conventions — model binding, [ApiController] automatic 400s, action filters, versioning conventions — where the structure pays for itself. Minimal APIs shine for small, focused services.