All 17 ASP.NET Core Middleware & Request Pipeline questions
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How do you write a custom middleware component as a reusable class instead of an inline lambda? What's the difference between app.Use, app.Run, and app.Map in configuring the middleware pipeline? How does short-circuiting work in middleware, and when would you deliberately not call next()? What is the purpose of UseExceptionHandler and UseDeveloperExceptionPage middleware, and how should they differ between environments? How does the HttpContext object serve as the shared state carrier through the middleware pipeline? What is middleware in ASP.NET Core, and how does the order in which it's registered affect request handling? What are Minimal APIs in ASP.NET Core, and when would you choose them over traditional MVC controllers? How do you inject scoped (per-request) services into middleware, given that middleware classes are constructed once at app startup? How does UseRouting and endpoint routing (via MapControllers, MapGet, etc.) fit into the middleware pipeline conceptually? How do minimal API endpoint filters (IEndpointFilter) compare to traditional middleware for cross-cutting logic?
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How do you conditionally apply middleware only to certain requests using UseWhen or MapWhen?

Intermediate

Answer

UseWhen(predicate, configuration) branches the pipeline conditionally based on a predicate evaluated per request, applying the given middleware chain only when the condition is true, then automatically rejoining the main pipeline afterward -- unlike Map/MapWhen, which create a fully separate branch that typically doesn't rejoin. This is useful for applying middleware (like extra logging) only to specific request types without duplicating the entire remaining pipeline.
app.UseWhen(
    context => context.Request.Path.StartsWithSegments("/api"),
    apiApp => apiApp.UseMiddleware<ApiKeyValidationMiddleware>()
);
// Non-API requests skip ApiKeyValidationMiddleware entirely, but both paths
// continue through the same remaining pipeline afterward
Real-world example An application applies API-key validation middleware only to requests under /api using UseWhen, while static file requests and health check endpoints skip that check entirely without needing a completely separate pipeline branch.

Common follow-ups: How does UseWhen's rejoining behavior differ from Map's branching?;What's a good use case for MapWhen instead of UseWhen?

Authentication & Authorization (Identity JWT OAuth);Health Checks & Readiness/Liveness Probes
All 17 ASP.NET Core Middleware & Request Pipeline questions