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Why does the order in which middleware is registered matter so much in ASP.NET Core? 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()? How do you conditionally apply middleware only to certain requests using UseWhen or MapWhen? 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?
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What is the purpose of UseExceptionHandler and UseDeveloperExceptionPage middleware, and how should they differ between environments?

Intermediate

Answer

UseDeveloperExceptionPage shows detailed stack traces and exception information directly in the browser, intended only for local development since it leaks sensitive implementation details. UseExceptionHandler is the production-safe alternative, catching unhandled exceptions and routing to a generic error page or a custom error-handling endpoint without exposing internal details to end users, typically registered conditionally based on the hosting environment.
if (app.Environment.IsDevelopment()) {
    app.UseDeveloperExceptionPage();
} else {
    app.UseExceptionHandler("/error");
    app.UseHsts();
}
Real-world example A production incident is avoided when a code review catches that UseDeveloperExceptionPage was accidentally left unconditional, which would have exposed database connection strings in stack traces to any end user triggering an unhandled exception.

Common follow-ups: What information does UseDeveloperExceptionPage expose that's dangerous in production?;How does the newer IExceptionHandler interface (added in .NET 8) provide more structured error handling?

Global Exception Handling & Middleware;Diagnostics & Performance
All 17 ASP.NET Core Middleware & Request Pipeline questions