Interview Q&A

Short answer: Mistake Fix Doing authentication inside action filters Use authorization filters Performing heavy logic Move to middleware Not registering filters as services Use DI for maintainability Returning inconsistent error messages Handle errors via global exception filter

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: pp.Run(async ctx => { wait ctx.Response.WriteAsync("This ends the pipeline"); }); pp.Run(async ctx => { wait ctx.Response.WriteAsync("This ends the pipeline"); }); pp.Run(async ctx => { wait ctx.Response.WriteAsync("This ends the pipeline"); }); pp.Run(async ctx => { wait ctx.Response.WriteAsync("This ends the pipeline"); });

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: middleware Type Description Terminal Ends the pipeline. Doesn’t call next(). E.g., app.Run() Non-Termi nal Calls next() and allows other middlewares to run after it. E.g., app.Use() Terminal middleware: app.Run(async ctx => { await ctx.Response.WriteAsync("This ends the pipeline"); });

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Yes—action filters have full access to ActionArguments.

Real-world example (ShopNest)

ShopNest registers AppDbContext as Scoped and IMemoryCache as Singleton. Putting DbContext in a Singleton causes threading bugs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: app.UseAuthentication(); // Validates user identity app.UseAuthorization(); // Applies policies/roles Order matters: must be after routing but before endpoints. app.UseRouting(); app.UseAuthentication(); app.UseAuthorization(); app.UseEndpoints(...);

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Filters provide clean, reusable cross-cutting logic for MVC pipelines. Use the right filter type for the right stage. Prefer middleware when you don’t need controller context. Exception filters are best for consistent error handling. Use DI for scalable, clean, testable filter implementations. Filters shine in enterprise applications: auditing, authorization, and response shaping.

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Exception filters only catch exceptions from MVC actions; middleware catches exceptions from the entire pipeline.

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: pp.UseCors("MyPolicy"); Must be placed before routing/endpoints. pp.UseCors("MyPolicy"); Must be placed before routing/endpoints. pp.UseCors("MyPolicy"); Must be placed before routing/endpoints. pp.UseCors("MyPolicy"); Must be placed before routing/endpoints.

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: builder.Services.AddCors(options => { options.AddPolicy("MyPolicy", policy => { policy.WithOrigins(" .AllowAnyHeader() .AllowAnyMethod(); }); }); app.UseCors("MyPolicy"); Must be placed before routing/endpoints.

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use: services.AddControllers(options => { options.Filters.Add(typeof(AuditLogFilter)); }); Advanced Level

Example code

Use: services.AddControllers(options => { options.Filters.Add(typeof(AuditLogFilter)); }); Advanced Level

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: wait _next(context); sw.Stop(); Console.WriteLine($"Request took {sw.ElapsedMilliseconds} ms"); } } Register: pp.UseMiddleware<TimingMiddleware>(); Dependency Injection (DI) wait _next(context); sw.Stop(); Console.WriteLine($"Request took {sw.ElapsedMilliseconds} ms"); } } Register: pp.UseMiddleware<TimingMiddleware>(); Dependency… Injection (DI) wait… _next(context); sw.Stop(); Console.WriteLine($"Request took…

Explain a bit more

{sw.ElapsedMilliseconds} ms"); } } Register: pp.UseMiddleware<TimingMiddleware>(); Dependency Injection (DI) wait _next(context); sw.Stop(); Console.WriteLine($"Request took {sw.ElapsedMilliseconds} ms"); } } Register: pp.UseMiddleware<TimingMiddleware>(); Dependency… Injection (DI)

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: execution time) Example custom middleware to measure time: public class TimingMiddleware

Example code

{
private readonly RequestDelegate _next;
public TimingMiddleware(RequestDelegate next) => _next = next; public async Task InvokeAsync(HttpContext context)
{
var sw = Stopwatch.StartNew();
await _next(context); sw.Stop(); Console.WriteLine($"Request took {sw.ElapsedMilliseconds} ms"); }
} Register: app.UseMiddleware<TimingMiddleware>(); Dependency Injection (DI)

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Caching expensive requests where model binding is unnecessary.

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Dependency Injection (DI) is a design pattern that allows you to inject dependencies (services) into classes instead of hard-coding them.

Real-world example (ShopNest)

ShopNest registers AppDbContext as Scoped and IMemoryCache as Singleton. Putting DbContext in a Singleton causes threading bugs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Filters specifically built for Minimal APIs (introduced in ASP.NET Core 7).

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: ✅ Capabilities: Constructor injection Lifetime management (Transient, Scoped, Singleton) Supports IEnumerable<T>, IServiceProvider, and open generics ⚠ Limitations: No support for named registrations Limited property injection Basic feature set compared to Autofac or other 3rd-party containers

Real-world example (ShopNest)

ShopNest registers AppDbContext as Scoped and IMemoryCache as Singleton. Putting DbContext in a Singleton causes threading bugs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Used for filters that require custom instantiation logic.

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Lifetime Description Use Case Example Transient New instance every time Lightweight stateless services Scoped One instance per request Database context, UoW Singleto One instance for the app's lifetime Logging, Config access

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Lower order executes first on entry but last on exit—like nested layers.

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: public void ConfigureServices(IServiceCollection services) { services.AddTransient<IMyService, MyService>(); services.AddScoped<IRepository, Repository>(); services.AddSingleton<ILoggerService, LoggerService>(); }

Example code

public void ConfigureServices(IServiceCollection services)
{ services.AddTransient<IMyService, MyService>(); services.AddScoped<IRepository, Repository>(); services.AddSingleton<ILoggerService, LoggerService>(); }

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use a mocked HttpContext and assert on changes.

Real-world example (ShopNest)

ShopNest registers AppDbContext as Scoped and IMemoryCache as Singleton. Putting DbContext in a Singleton causes threading bugs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Controllers: Constructor injection Razor Pages: Constructor injection in PageModel Middleware: Inject via constructor or use IApplicationBuilder.ApplicationServices

Real-world example (ShopNest)

ShopNest pipeline order matters: exception handling → HTTPS → auth → authorization → endpoints. Auth must run before protected APIs.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use [SkipFilter] custom attribute or exclude via filter predicates.

Real-world example (ShopNest)

A ShopNest ValidateModelAttribute runs before actions and returns 400 if ModelState is invalid—same rule for every controller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Constructor Injection: Supported and preferred in ASP.NET Core. Property Injection: Not supported natively in built-in DI; can be achieved via custom logic or 3rd-party containers. ✅ Use constructor injection for immutability and clarity.

Example code

Constructor Injection: Supported and preferred in ASP.NET Core. Property Injection: Not supported natively in built-in DI; can be achieved via custom logic or 3rd-party containers. ✅ Use constructor injection for immutability and clarity.

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: No. But middleware can replace some filter use cases.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.