Interview Q&A

Short answer: Async filters implement IAsyncActionFilter and support await, improving scalability.

Example code

Async filters implement IAsyncActionFilter and support await, improving scalability.

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: 🏦 Fintech API – Global Exception Filter Ensures consistent error envelopes across microservices. 🛠 Microservices – Audit Logging Tracks when sensitive controller actions are executed. 👮 Role-based Authorization Custom authorization filter validating role claims dynamically. 🚀 Performance Profiling Times how long each controller method takes. public class ProfilingFilter : IActionFilter

Example code

{
private Stopwatch _watch;
public void OnActionExecuting(ActionExecutingContext context)
{
_watch = Stopwatch.StartNew();
}
public void OnActionExecuted(ActionExecutedContext context)
{ _watch.Stop(); Console.WriteLine($"Action took {_watch.ElapsedMilliseconds} ms"); }
}

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: custom page or handler. if (env.IsDevelopment()) pp.UseDeveloperExceptionPage(); else pp.UseExceptionHandler("/Error"); Or inline: pp.UseExceptionHandler(errorApp => { errorApp.Run(async context => { context.Response.StatusCode = 500; wait context.Response.WriteAsync("An error occurred"); }); }); custom page or handler.…

Example code

if… (env.IsDevelopment()) pp.UseDeveloperExceptionPage(); else pp.UseExceptionHandler("/Error"); Or inline: pp.UseExceptionHandler(errorApp => { errorApp.Run(async context => { context.Response.StatusCode = 500; wait context.Response.WriteAsync("An error occurred"); }); }); custom page or handler. Example: if (env.IsDevelopment()) pp.UseDeveloperExceptionPage(); else pp.UseExceptionHandler("/Error"); Or inline: pp.UseExceptionHandler(errorApp => { errorApp.Run(async context => { context.Response.StatusCode = 500; wait context.Response.WriteAsync("An error occurred"); }); }); custom page or handler.… Example: if (env.IsDevelopment()) pp.UseDeveloperExceptionPage(); else pp.UseExceptionHandler("/Error"); Or inline: pp.UseExceptionHandler(errorApp => { errorApp.Run(async context => { context.Response.StatusCode = 500; wait context.Response.WriteAsync("An error occurred"); }); });

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: (UseExceptionHandler, UseDeveloperExceptionPage) UseDeveloperExceptionPage() shows detailed errors (development only). UseExceptionHandler("/Error") handles errors in production with a custom page or handler. Example: if (env.IsDevelopment()) app.UseDeveloperExceptionPage(); else app.UseExceptionHandler("/Error"); Or inline: app.UseExceptionHandler(errorApp => { errorApp.Run(async context => {

Example code

context.Response.StatusCode = 500;
await context.Response.WriteAsync("An error occurred"); }); });

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. They form a pipeline based on their defined order.

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: Filters execute per controller action → use sparingly. Avoid heavy DB calls. Avoid creating new HttpClients or DbContexts inside filters. Prefer asynchronous filters.

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.UseHttpsRedirection(); dd early in the pipeline, before auth or routing.

Explain a bit more

You can configure HTTPS ports in launchSettings.json or ppsettings.json. pp.UseHttpsRedirection(); dd early in the pipeline, before auth or routing. You can configure HTTPS ports in launchSettings.json or ppsettings.json. pp.UseHttpsRedirection(); dd early in the pipeline, before auth or routing. You can configure HTTPS ports in launchSettings.json or ppsettings.json. pp.UseHttpsRedirection(); dd early in the pipeline, before auth or routing. You can configure HTTPS ports in launchSettings.json or ppsettings.json.

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: Redirects HTTP requests to HTTPS. app.UseHttpsRedirection(); Add early in the pipeline, before auth or routing. You can configure HTTPS ports in launchSettings.json or appsettings.json.

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: ActionExecutingContext mocks ActionExecutedContext mocks DefaultHttpContext var httpContext = new DefaultHttpContext(); var context = new ActionExecutingContext(...); Focus tests on: Expected results Short-circuiting behavior Context manipulation

Example code

Use: ActionExecutingContext mocks ActionExecutedContext mocks DefaultHttpContext var httpContext = new DefaultHttpContext();
var context = new ActionExecutingContext(...); Focus tests on: Expected results Short-circuiting behavior Context manipulation

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: app.UseStaticFiles(new StaticFileOptions { FileProvider = new PhysicalFileProvider(Path.Combine(env.ContentRootPath, "MyFiles")), RequestPath = "/Files", OnPrepareResponse = ctx => { ctx.Context.Response.Headers.Append("Cache-Control", "public,max-age=600"); } }); For directory browsing: app.UseDirectoryBrowser(new DirectoryBrowserOptions { FileProvider = new PhysicalFileProvider("path"), RequestPath = "/browse" });

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: When your logic depends on controller/action context, such as reading route values or action arguments.

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: 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: 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.