ASP.NET CORE MVC - Topic-wise Practice

P1. Predict Output: What HTTP status? Why?

// Program.cs
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.MapGet("/admin", [Authorize] () => "Secret");

// Request: GET /admin without login

P2. Spot the Bug: All requests return 404. Why?

// Program.cs
app.UseAuthentication();
app.UseAuthorization();
app.UseRouting(); // Sanju's order
app.MapControllers();

P3. Write Code: Middleware that logs request path + time taken. Must not break pipeline.

public class RequestLogMiddleware {
    private readonly RequestDelegate _next;
    private readonly ILogger<RequestLogMiddleware> _log;
    
    public RequestLogMiddleware(RequestDelegate next, ILogger<RequestLogMiddleware> log) {
        _next = next; _log = log;
    }
    
    public async Task InvokeAsync(HttpContext ctx) {
        var sw = Stopwatch.StartNew();
        try {
            await _next(ctx); // Must call next
        } finally {
            sw.Stop();
            _log.LogInformation("{Path} took {Ms}ms", ctx.Request.Path, sw.ElapsedMilliseconds);
        }
    }
}
// Program.cs: app.UseMiddleware<RequestLogMiddleware>();

P4. Predict Output: What is vm.Name?

// Action
public IActionResult Create(UserVM vm) => Json(vm.Name);

// POST /users
// Body: name=John&name=Kunal

P5. Spot the Bug: User posts Price="abc". Why does DB save 0?

[HttpPost] public IActionResult Create(ProductVM vm) {
    // ModelState.IsValid = false here, but we forgot to check
    _db.Products.Add(new Product { Price = vm.Price });
    _db.SaveChanges(); // Prerna's bug
    return RedirectToAction("Index");
}

P6. Write Code: Custom validation attribute [NotFutureDate] for DateTime.

public class NotFutureDateAttribute : ValidationAttribute {
    protected override ValidationResult IsValid(object value, ValidationContext ctx) {
        if (value is DateTime date && date > DateTime.Now) {
            return new ValidationResult(ErrorMessage?? "Date cannot be in future");
        }
        return ValidationResult.Success;
    }
}
// Usage: [NotFutureDate] public DateTime BirthDate {get;set;}

P7. Predict Output: Browser View Source shows what?

@model UserVM
<input asp-for="Email" />
// No @addTagHelper in view or _ViewImports.cshtml

P8. Spot the Bug: NullReferenceException on production. Why?

// Action
public IActionResult Details(int id) {
    var user = _db.Users.Find(id);
    return View(user); // user might be null
}
// View
@model User
<h1>@Model.Name</h1> // Kunal's view

P9. Write Code: Tag Helper <email>kunal@test.com</email> that renders <a href="mailto:kunal@test.com">kunal@test.com</a>

[HtmlTargetElement("email")]
public class EmailTagHelper : TagHelper {
    public override async Task ProcessAsync(TagHelperContext ctx, TagHelperOutput output) {
        var content = await output.GetChildContentAsync();
        var email = content.GetContent();
        output.TagName = "a";
        output.Attributes.SetAttribute("href", $"mailto:{email}");
        output.Content.SetContent(email);
    }
}
// _ViewImports.cshtml: @addTagHelper *, YourAssembly

P10. Predict Output: How many times does "Ctor" print?

builder.Services.AddTransient<IService, MyService>();
public class MyService { public MyService() => Console.Write("Ctor"); }

public class HomeController {
    public HomeController(IService s1, IService s2) {}
}
// GET / requests HomeController twice

P11. Spot the Bug: Random data leaks. User A sees User B data. Why?

builder.Services.AddSingleton<IUserService, UserService>();
public class UserService : IUserService {
    private readonly AppDbContext _db;
    public UserService(AppDbContext db) => _db = db; // Kaushal's service
}

P12. Write Code: BackgroundService that runs every 5min, needs DbContext. Handle lifetime correctly.

public class CleanupService : BackgroundService {
    private readonly IServiceScopeFactory _scopeFactory;
    public CleanupService(IServiceScopeFactory scopeFactory) => _scopeFactory = scopeFactory;
    
    protected override async Task ExecuteAsync(CancellationToken ct) {
        while (!ct.IsCancellationRequested) {
            using var scope = _scopeFactory.CreateScope(); // New scope per run
            var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
            await db.Users.Where(u => u.LastLogin < DateTime.UtcNow.AddYears(-1)).ExecuteDeleteAsync(ct);
            await Task.Delay(TimeSpan.FromMinutes(5), ct);
        }
    }
}
// Program.cs: builder.Services.AddHostedService<CleanupService>();

P13. Predict Output: Action runs or not?

public class BlockFilter : IAsyncActionFilter {
    public async Task OnActionExecutionAsync(ActionExecutingContext ctx, ActionExecutionDelegate next) {
        ctx.Result = new StatusCodeResult(403); // Don't call next()
    }
}
[ServiceFilter(typeof(BlockFilter))]
public IActionResult Index() => Content("Hi"); // Will "Hi" print?

P14. Spot the Bug: Thread pool starvation. Why?

public class SlowFilter : ActionFilterAttribute {
    public override void OnActionExecuting(ActionExecutingContext ctx) {
        Thread.Sleep(5000); // Sanju's logging filter
    }
}

P15. Write Code: Resource Filter that adds "X-Response-Time" header with action execution time.

public class TimingFilter : IAsyncResourceFilter {
    public async Task OnResourceExecutionAsync(ResourceExecutingContext ctx, ResourceExecutionDelegate next) {
        var sw = Stopwatch.StartNew();
        var executed = await next(); // Runs rest of pipeline
        sw.Stop();
        executed.HttpContext.Response.Headers["X-Response-Time"] = $"{sw.ElapsedMilliseconds}ms";
    }
}
// Program.cs: builder.Services.AddControllers(opt => opt.Filters.Add<TimingFilter>());