Advanced .NET Scenario-Based Questions and Answers

 Memory Leak in a WPF Application

Scenario:
A WPF application's memory usage increases over time, even when idle. The application uses event subscriptions, but events are not unsubscribed. How would you diagnose and resolve this issue?

Answer:

• Cause: Event subscribers hold strong references to publishers. If not unsubscribed, publishers outlive subscribers, keeping them in memory.

• Diagnosis: Use a memory profiler (e.g., dotMemory, Visual Studio Diagnostic Tools) to inspect object retention graphs. Look for lingering EventHandler references.

• Fix:

  • Unsubscribe events explicitly when no longer needed.

  • Use weak event patterns (e.g., WeakEventManager).

  csharp

  // Before (leaks):
  publisher.Event += SubscriberMethod;
  
  // Fix:
  publisher.Event -= SubscriberMethod; // Unsubscribe when done
  
  // Or use WeakEventManager:
  WeakEventManager<PublisherType, EventArgs>.AddHandler(publisher, nameof(publisher.Event), SubscriberMethod);

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2. Async Deadlock in ASP.NET

Scenario:
An ASP.NET MVC action uses Task.Result to call an async method, causing deadlocks. How would you refactor this?

Answer:

• Cause: Blocking on async code (e.g., Result/Wait()) in a synchronous context deadlocks due to thread pool exhaustion.

• Fix: Replace blocking calls with async/await throughout the call chain.

  csharp

  
  

  // Before (deadlocks):
  public ActionResult GetData() {
      var data = SomeAsyncMethod().Result;
      return View(data);
  }
  
  // After (async all the way):
  public async Task<ActionResult> GetData() {
      var data = await SomeAsyncMethod();
      return View(data);
  }

---

3. High CPU in Parallel.ForEach

Scenario:
A Parallel.ForEach loop processes a large collection but causes high CPU due to thread contention. How do you optimize it?

Answer:

• Cause: Shared resources or fine-grained work items cause excessive locking.

• Optimizations:

  • Use Partitioner.Create to reduce overhead.

  • Avoid shared state; use thread-local variables (ThreadLocal<T>).

  csharp

  
  

  var partitioner = Partitioner.Create(data, EnumerablePartitionerOptions.NoBuffering);
  Parallel.ForEach(partitioner, item => Process(item));
  
  // Or use concurrent collections:
  var results = new ConcurrentBag<ResultType>();
  Parallel.ForEach(data, item => results.Add(Process(item)));

---

4. Captive Dependency in DI

Scenario:
A singleton service in ASP.NET Core injects a scoped service, causing the scoped instance to behave like a singleton. How do you fix this?

Answer:

• Cause: Scoped services injected into singletons become captive, as their lifespan is extended.

• Fix: Use IServiceScopeFactory to resolve scoped services within the singleton.

  csharp

  
  

  public class SingletonService {
      private readonly IServiceScopeFactory _scopeFactory;
      public SingletonService(IServiceScopeFactory scopeFactory) => _scopeFactory = scopeFactory;
  
      public void Method() {
          using var scope = _scopeFactory.CreateScope();
          var scopedService = scope.ServiceProvider.GetRequiredService<IScopedService>();
          scopedService.DoWork();
      }
  }

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5. Optimizing Slow LINQ Queries on Large Data

Scenario:
A LINQ-to-Objects query on a 1-million-item collection is slow. How would you optimize it?

Answer:

• Optimizations:

  • Use PLINQ (AsParallel()) for CPU-bound work.

  • Replace class with struct and use Span<T> to reduce heap allocations.

  csharp

  
  

  // PLINQ example:
  var results = data.AsParallel()
                   .Where(item => item.IsValid)
                   .Select(item => Transform(item))
                   .ToList();
  
  // Span<T> example (for stack-based processing):
  Span<DataItem> span = dataArray.AsSpan();
  foreach (ref var item in span) {
      Process(ref item); // Avoids boxing and heap allocations
  }

---

6. Secure Password Handling

Scenario:
How would you securely hash passwords in a .NET application to prevent exposure if the database is compromised?

Answer:

• Best Practice: Use Rfc2898DeriveBytes (PBKDF2) with a high iteration count.

  csharp

  
  

  public string HashPassword(string password) {
      byte[] salt = new byte[16];
      using (var rng = RandomNumberGenerator.Create()) {
          rng.GetBytes(salt);
      }
      var pbkdf2 = new Rfc2898DeriveBytes(password, salt, 100000, HashAlgorithmName.SHA256);
      byte[] hash = pbkdf2.GetBytes(32);
      return Convert.ToBase64String(salt) + ":" + Convert.ToBase64String(hash);
  }