{"@context":"https://schema.org","@type":"FAQPage","mainEntity":[{"@type":"Question","name":"Why use async/await in ASP.NET Core controller actions?","acceptedAnswer":{"@type":"Answer","text":"Using async/await in controllers frees thread pool threads during I/O operations (database, external APIs). This improves server scalability by allowing the thread to process other requests while waiting. Without async, the thread remains blocked and consumes resources unnecessarily, limiting the number of concurrent requests."}},{"@type":"Question","name":"What happens if a controller action returns Task without using async/await?","acceptedAnswer":{"@type":"Answer","text":"Returning Task without async/await works if you directly return an existing Task (return dbContext.SaveChangesAsync();). However, without the async keyword, you cannot use await in the method body. Most of the time, using async allows awaiting multiple async operations and handling errors with try/catch."}},{"@type":"Question","name":"What is the difference between ToList() and ToListAsync() in a controller action?","acceptedAnswer":{"@type":"Answer","text":"ToList() is synchronous and blocks the thread during the database query, limiting scalability. ToListAsync() is asynchronous and frees the thread during I/O operations, allowing the server to process other requests. Always use ToListAsync() in controllers to maximize supported concurrent requests."}},{"@type":"Question","name":"What is the role of CancellationToken in controller actions?","acceptedAnswer":{"@type":"Answer","text":"CancellationToken allows canceling an async operation when the client disconnects (closes browser, network timeout). ASP.NET Core automatically provides a token linked to HttpContext. Passing this token to async methods (ToListAsync, SaveChangesAsync) allows stopping unnecessary I/O operations, saving server resources."}},{"@type":"Question","name":"How to obtain the CancellationToken in an ASP.NET Core controller action?","acceptedAnswer":{"@type":"Answer","text":"ASP.NET Core automatically injects CancellationToken via model binding if added as an action parameter. No special attribute needed. The framework detects the CancellationToken type and provides the token linked to HttpContext.RequestAborted, which is triggered when the client cancels the request."}},{"@type":"Question","name":"What is the advantage of returning IAsyncEnumerable instead of Task> in a Web API?","acceptedAnswer":{"@type":"Answer","text":"IAsyncEnumerable enables result streaming: each element is serialized and sent to the client as soon as available, without waiting for the end of the query. This reduces memory consumption (no buffering of the entire collection) and improves Time To First Byte (TTFB). Ideal for large collections or progressively generated results."}},{"@type":"Question","name":"What syntax to use for returning an IAsyncEnumerable in a controller action?","acceptedAnswer":{"@type":"Answer","text":"Use async IAsyncEnumerable as return signature (not Task>). In the body, use yield return for each element instead of return. The yield keyword returns an element and suspends the method until the client requests the next element. Requires C# 8.0 or higher."}},{"@type":"Question","name":"How to consume an IAsyncEnumerable in a controller action?","acceptedAnswer":{"@type":"Answer","text":"Use the await foreach (var item in asyncEnumerable) syntax to iterate over each async element. This syntax introduced in C# 8.0 automatically awaits each element without blocking the thread. Alternative: call ToListAsync() to materialize the entire collection in memory, but this loses the streaming advantage."}},{"@type":"Question","name":"In which scenario is streaming with IAsyncEnumerable particularly useful?","acceptedAnswer":{"@type":"Answer","text":"Streaming is ideal for returning large collections (thousands of records) where waiting for the end of the query would increase latency and memory consumption. Also useful for progressively generated results (real-time reports, logs, large CSV exports). The client starts receiving data immediately, improving user experience."}},{"@type":"Question","name":"What is the difference between IActionFilter and IAsyncActionFilter?","acceptedAnswer":{"@type":"Answer","text":"IActionFilter provides two synchronous methods (OnActionExecuting, OnActionExecuted). IAsyncActionFilter provides a single async method OnActionExecutionAsync that wraps action execution. Use IAsyncActionFilter if the filter performs I/O operations (database logging, API calls) to avoid blocking the thread. Never implement both interfaces simultaneously."}},{"@type":"Question","name":"How to call the next action in OnActionExecutionAsync of an IAsyncActionFilter?","acceptedAnswer":{"@type":"Answer","text":"Call await next() where next is the ActionExecutionDelegate delegate passed as parameter. This call executes the action and returns ActionExecutedContext. Code before await next() executes before the action (equivalent OnActionExecuting), code after executes after the action (equivalent OnActionExecuted). If next() is not called, the action is short-circuited."}},{"@type":"Question","name":"How to implement a timeout for an async operation in a controller action?","acceptedAnswer":{"@type":"Answer","text":"Use CancellationTokenSource.CancelAfter(timeSpan) to create a token that cancels after a delay. Combine with the request token via CancellationTokenSource.CreateLinkedTokenSource to support both timeout and client cancellation. Pass the combined token to async methods. Alternative: Task.WhenAny with Task.Delay for manual timeout, but CancelAfter is simpler."}},{"@type":"Question","name":"How to handle a TaskCanceledException in an async controller action?","acceptedAnswer":{"@type":"Answer","text":"TaskCanceledException is thrown when an async operation is canceled via CancellationToken. Handle with try/catch around async operations. Check cancellationToken.IsCancellationRequested before continuing processing. Return StatusCode 499 (Client Closed Request) or log and ignore if cancellation is expected (client disconnected). Don't return error 500."}},{"@type":"Question","name":"When to use Task.WhenAll in a controller action?","acceptedAnswer":{"@type":"Answer","text":"Task.WhenAll allows executing multiple independent async operations in parallel (calling multiple external APIs, independent database queries). Reduces total latency by awaiting all tasks simultaneously instead of sequentially. Caution: verify operations have no mutual dependencies and handle exceptions with try/catch as WhenAll aggregates errors."}},{"@type":"Question","name":"Should you use ConfigureAwait(false) in ASP.NET Core controller actions?","acceptedAnswer":{"@type":"Answer","text":"No, ConfigureAwait(false) is useless in ASP.NET Core. The framework doesn't use SynchronizationContext, so code after await already executes on a thread pool without synchronization cost. ConfigureAwait(false) was necessary in .NET Framework with SynchronizationContext (UI apps, classic ASP.NET). In ASP.NET Core, it provides no performance gain."}},{"@type":"Question","name":"What is the difference between Task and ValueTask in ASP.NET Core?","acceptedAnswer":{"@type":"Answer","text":"ValueTask is a struct that avoids heap allocation when the result is already available (cache hit, fast synchronous operation). Task is a class that always allocates on the heap. Use ValueTask for hot paths where results are often synchronous (cache, pooling). Caution: ValueTask can only be awaited once, unlike Task."}},{"@type":"Question","name":"How to pass a CancellationToken to a method returning IAsyncEnumerable?","acceptedAnswer":{"@type":"Answer","text":"Use the [EnumeratorCancellation] attribute on the CancellationToken parameter of the method. This special attribute allows the compiler to propagate the token from await foreach to the async iterator. Without this attribute, the token passed to the method parameter would not be propagated to GetAsyncEnumerator. Alternative: manually pass the token to GetAsyncEnumerator."}},{"@type":"Question","name":"How to stream a large file without loading all content into memory?","acceptedAnswer":{"@type":"Answer","text":"Use FileStreamResult or File(stream, contentType) with a FileStream created with FileOptions.Asynchronous and FileOptions.SequentialScan. ASP.NET Core reads and sends the file in chunks without loading everything into memory. Alternative: return IAsyncEnumerable and yield return chunks, but FileStreamResult is optimized for files. Never use File.ReadAllBytes for large files."}},{"@type":"Question","name":"Why avoid parallelizing multiple operations on the same DbContext with Task.WhenAll?","acceptedAnswer":{"@type":"Answer","text":"DbContext is not thread-safe and supports only one async operation at a time. Parallelizing with Task.WhenAll can cause InvalidOperationException or data corruption. If multiple queries are needed, execute them sequentially or create multiple DbContext instances (but watch out for scope in DI). Alternative: use AsSplitQuery() to optimize complex queries."}},{"@type":"Question","name":"In which case should you avoid async/await in a controller action?","acceptedAnswer":{"@type":"Answer","text":"Avoid async/await for purely CPU-bound operations (calculations, in-memory transformations) without I/O. The async state machine overhead is useless if no thread is freed during processing. For CPU-intensive operations, use Task.Run only if really necessary (long operations > 50ms), otherwise leave synchronous. Always use async for I/O (database, files, network)."}}]}