Dotnet Csharp Interview Questions Scenario Based 2025 Interview Questions & Answers

Task: Always allocates; best for long-running async operations. ValueTask: Avoids allocation when result is synchronous; but dangerous if awaited twice. IAsyncEnumerable: Async streaming; uses await foreach and yields values over time.

.NET uses the Hill Climbing Algorithm: Measures throughput Adjusts thread count dynamically Increases threads until throughput drops Then finds an optimal stable point ThreadPool adapts to CPU cores, blocking calls, and workload patterns.

They eliminate unnecessary memory allocations: Work with stack/heap/native memory No copying of buffers High-performance parsing, serialization, pipelines Span cannot escape the stack ? improves safety & performance.

Because: Exceptions bypass the caller and crash the process Caller cannot await completion Execution becomes fire-and-forget Only valid for event handlers.

struct ? value type readonly struct ? immutable + avoids defensive copies ref struct ? must remain on stack (e.g., Span) record struct ? value semantics + synthesized equality

Generations: Gen0 ? short-lived objects Gen1 ? mid-lived Gen2 ? long-lived (full GC expensive) LOH: Objects > 85 KB Not compacted by default ? fragmentation .NET 5+ adds optional LOH compaction.

When using async/await: Compiler generates a hidden state machine struct Stores: locals awaiters continuation context Executes via MoveNext() Optimized heavily in .NET 7/8.

Common deadlock scenario: var x = Task.Run(() => DoWorkAsync()).Result; Because: UI/ASP.NET sync context waits Async method attempts to resume on same context Results in deadlock Fix: Use ConfigureAwait(false) Avoid .Result or .Wait().

Because: External callers may also lock the same reference Causes global deadlocks Exposes private synchronization Use: private readonly object _lock = new();

Singleton ? one instance for entire application Scoped ? one instance per web request Transient ? new instance every resolution DI container creates an object graph and disposes scoped services at request end.

DisposeAsync allows async cleanup (DB, IO, network). Works with: await using var x = new Resource(); Useful for pooled or high-latency resources.

Because: $"{a}{b}" creates: Hidden string builder Extra allocations For high-performance logging: logger.LogInformation("Value {a} {b}", a, b);

yield return: Synchronous iterator Lazily returns data IAsyncEnumerable: Async streaming Requires await foreach Ideal for large data sets over network/DB.

Reflection: Scans metadata Creates objects dynamically Costly per call Source Generators: Compile-time code generation Zero reflection Used in System.Text.Json, DI, gRPC.

Custom struct overriding Equals() incorrectly: Ignoring fields Not overriding GetHashCode() Mutable fields changing after insert in dictionary.

ASP.NET Core does NOT use SynchronizationContext. It uses TaskScheduler.Default ? no thread affinity. Prevents deadlocks typical in MVC/WebForms.

Uses libuv (older) or managed sockets (newer) Zero-copy pipeline I/O polling Pooled buffers Single writer per connection Multiple event loops Designed for millions of requests.

Span-based parsing No boxing/unboxing Source-generated serializers Fewer virtual calls Less reflection Less memory churn.

IHostedService: Background process inside ASP.NET Core host Worker Service: Template for standalone background apps Uses generic host Ideal for schedulers, queues, Kafka, RabbitMQ.

Causes: Too many blocking calls Long-running sync code on threadpool Misconfigured MinThreads Tasks queued faster than executed Diagnosis: dotnet-trace PerfView Visual Studio Concurrency.