C# Async and Concurrency

Apply these rules when adding or changing asynchronous APIs, background work, or concurrent processing.

Use asynchronous APIs for genuinely asynchronous I/O; do not wrap synchronous work in unnecessary tasks.
Keep asynchronous operations async through the call chain. Avoid .Result, .Wait(), and other synchronous waits that can block threads or deadlock.
Accept and propagate CancellationToken through work that can take meaningful time or be cancelled by its caller.
Bound concurrency for large or unbounded workloads. Avoid creating a task per item or using uncontrolled Task.WhenAll; use an appropriate queue or concurrency limiter when work must be throttled.
Use ValueTask only when profiling supports it and its usage contract is clear; otherwise prefer Task.
Keep shared state safe under concurrent access and preserve cancellation and exception behavior when coordinating tasks. Prefer immutable messages or synchronization primitives with clear ownership over unsynchronized shared mutation.
Avoid holding locks across await, blocking thread-pool threads, and starting background work whose lifetime is detached from its owner.
For producer/consumer work, apply backpressure and bound buffered items so a fast producer cannot exhaust memory.
Retry only transient failures, with bounded attempts and delay. Preserve idempotency and cancellation, and avoid multiplying retries across nested layers.