Context capture, bounded concurrency, anti-patterns.

.NET 9 Async/Await — ConfigureAwait, Task.WhenEach, Concurrency

async/await in .NET compiles to a state machine that resumes on a captured SynchronizationContext or ThreadPool. ConfigureAwaitOptions (enum in .NET 8) replaces .ConfigureAwait(false). .NET 9's Task.WhenEach streams results as tasks complete. Channel<T> and SemaphoreSlim implement bounded concurrency. Understanding context capture, deadlocks, and CancellationToken propagation separates reliable async code from the fragile kind.

Interactive Pipeline Flow

FoundationCore LogicProduction

⚡ async Method — state machine

The compiler transforms async methods into a state machine class implementing IAsyncStateMachine. Each await becomes a state transition.

Architecture Diagrams

WhenAll fan-out vs WhenEach streaming vs Channel bounded concurrency.

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Key Design Decisions

🔥Never .Result or .Wait() in async context

Calling .Result or .Wait() on a Task in an async context will deadlock when a SynchronizationContext is present.

🌊WhenEach over WhenAll for streaming

Task.WhenEach (.NET 9) lets you process results as tasks complete. Ideal for showing partial results in a loop.

🏎️ValueTask for hot paths only

ValueTask is an optimisation for methods that are frequently synchronous (cache hit). Do not use by default — only measure then apply.

Discussion