Async streams and IAsyncDisposable
IAsyncEnumerable pipelines, await using, and backpressure between async producers and consumers.
Async streams and IAsyncDisposable
IAsyncEnumerable<T> is the async IEnumerable: items arrive over time,
and await foreach consumes them. An async iterator combines yield return
with await:
static async IAsyncEnumerable<Trade> StreamAsync(
IAsyncEnumerable<string> lines, [EnumeratorCancellation] CancellationToken ct = default)
{
await foreach (var line in lines.WithCancellation(ct).ConfigureAwait(false))
if (Trade.TryParse(line, out var t)) yield return t;
}
Backpressure is inherent: the consumer pulls; the producer only produces
when pulled (unlike Task-based fan-out, which produces as fast as it can).
For bounded buffering semantics, compose streams with Channel<T>
(Module 13).
IAsyncDisposable + await using close the lifecycle for resources whose
release is itself asynchronous (network connections, streams, DB readers):
await using var conn = await pool.RentAsync(ct);
// DisposeAsync runs on scope exit — even on exception
The trap: DisposeAsync is NOT awaited if you use using instead of
await using — the compiler inserts a sync-over-async wait, or worse,
fire-and-forget semantics depending on the type. And ConfigureAwait(false)
belongs on the await foreach line, applied to the enumerator, not the
items.