Why Task.WhenAll can crash services when processing 10,000 items concurrently:

Task.WhenAll spawns all 10,000 tasks immediately, overwhelming downstream databases and triggering HTTP 429 rate limits.

Controlled Concurrency with Parallel.ForEachAsync:

await Parallel.ForEachAsync(itemUrls, new ParallelOptions
{
    MaxDegreeOfParallelism = 8, // Run at most 8 tasks concurrently
    CancellationToken = cancellationToken
}, async (url, ct) =>
{
    await DownloadAndProcessAsync(url, ct);
});

Maintains a steady pool of active workers with built-in cancellation and error aggregation!

Parallel.ForEachAsync in .NET 6+ completely replaced our manual SemaphoreSlim throttling boilerplate.

Handles task partition balancing across thread pool workers automatically.