How do you process asynchronous work in parallel with Parallel.ForEachAsync in .NET 6+?

Short answer

It provides a built-in way to process a collection with async code, set a precise maximum concurrency with MaxDegreeOfParallelism, and support cancellation.

Explanation

The older Parallel.ForEach supported only synchronous code and blocked threads. Parallel.ForEachAsync was designed for async/await: it processes a large array while limiting how many async operations run at once (for example, eight in parallel), and accepts a CancellationToken to cancel immediately.

Code example

await Parallel.ForEachAsync(urls, new ParallelOptions { MaxDegreeOfParallelism = 8 }, async (url, ct) => {
    var content = await httpClient.GetStringAsync(url, ct);
    await ProcessContentAsync(content, ct);
});

Common mistakes

• Using old Parallel.ForEach with asynchronous code through async void in its delegate, so it does not wait for the operations to finish.

Follow-up questions

• How do you pass and handle CancellationToken through asynchronous call chains in ASP.NET Core?
• What is the fundamental difference between ThreadLocal<T> and AsyncLocal<T> in asynchronous .NET code?

Sources

• Microsoft Learn — Parallel.ForEachAsync Method