Can Parallel.ForEach Effectively Utilize Asynchronous Lambda Expressions?

Parallelism and Asynchronous Lambdas: A Balancing Act

Parallel processing significantly boosts performance by distributing workloads across multiple cores. However, integrating asynchronous lambda expressions within parallel loops presents unique challenges.

The Problem: Lost Tasks

Consider this example:

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, async item =>
{
  // Pre-processing
  var response = await GetData(item);
  bag.Add(response);
  // Post-processing
});
var count = bag.Count; // count is often 0

The count frequently remains 0 because Parallel.ForEach's background threads don't wait for asynchronous operations to complete. A naive attempt to fix this:

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, item =>
{
  // Pre-processing
  var responseTask = GetData(item);
  responseTask.Wait();
  var response = responseTask.Result;
  bag.Add(response);
  // Post-processing
});
var count = bag.Count;

While this ensures task completion, it negates the advantages of asynchronous programming by blocking threads and requiring manual exception handling.

Effective Solutions

For straightforward parallel processing with asynchronous lambdas:

var bag = new ConcurrentBag<object>();
var tasks = myCollection.Select(async item =>
{
  // Pre-processing
  var response = await GetData(item);
  bag.Add(response);
  // Post-processing
});
await Task.WhenAll(tasks);
var count = bag.Count;

This approach creates a task for each item, enabling asynchronous execution. Task.WhenAll ensures all tasks finish before continuing.

For more intricate scenarios, Stephen Toub's blog post on ForEachAsync offers advanced techniques and best practices. This provides a more robust and efficient solution for handling asynchronous operations within parallel loops.

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