How Can I Properly Await Asynchronous Operations within a Parallel.ForEach Loop in C#?

Parallel Asynchronous Operations with Parallel.ForEach in C#

Improving application performance often involves parallel processing. However, combining asynchronous operations (async/await) with parallel loops like Parallel.ForEach presents unique challenges. This article addresses these challenges and provides solutions for correctly awaiting asynchronous tasks within a parallel loop.

The Problem: Asynchronous Tasks and Parallel.ForEach

The async keyword in C# enables asynchronous programming, returning a Task object. The problem arises when using async methods within Parallel.ForEach because background threads don't inherently wait for asynchronous tasks to finish before the loop completes.

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!

count frequently returns 0 because the main thread proceeds before the background threads complete their asynchronous operations.

Solution: Using Select and Task.WhenAll

A robust solution involves Select to create a collection of Task objects and Task.WhenAll to wait for all tasks to finish:

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; // count will be correct

This approach initiates asynchronous operations concurrently using Select and then uses Task.WhenAll to ensure all tasks complete before accessing the bag.

Advanced Solutions: ForEachAsync

For more intricate scenarios, Stephen Toub's blog post on ForEachAsync provides a more sophisticated and adaptable solution:

https://www.php.cn/link/33edf41c0becd0d57c35c4e27276617b

This offers a more controlled and efficient method for handling parallel asynchronous operations, especially when dealing with potential exceptions or cancellation. It provides a more robust and flexible approach compared to the simple Select/Task.WhenAll method.

The above is the detailed content of How Can I Properly Await Asynchronous Operations within a Parallel.ForEach Loop in C#?. For more information, please follow other related articles on the PHP Chinese website!