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.NET 6 - Parallel.ForEachAsync

You maybe used Parallel.ForEach() before. It allows to iterate over a collection in a parallel way. It works similar to a Parallel.For loop. The loop partitions the source collection and schedules the work on multiple threads based  on the available processors in a system.

Unfortunately the Parallel.ForEach() cannot be used for asynchronous work.

Async vs parallel

It is important to understand that "async" and "parallel" are two different concepts. Although they are both related to concurrent programming, they serve different purposes and are used in different contexts.

Async is used to make non-blocking I/O operations and asynchronous code execution. It is primarily used for tasks that may take some time to complete, like reading from a file, making a network request, or performing database operations.

Parallel however refers to parallel programming, which is about executing multiple tasks or operations simultaneously to improve performance and take advantage of multi-core processors. Parallelism is used when you have independent, CPU-bound tasks that can be executed concurrently.


Although it should be clear by now that async and parallel are 2 different concepts, they can still be used together.

With .Net 6 we got the new Parallel.ForEachAsync method from the parallel library. Using this method you can now use the await key word inside of the action that you execute on each element in the list. Just like in ForEach it executes the actions in parallel but waits for all of them to finish.

More information

Write a simple parallel program using Parallel.ForEach - .NET | Microsoft Learn

Parallel.ForEachAsync Method (System.Threading.Tasks) | Microsoft Learn

Parallel.ForEachAsync Deep Dive :: Greg Bair

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