Async call within a lambda foreach is not awaited

Question

I have the following method that returns before it completes (method cut down for brevity):

private void ProcessAllItems(List<Item> items) { items.ForEach(async item => { var response = await Processor.IOBoundTaskAsync(item); // Some further processing on the response object. }); }

The Processor.IOBoundTaskAsync method returns a Task<SomeResponseObject>, so not a void.

So, what is happening?

I want the lambda foreach to process each item one at a time, so not in parallel. For the first iteration, the code goes into the IOBoundTaskAsync and gets as far as the first "await" (where it calls the first of 4 awaited web service methods) and then the calling method "ProcessAllItems" exits.

I'm only using test data, so my list only has 1 item in it.

What am I missing?

Community · Accepted Answer · 2017-05-23 11:53:49Z

List<T>.ForEach converts your async lambda into async void, as it accepts an Action<T>. Although it does that and that has some complications with exception handling, your ForEach call should work properly. If not, you might not be using the async pattern correctly. I would advise you to set a breakpoint after the first await and see the continuation executing.

I would advise you to use a regular foreach instead:

private async Task ProcessAllItemsAsync(List<Item> items) { foreach (var item in items) { var response = await Processor.IOBoundTaskAsync(item); // Some further processing on the response object. } }

Read How can I use Async with ForEach? for more.

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