ForEach Async Woes

Software Engineering

I was investigating a bug last week when I stumbled upon a block of code that looked like this:

public async Task Method(List<Animal> collection)
{ 
    // do something async

    collection.ForEach(async v => await _mediator.Send(new Command(v.Id), cancellationToken));
}

I wrote this without thinking, I very rarely use List.ForEach() for enumerating through a list, but for some reason I did here. I probably got carried away with the because I can, rather than because I should.

Unfortunately this code is broken, it does not work the way I expected. Because there is no await on the collection.ForEach... execution continues on past and the method exits.

Switching to an old school foreach(...) loop and awaiting inside in each enumeration will solve this problem.

public async Task Method(List<Animal> collection)
{
    // do something async
    
    foreach(var v in collection)
    {
        await _mediator.Send(new Command(v.Id), cancellationToken);
    }
}