Async await in linq select

Question

I need to modify an existing program and it contains following code:

var inputs = events.Select(async ev => await ProcessEventAsync(ev))
                          


        

Answer 1

I wanted to call Select(...) but ensure it ran in sequence because running in parallel would cause some other concurrency problems, so I ended up with this. I cannot call .Result because it will block the UI thread.

public static class TaskExtensions
{
    public static async Task<IEnumerable<TResult>> SelectInSequenceAsync<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, Task<TResult>> asyncSelector)
    {
        var result = new List<TResult>();
        foreach (var s in source)
        {
            result.Add(await asyncSelector(s));
        }
        
        return result;
    }
}

Usage:

var inputs = events.SelectInSequenceAsync(ev => ProcessEventAsync(ev))
                   .Where(i => i != null)
                   .ToList();

I am aware that Task.WhenAll is the way to go when we can run in parallel.

Answer 2

With current methods available in Linq it looks quite ugly:

var tasks = items.Select(
    async item => new
    {
        Item = item,
        IsValid = await IsValid(item)
    });
var tuples = await Task.WhenAll(tasks);
var validItems = tuples
    .Where(p => p.IsValid)
    .Select(p => p.Item)
    .ToList();

Hopefully following versions of .NET will come up with more elegant tooling to handle collections of tasks and tasks of collections.

Answer 3

var inputs = events.Select(async ev => await ProcessEventAsync(ev))
                   .Select(t => t.Result)
                   .Where(i => i != null)
                   .ToList();

But this seems very weird to me, first of all the use of async and await in the select. According to this answer by Stephen Cleary I should be able to drop those.

The call to Select is valid. These two lines are essentially identical:

events.Select(async ev => await ProcessEventAsync(ev))
events.Select(ev => ProcessEventAsync(ev))

(There's a minor difference regarding how a synchronous exception would be thrown from ProcessEventAsync, but in the context of this code it doesn't matter at all.)

Then the second Select which selects the result. Doesn't this mean the task isn't async at all and is performed synchronously (so much effort for nothing), or will the task be performed asynchronously and when it's done the rest of the query is executed?

It means that the query is blocking. So it is not really asynchronous.

Breaking it down:

var inputs = events.Select(async ev => await ProcessEventAsync(ev))

will first start an asynchronous operation for each event. Then this line:

                   .Select(t => t.Result)

will wait for those operations to complete one at a time (first it waits for the first event's operation, then the next, then the next, etc).

This is the part I don't care for, because it blocks and also would wrap any exceptions in AggregateException.

and is it completely the same like this?

var tasks = await Task.WhenAll(events.Select(ev => ProcessEventAsync(ev)));
var inputs = tasks.Where(result => result != null).ToList();

var inputs = (await Task.WhenAll(events.Select(ev => ProcessEventAsync(ev))))
                                       .Where(result => result != null).ToList();

Yes, those two examples are equivalent. They both start all asynchronous operations (events.Select(...)), then asynchronously wait for all the operations to complete in any order (await Task.WhenAll(...)), then proceed with the rest of the work (Where...).

Both of these examples are different from the original code. The original code is blocking and will wrap exceptions in AggregateException.

Answer 4

I used this code:

public static async Task<IEnumerable<TResult>> SelectAsync<TSource,TResult>(this IEnumerable<TSource> source, Func<TSource, Task<TResult>> method)
{
      return await Task.WhenAll(source.Select(async s => await method(s)));
}

like this:

var result = await sourceEnumerable.SelectAsync(async s=>await someFunction(s,other params));

Answer 5

Existing code is working, but is blocking the thread.

.Select(async ev => await ProcessEventAsync(ev))

creates a new Task for every event, but

.Select(t => t.Result)

blocks the thread waiting for each new task to end.

In the other hand your code produce the same result but keeps asynchronous.

Just one comment on your first code. This line

var tasks = await Task.WhenAll(events...

will produce a single Task so the variable should be named in singular.

Finally your last code make the same but is more succinct

For reference: Task.Wait / Task.WhenAll

Answer 6

I prefer this as an extension method:

public static async Task<IEnumerable<T>> WhenAll<T>(this IEnumerable<Task<T>> tasks)
{
    return await Task.WhenAll(tasks);
}

So that it is usable with method chaining:

var inputs = await events
  .Select(async ev => await ProcessEventAsync(ev))
  .WhenAll()