ForEachAsync with Result

Question

I'm trying to change Stephen Toub's ForEachAsync<T> extension method into an extension which returns a result...

Stephen's extension:

public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body) 
{ 
    return Task.WhenAll( 
        from partition in Partitioner.Create(source).GetPartitions(dop) 
        select Task.Run(async delegate { 
            using (partition) 
                while (partition.MoveNext()) 
                    await body(partition.Current); 
        })); 
}

My approach (not working; tasks get executed but result is wrong)

public static Task<TResult[]> ForEachAsync<T, TResult>(this IList<T> source,
    int degreeOfParallelism, Func<T, Task<TResult>> body)
{
    return Task.WhenAll<TResult>(
        from partition in Partitioner.Create(source).GetPartitions(degreeOfParallelism)
        select Task.Run<TResult>(async () = 
        {
            using (partition)
                while (partition.MoveNext())
                    await body(partition.Current); // When I "return await",
                        // I get good results but only one per partition 
            return default(TResult);
        }));
}

I know I somehow have to return (WhenAll?) the results from the last part but I didn't yet figure out how to do it...

Update: The result I get is just degreeOfParallelism times null (I guess because of default(TResult)) even though all the tasks get executed. I also tried to return await body(...) and then the result was fine, but only degreeOfParallelism number of tasks got executed.

Answer 1

Your LINQ query can only ever have the same number of results as the number of partitions - you're just projecting each partition into a single result.

If you don't care about the order, you just need to assemble the results of each partition into a list, then flatten them afterwards.

public static async Task<TResult[]> ExecuteInParallel<T, TResult>(this IList<T> source, int degreeOfParalleslism, Func<T, Task<TResult>> body)
{
    var lists = await Task.WhenAll<List<TResult>>(
        Partitioner.Create(source).GetPartitions(degreeOfParalleslism)
            .Select(partition => Task.Run<List<TResult>>(async () =>
                    {
                        var list = new List<TResult>();
                        using (partition)
                        {
                            while (partition.MoveNext())
                            {
                                list.Add(await body(partition.Current));
                            }
                        }
                        return list;
                   })));
     return lists.SelectMany(list => list).ToArray();
}

(I've renamed this from ForEachAsync, as ForEach sounds imperative (suitable for the Func<T, Task> in the original) whereas this is fetching results. A foreach loop doesn't have a result - this does.)

Answer 2

Now that the Parallel.ForEachAsync API has become part of the standard libraries (.NET 6), it makes sense to implement a variant that returns a Task<TResult[]>, based on this API. Here is an implementation:

public static Task<TResult[]> ForEachAsync<TSource, TResult>(
    this IEnumerable<TSource> source,
    ParallelOptions parallelOptions,
    Func<TSource, CancellationToken, ValueTask<TResult>> body)
{
    var results = new List<TResult>();
    if (source.TryGetNonEnumeratedCount(out int count)) results.Capacity = count;
    var withIndexes = source.Select((item, index) => (item, index));
    return Parallel.ForEachAsync(withIndexes, parallelOptions, async (entry, ct) =>
    {
        var (item, index) = entry;
        var result = await body(item, ct).ConfigureAwait(false);
        lock (results)
        {
            while (results.Count <= index) results.Add(default);
            results[index] = result;
        }
    }).ContinueWith(t =>
    {
        if (t.IsCanceled)
        {
            // Propagate the correct token
            CancellationToken ct = default;
            try { t.GetAwaiter().GetResult(); }
            catch (OperationCanceledException oce) { ct = oce.CancellationToken; }
            return Task.FromCanceled<TResult[]>(ct);
        }
        if (t.IsFaulted)
        {
            var tcs = new TaskCompletionSource<TResult[]>();
            tcs.SetException(t.Exception.InnerExceptions);
            return tcs.Task;
        }
        lock (results) return Task.FromResult(results.ToArray());
    }, default, TaskContinuationOptions.DenyChildAttach, TaskScheduler.Default)
        .Unwrap();
}

This implementation supports all the options and the functionality of the Parallel.ForEachAsync overload that has an IEnumerable<T> as source. Its behavior in case of errors and cancellation is identical. The results are arranged in the same order as the associated elements in the source sequence.