How can I asynchronously transform one IEnumerable to another, just like LINQ's Select(), but using await on every transformed item?

Question

Consider this situation:

class Product { }

interface IWorker
{
    Task<Product> CreateProductAsync();
}

I am now given an IEnumerable<IWorker> workers and am supposed to create an IEnumerable<Product> from it that I have to pass to some other function that I cannot alter:

void CheckProducts(IEnumerable<Product> products);

This methods needs to have access to the entire IEnumerable<Product>. It is not possible to subdivide it and call CheckProducts on multiple subsets.

One obvious solution is this:

CheckProducts(workers.Select(worker => worker.CreateProductAsync().Result));

But this is blocking, of course, and hence it would only be my last resort. Syntactically, I need precisely this, just without blocking.

I cannot use await inside of the function I'm passing to Select() as I would have to mark it as async and that would require it to return a Task itself and I would have gained nothing. In the end I need an IEnumerable<Product> and not an IEnumerable<Task<Product>>.

It is important to know that the order of the workers creating their products does matter, their work must not overlap. Otherwise, I would do this:

async Task<IEnumerable<Product>> CreateProductsAsync(IEnumerable<IWorker> workers)
{
    var tasks = workers.Select(worker => worker.CreateProductAsync());
    return await Task.WhenAll(tasks);
}

But unfortunately, Task.WhenAll() executes some tasks in parallel while I need them executed sequentially.

Here is one possibility to implement it if I had an IReadOnlyList<IWorker> instead of an IEnumerable<IWorker>:

async Task<IEnumerable<Product>> CreateProductsAsync(IReadOnlyList<IWorker> workers)
{
    var resultList = new Product[workers.Count];
    for (int i = 0; i < resultList.Length; ++i)
        resultList[i] = await workers[i].CreateProductAsync();
    return resultList;
}

But I must deal with an IEnumerable and, even worse, it is usually quite huge, sometimes it is even unlimited, yielding workers forever. If I knew that its size was decent, I would just call ToArray() on it and use the method above.

The ultimate solution would be this:

async Task<IEnumerable<Product>> CreateProductsAsync(IEnumerable<IWorker> workers)
{
    foreach (var worker in workers)
        yield return await worker.CreateProductAsync();
}

But yield and await are incompatible as described in this answer. Looking at that answer, would that hypothetical IAsyncEnumerator help me here? Does something similar meanwhile exist in C#?

A summary of the issues I'm facing:

• I have a potentially endless IEnumerable<IWorker>
• I want to asynchronously call CreateProductAsync() on each of them in the same order as they are coming in
• In the end I need an IEnumerable<Product>

A summary of what I already tried, but doesn't work:

• I cannot use Task.WhenAll() because it executes tasks in parallel.
• I cannot use ToArray() and process that array manually in a loop because my sequence is sometimes endless.
• I cannot use yield return because it's incompatible with await.

Does anybody have a solution or workaround for me? Otherwise I will have to use that blocking code...

Answer 1

IEnumerator<T> is a synchronous interface, so blocking is unavoidable if CheckProducts enumerates the next product before the next worker has finished creating the product.

Nevertheless, you can achieve parallelism by creating products on another thread, adding them to a BlockingCollection<T>, and yielding them on the main thread:

static IEnumerable<Product> CreateProducts(IEnumerable<IWorker> workers)
{
    var products = new BlockingCollection<Product>(3);

    Task.Run(async () => // On the thread pool...
    {
        foreach (IWorker worker in workers)
        {
            Product product = await worker.CreateProductAsync(); // Create products serially.
            products.Add(product); // Enqueue the product, blocking if the queue is full.
        }

        products.CompleteAdding(); // Notify GetConsumingEnumerable that we're done.
    });

    return products.GetConsumingEnumerable();
}

To avoid unbounded memory consumption, you can optionally specify the capacity of the queue as a constructor argument to BlockingCollection<T>. I used 3 in the code above.

Answer 2

The Situation:

Here you're saying you need to do this synchronously, because IEnumerable doesn't support async and the requirements are you need an IEnumerable<Product>.

I am now given an IEnumerable workers and am supposed to create an IEnumerable from it that I have to pass to some other function that I cannot alter:

Here you say the entire product set needs to be processed at the same time, presumably making a single call to void CheckProducts(IEnumerable<Product> products).

This methods needs to check the entire Product set as a whole. It is not possible to subdivide the result.

And here you say the enumerable can yield an indefinite number of items

But I must deal with an IEnumerable and, even worse, it is usually quite huge, sometimes it is even unlimited, yielding workers forever. If I knew that its size was decent, I would just call ToArray() on it and use the method above.

So lets put these together. You need to do asynchronous processing of an indefinite number of items within a synchronous environment and then evaluate the entire set as a whole... synchronously.

The Underlying Problems:

• 1: To evaluate a set as a whole, it must be completely enumerated. To completely enumerate a set, it must be finite. Therefore it is impossible to evaluate an infinite set as a whole.
• 2: Switching back and forth between sync and async forces the async code to run synchronously. that might be ok from a requirements perspective, but from a technical perspective it can cause deadlocks (maybe unavoidable, I don't know. Look that up. I'm not the expert).

Possible Solutions to Problem 1:

• 1: Force the source to be an ICollection<T> instead of IEnumerable<T>. This enforces finiteness.
• 2: Alter the CheckProducts algorithm to process iteratively, potentially yielding intermediary results while still maintaining an ongoing aggregation internally.

Possible Solutions to Problem 2:

• 1: Make the CheckProducts method asynchronous.
• 2: Make the CreateProduct... method synchronous.

Bottom Line

You can't do what you're asking how you're asking, and it sounds like someone else is dictating your requirements. They need to change some of the requirements, because what they're asking for is (and I really hate using this word) impossible. Is it possible you have misinterpreted some of the requirements?

Answer 3

From your requirements I can put together the following:

1) Workers processed in order

2) Open to receive new Workers at any time

So using the fact that a dataflow TransformBlock has a built in queue and processes items in order. Now we can accept Workers from the producer at any time.

Next we make the result of the TransformBlockobservale so that the consumer can consume Products on demand.

Made some quick changes and started the consumer portion. This simply takes the observable produced by the Transformer and maps it to an enumerable that yields each product. For background here is the ToEnumerable().

The ToEnumerator operator returns an enumerator from an observable sequence. The enumerator will yield each item in the sequence as it is produced

Source

using System;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace ClassLibrary1
{
    public class WorkerProducer
    {
        public async Task ProduceWorker()
        {
            //await ProductTransformer_Transformer.SendAsync(new Worker())
        }
    }

    public class ProductTransformer
    {
        public IObservable<Product> Products { get; private set; }
        public TransformBlock<Worker, Product> Transformer { get; private set; }

        private Task<Product> CreateProductAsync(Worker worker) => Task.FromResult(new Product());

        public ProductTransformer()
        {
            Transformer = new TransformBlock<Worker, Product>(wrk => CreateProductAsync(wrk));
            Products = Transformer.AsObservable();
        }
    }

    public class ProductConsumer
    {
        private ThirdParty ThirdParty { get; set; } = new ThirdParty();
        private ProductTransformer Transformer { get; set; }

        public ProductConsumer()
        {
            ThirdParty.CheckProducts(Transformer.Products.ToEnumerable());  
        }

    public class Worker { }
    public class Product { }

    public class ThirdParty
    {
        public void CheckProducts(IEnumerable<Product> products)
        {
        }
    }
}

Answer 4

Two ideas for you OP

Multiple call solution

If you are allowed to call CheckProducts more than once, you could simply do this:

foreach (var worker in workers)
{
    var product = await worker.CreateProductAsync();
    CheckProducts(new [] { product } );
}

If it adds value, I'm pretty sure you could work out a way to do it in batches of, say, 100 at a time, too.

Thread pool solution

If you are not allowed to call CheckProducts more than once, and not allowed to modify CheckProducts, there is no way to force it to yield control and allow other continuations to run. So no matter what you do, you cannot force asynchronousness into the IEnumerable that you pass to it, not just because of the compiler checking, but because it would probably deadlock.

So here is a thread pool solution. The idea is to create one separate thread to process the products in series; the processor is async, so a call to CreateProductAsync() will still yield control to anything else that has been posted to the synchronization context, as needed. However it can't magically force CheckProduct to give up control, so there is still some possibility that it will block occasionally if it is able to check products faster than they are created. In my example I'm using Monitor.Wait() so the O/S won't schedule the thread until there is something waiting for it. You'll still be using up a thread resource while it blocks, but at least you won't be wasting CPU time in a busy-wait loop.

public static IEnumerable<Product> CreateProducts(IEnumerable<Worker> workers)
{
    var queue = new ConcurrentQueue<Product>();
    var task = Task.Run(() => ConvertProducts(workers.GetEnumerator(), queue));
    while (true)
    {
        while (queue.Count > 0)
        {
            Product product;
            var ok = queue.TryDequeue(out product);
            if (ok) yield return product;
        }
        if (task.IsCompleted && queue.Count == 0) yield break;
        Monitor.Wait(queue, 1000);
    }
}

private static async Task ConvertProducts(IEnumerator<Worker> input, ConcurrentQueue<Product> output)
{
    while (input.MoveNext())
    {
        var current = input.Current;
        var product = await current.CreateProductAsync();
        output.Enqueue(product);
        Monitor.Pulse(output);
    }
}

Answer 5

Unless I misunterstood something, I don't see why you don't simply do it like this:

var productList = new List<Product>(workers.Count())
foreach(var worker in workers)
{
    productList.Add(await worker.CreateProductAsync());
}
CheckProducts(productList);

What about if you simply keep clearing a List of size 1?

var productList = new List<Product>(1);
var checkTask = Task.CompletedTask;
foreach(var worker in workers)
{
    await checkTask;
    productList.Clear();
    productList.Add(await worker.CreateProductAsync());
    checkTask = Task.Run(CheckProducts(productList));
}
await checkTask;

Answer 6

You can use Task.WhenAll, but instead of returning result of Task.WhenAll, return collection of tasks transformed to the collection of results.

async Task<IEnumerable<Product>> CreateProductsAsync(IEnumerable<IWorker> workers)
{
    var tasks = workers.Select(worker => worker.CreateProductAsync()).ToList();
    await Task.WhenAll(tasks);

    return tasks.Select(task => task.Result);
}

Order of tasks will be persisted.

And seems like should be ok to go with just return await Task.WhenAll()
From docs of Task.WhenAll Method (IEnumerable>)

The Task.Result property of the returned task will be set to an array containing all of the results of the supplied tasks in the same order as they were provided...

If workers need to be executed one by one in the order they were created and based on requirement that another function need whole set of workers results

async Task<IEnumerable<Product>> CreateProductsAsync(IEnumerable<IWorker> workers)
{
    var products = new List<product>();
    foreach (var worker in workers)
    {
        product = await worker.CreateProductAsync();
        products.Add(product);
    }

    return products;
}

Answer 7

You can do this now with async, IEnumerable and LINQ but every method in the chain after the async would be a Task<T>, and you need to use something like await Task.WhenAll at the end. You can use async lambdas in the LINQ methods, which return Task<T>. You don't need to wait synchronously in these.

The Select will start your tasks sequentially i.e. they won't even exist as tasks until the select enumerates each one, and won't keep going after you stop enumerating. You could also run your own foreach over the enumerable of tasks if you want to await them all individually.

You can break out of this like any other foreach without it starting all of them, so this will also work on an infinite enumerable.

public async Task Main()
{
    // This async method call could also be an async lambda
    foreach (var task in GetTasks())
    {
        var result = await task;
        Console.WriteLine($"Result is {result}");
        if (result > 5) break;
    }
}

private IEnumerable<Task<int>> GetTasks()
{
    return GetNumbers().Select(WaitAndDoubleAsync);
}

private async Task<int> WaitAndDoubleAsync(int i)
{
    Console.WriteLine($"Waiting {i} seconds asynchronously");
    await Task.Delay(TimeSpan.FromSeconds(i));
    return i * 2;
}

/// Keeps yielding numbers
private IEnumerable<int> GetNumbers()
{
    var i = 0;
    while (true) yield return i++;
}

Outputs, the following, then stops:

Waiting 0 seconds asynchronously
Result is 0
Waiting 1 seconds asynchronously
Result is 2
Waiting 2 seconds asynchronously
Result is 4
Waiting 3 seconds asynchronously
Result is 6

The important thing is that you can't mix yield and await in the same method, but you can yield Tasks returned from a method that uses await absolutely fine, so you can use them together just by splitting them into separate methods. Select is already a method that uses yield, so you may not need to write your own method for this.

In your post you were looking for a Task<IEnumerable<Product>>, but what you can actually use is a IEnumerable<Task<Product>>.

You can go even further with this e.g. if you had something like a REST API where one resource can have links to other resources, like if you just wanted to get a list of users of a group, but stop when you found the user you were interested in:

public async Task<IEnumerable<Task<User>>> GetUserTasksAsync(int groupId)
{
    var group = await GetGroupAsync(groupId);
    return group.UserIds.Select(GetUserAsync);
}

foreach (var task in await GetUserTasksAsync(1))
{
    var user = await task;
    ...
}

Answer 8

There is no solution to your problem. You can't transform a deferred IEnumerable<Task<Product>> to a deferred IEnumerable<Product>, such that the consuming thread will not get blocked while enumerating the IEnumerable<Product>. The IEnumerable<T> is a synchronous interface. It returns an enumerator with a synchronous MoveNext method. The MoveNext returns bool, which is not an awaitable type. An asynchronous interface IAsyncEnumerable<T> exists, whose enumerator has an asynchronous MoveNextAsync method, with a return type of ValueTask<bool>. But you have explicitly said that you can't change the consuming method, so you are stuck with the IEnumerable<T> interface. No solution then.

Answer 9

try

workers.ForEach(async wrkr => 
{
    var prdlist = await wrkr.CreateProductAsync();
    //Remaing tasks....
});