How to aggregate results of an IAsyncEnumerable

Question

I want to ask if there is a plan or exists a method to aggregate the return(s) of an IAsyncEnumerable? So given the below method why is there no terse way to aggregate its result?

public async IAsyncEnumerable<bool> GenerateAsyncEnumerable(int range)
{
     foreach (var x in Enumerable.Range(0, range))
     {
           await Task.Delay(500 * x);
           yield return x % 2 == 0;
     }
}

Current Scenario

public async Task Main()
{

    bool accum = true;
    await foreach (var item in GenerateAsyncEnumerable(3))
    {
         accum &= item;
         //have some side effects on each particular item of the sequence
    }
    Console.WriteLine($"Accumulator:{accum}");
}

Desired Scenario

I would like to aggregate the results of the IAsyncEnumerable given a custom aggregation Func

public async Main()
{

    bool result = await foreach (var item in GenerateAsyncEnumerable(3).Aggregate(true, (x, y) => x & y))
    {
        //have some side effects on each particular item of the sequence
    }
}

P.S I do not like (in the first scenario) that I have to add an extra local variable accum that collects the results of the enumerable. Have I missed something, is there some syntactic sugar that I am not aware of ?

You could write your own extension method that returns a new `IAsyncEnumerable`? — DavidG, Feb 25 '20 at 11:08
All those methods are part of `System.Linq.Async`, including [AggregateAsync](https://github.com/dotnet/reactive/blob/master/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Aggregate.cs). — Panagiotis Kanavos, Feb 25 '20 at 12:54
As @PanagiotisKanavos mentioned there are extensions methods under in library `System.Linq.Async`. One of them is `ToAsyncEnumerable` but exercise caution when using it. See https://stackoverflow.com/questions/59956623/using-iasyncenumerable-with-dapper — Ankit Vijay, Feb 25 '20 at 12:59

score 4 · Accepted Answer · answered Feb 25 '20 at 12:58

4

You could use the AggregateAsync method, from the System.Linq.Async package:

bool result = await GenerateAsyncEnumerable(3).AggregateAsync(true, (x, y) => x & y);
Console.WriteLine($"Result: {result}");

Output:

Result: False

answered Feb 25 '20 at 12:58

Theodor Zoulias

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score 3 · Answer 2 · answered Feb 25 '20 at 13:07

The System.Linq.Async package developed by the ReactiveX team provides LINQ operators for IAsyncEnumerable that are equivalent to those LINQ provides for IEnumerable.

This includes the common operators like Select(), Where(), Take() etc. Aggregate is implemented by AggregateAsync.

The overloads are similar to Enumerable.Aggregate , which means you can write:

bool result=await GenerateAsyncEnumerable(3).AggregateAsync(true, (x, y) => x & y);

AggregateAsync is named this way because it consumes the entire enumerable and produces a single result. It needs an await call to work. Other operators like Select though, accept an IAsyncEnumerable and produce a new one. There's no need to await them.

You can use this naming convention to find the Linq.Async operators you need based on the name of the equivalent Enumerable operator

How to aggregate results of an IAsyncEnumerable

2 Answers2

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