How to assign the results of WaitAll tasks to variables [duplicate]

Question

I have a situation like this:

var retrievalTasks = new Task[2];

retrievalTasks[0] = GetNodesAsync();

retrievalTasks[1] = GetAssetsToHandleAsync();

Task.WaitAll(retrievalTasks);

And I would like the result of retrievalTasks[0] and retrievalTasks[1] to be stored in variables.

I could achieve this with:

 var a = await GetNodesAsync();
 var b = await GetAssetsToHandleAsync();;

But I would rather not await both like that, because then they're not fired at the same time right? Or am I misunderstanding?

I've seen this as a reference: Awaiting multiple Tasks with different results

But I think this is a slightly different scenario that wouldn't work in my case.

Any ideas?

Thanks

EDIT:

await Task.WhenAll(catTask, houseTask, carTask);

var cat = await catTask;

var house = await houseTask;

var car = await carTask;

This just seemed to wait four times to get the three results. However, @armenm has shown how to avoid that.

Bibliolatry

Return type of both the functions should be same. – Mitchelmitchell

What makes you think the one you've found doesn't work in your case? – Takeshi

Because the answer is waiting all individually, i.e. await a, await b – Bibliolatry

which makes me think they won't be fired at the same time – Bibliolatry

That's just syntactic stuff at that point, they've been awaited in the WhenAll, in practice all the following awaits will run synchronously – Takeshi

so are they awaiting twice in that example, or is it because it's already awaited it won't make a difference? – Bibliolatry

Read through the comments etc under the answers - they go into more detail than is worth repeating here. If you still think it doesn't work for you, please edit your question with reasoning as to why not. – Takeshi

I read through those comments, but it just looked like it would still be different. Maybe it's just my misunderstanding. However, this answer below is more suited to what I was attempting I think. – Bibliolatry

nuget.org/packages/TaskTupleAwaiter – Beaudoin

Answer 1

Here we go, you need to have separate variables for the tasks having respective types:

var task1 = GetNodesAsync();
var task2 = GetAssetsToHandleAsync();

Task.WaitAll(new Task[] { task1, task2 });

var result1 = task1.Result;
var result2 = task2.Result;

But I would recommend to make it asynchronous:

var task1 = GetNodesAsync();
var task2 = GetAssetsToHandleAsync();

await Task.WhenAll(new Task[] { task1, task2 });

var result1 = task1.Result;
var result2 = task2.Result;

Iguanodon

Thank you. I was trying to use result before. But I think the way I had it set up wouldn't allow it. – Bibliolatry

Answer 2

You can also wrap the methods in Task.Run to avoid using task variables. For example:

private async Task DoStuff()
{
    int i1, i2;

    await Task.WhenAll(
        Task.Run(async () => i1 = await Calc1Async()),
        Task.Run(async () => i2 = await Calc2Async()));
}

Menstruum

Why choose to reopen a 6 year old question with a shockingly wrong answer? – Turret

@Turret questions here are not closed after a period of inactivity. This is not Microsoft's GitHub. It's perfectly fine to answer a 15 years old question. – Dusky

@Turret also would you like to explain why you think that this answer is wrong? – Dusky

@TheodorZoulias It makes no sense to add Task.Run as a wrapper around method calls that are already async. The original question doesn't even state that the return types are integers. – Turret

@Turret you said that the answer is wrong. Shockingly wrong to be precise. The Task.Run doesn't affect the correctness of the operation. At least not in general. There are only a couple of cases that wrapping the asynchronous calls in Task.Run would compromise the correctness of the operation, namely the Calc1Async and Calc2Async including non-thread-safe side-effects, or including interactions with thread-affine components. The Task.Run could (potentially) affect the performance of the combined operation in a positive way. I see nothing shockingly wrong with this answer. – Dusky