Task.WaitAny to accept Task instead of Task [ ]

Question

I have a collection of generic Tasks those are marked for execution. As and when a task complete (using Task.WaitAny), I am adding it to an ObservableCollection.

But, the issue is at Task.WaitAny(...) line, which says Co-variant array conversion from Task<MyType>[] to Task[] can cause run-time exception on write operation.

I am fairly knowledgeable on what this exception means and why it complains at this stage.

Question: Is there any generic version of Task.WaitAny(), which can take Task<T> as parameter instead of Task[].

Thanks in advance.

Possible duplicate of [Is there a generic Task.WaitAll?](http://stackoverflow.com/questions/10044099/is-there-a-generic-task-waitall) — chue x, Oct 07 '15 at 18:49
@chuex The answers to that questions are outdated. There is a better way now. — Jakub Lortz, Oct 07 '15 at 19:03
@Nair: Why are the tasks in an *observable collection* to begin with? Surely they aren't being data-bound... — Stephen Cleary, Oct 07 '15 at 19:11
@StephenCleary, This code is just a stripped version to demonstrate the issue. Now to answer your question, the result from individual task has to be bound to a control in WPF and intention is to have responsive UI. — S.N, Oct 07 '15 at 19:28
@Nair: You can't bind to `Task.Result` and have a responsive UI. — Stephen Cleary, Oct 07 '15 at 19:35

score 2 · Accepted Answer · answered Oct 07 '15 at 18:57

2

There is a generic Task.WhenAny

public static Task<Task<TResult>> WhenAny<TResult>(IEnumerable<Task<TResult>> tasks);
public static Task<Task<TResult>> WhenAny<TResult>(params Task<TResult>[] tasks);

await it to get the completed task.

answered Oct 07 '15 at 18:57

Jakub Lortz

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Appreciate your support. Sorry I am using .NET 4. – S.N Oct 07 '15 at 19:21
@Nair If you're using .NET 4 in VS 2012 or higher, you can get Microsoft.Bcl.Async package from nuget and use TaskEx.WhenAny. – Jakub Lortz Oct 07 '15 at 19:34

score 2 · Answer 2 · answered Oct 07 '15 at 19:07

2

You can use Task.WhenAny which in your case will even simplify your code like this

var result = new ObservableCollection<LenderScoreCard>();
var parallelScoreList = parallelScore.ToList();
while (parallelScoreList.Count != 0)
{
    var completedTask = Task.WhenAny(parallelScoreList).Result;
    result.Add(completedTask.Result);
    parallelScoreList.Remove(completedTask);
}

answered Oct 07 '15 at 19:07

Ivan Stoev

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Appreciate your support. Sorry I am using .NET 4. – S.N Oct 07 '15 at 19:21
@Nair: Np, then just ignore the warning - `Task.WaitAny` does not modify the passed array anyway. – Ivan Stoev Oct 07 '15 at 19:26

Task.WaitAny to accept Task instead of Task [ ]

2 Answers2