How to WhenAll when some tasks can be null?

Question

I would like to wait all task, but some of them can be null. It is a code like that:

Task<MyType1> myTask1 = getData01Async();
Task<MyTyp2> myTask2 = null;
Task<MyType3> myTask3 = null;

if(myVariable == true)
{
    myTask2 = getData02Async();
}
else
{
    myTask3 = getData03Async();
}


wait Task.WhenAll(myTask1, myTask2, myTask3);

The idea is, task1 always exists, but task2 and task3 depends of a variable. So I would like to run all the task in parallel and wait when all of them are finished. And if one task is null, the treat it as it is finished.

The problem with this code is that I get a null reference exception when it runs.

There are some way to do that? Or what another alternatives could I use?

How about putting the tasks into an explicit collection and using `myTasks.Where(t => t!=null)` to pass to `Task.WhenAll`? — Uwe Keim, Jun 11 '18 at 14:47
@Fabjan meh, since they're all different `T` in the `Task` sense, that won't matter - it is the `params Task[]` overload, and `Task.CompletedTask` is definitely a `Task` — Marc Gravell, Jun 11 '18 at 15:01
@MarcGravell I only meant that it's not really possible to do this assignment: `Task myTask1 = Task.CompletedTask;` — Fabjan, Jun 11 '18 at 15:04
@UweKeim That's exactly what I was thinking, could make a good answer. — JSteward, Jun 11 '18 at 15:35

score 17 · Answer 1 · answered Jun 11 '18 at 16:28

17

Just filter the null tasks out:

await Task.WhenAll(new Task[] { task1, task2, task3 }.Where(i => i != null));

answered Jun 11 '18 at 16:28

armenm

922
6
9

Was this meant to be different than the answer I gave? – JSteward Jun 11 '18 at 16:37
@JSteward I didn't look at your answer (the purpose of mine was to make it short so I didn't dig the long ones), but yes - it's the same. – armenm Jun 11 '18 at 17:08
Fair enough, just had to ask – JSteward Jun 11 '18 at 17:09

Fabjan · Answer 2 · 2020-05-14T07:23:09.997

11

Well, depending on your scenario you could assign completed tasks or put them in an array/list and then pass this list to WhenAll:

Task<MyType1> myTask1 = getData01Async();
Task<MyType2> myTask2 = Task.FromResult((MyType2) null);
Task<MyType3> myTask3 = Task.FromResult((MyType3) null);

...

await Task.WhenAll(myTask1, myTask2, myTask3);

edited May 14 '20 at 07:23

answered Jun 11 '18 at 14:51

Fabjan

13,506
4
25
52

1

Thanks, This really helped me out. I had multiple tasks with different result types of which I was not sure the tasks should trigger. And with this solution I could still work with the results without worrying about what type the generic Task returned when adding everything to a list! – gogessj4 Jun 12 '19 at 15:51
The only non-obvious suggestion. Thanks! – Andriy Volkov Oct 29 '19 at 15:22
1

This is the best solution. Very easy to just do myTask = x != null ? MyAsync() : Task.CompletedResult instead of creating arrays and filtering on them. – Erndob May 14 '20 at 04:56

score 7 · Accepted Answer · answered Jun 11 '18 at 14:50

Use a collection to track the tasks that aren't null. Then pass that list to Task.WhenAll method like below:

var tasks = new List<Task>();

Task<MyType1> myTask1 = getData01Async();
tasks.Add(myTask1);

Task<MyTyp2> myTask2 = null;
Task<MyType3> myTask3 = null;

if(myVariable == true)
{
    myTask2 = getData02Async();
    tasks.Add(myTask2);
}
else
{
    myTask3 = getData03Async();
    tasks.Add(myTask3);
}

await Task.WhenAll(tasks);

JSteward · Answer 4 · 2018-06-11T16:32:54.423

3

To build on what @UweKeim suggested, why not simply filter out the null tasks when you pass them to the WhenAll.

public async Task FilterNullTasks()
{
    Task<string> myTask1 = Task.Delay(1000).ContinueWith(tsk => string.Empty);
    Task<int> myTask2 = null;
    Task<bool> myTask3 = null;

    await Task.WhenAll(new Task[]
    {
        myTask1, myTask2, myTask3
    }.Where(tsk => tsk != null));
}

edited Jun 11 '18 at 16:32

answered Jun 11 '18 at 16:20

JSteward

6,833
2
21
30

How to WhenAll when some tasks can be null?

4 Answers4

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