Return Task instantly

Question

I have a list of tasks, which i'd like to wait for. I'm waiting like

    await TaskEx.WhenAll(MyViewModel.GetListOfTasks().ToArray());

MyViewModel.GetListOfTasks() returns List of Task:

    var tasksList = new List<Task>();
    foreach (var item in Items)
    {                
        tasksList.Add(item.MyTask());
    }

Now, i'd like to return dummy task, which would be done immediately. However, TaskEx.WhenAll would wait it forever:

    public Task<bool> MyTask()
    {
        return new Task<bool>(() => false);
    }

How can i return Task, which would be finished instantly?

Should you be mocking this out with a different service layer? — David L, Nov 07 '13 at 20:57
@DavidL Yep, that's what i'm going to do, but at the current moment i'm just playing with new architecture, so i'd like to first fill it with dummies, and then starting working on implementation. — Vitalii Vasylenko, Nov 07 '13 at 21:00
@FabioMarcolini Thanks, that works! For wp7, it would be return TaskEx.FromResult(false); but you're rignt. — Vitalii Vasylenko, Nov 07 '13 at 21:05
@user2674389 why not? If to put there task with await, it works fine. — Vitalii Vasylenko, Nov 07 '13 at 21:06

David L · Accepted Answer · 2017-02-10T06:49:38.663

116

in .NET 4.5 you can use FromResult to immediately return a result for your task.

public Task<bool> MyTask()
{
    return TaskEx.FromResult(false);
}

http://msdn.microsoft.com/en-us/library/hh228607%28v=vs.110%29.aspx

For Windows Phone 8.1 and higher, the API has been merged to be consistent with other platforms:

public Task<bool> MyTask()
{
    return Task.FromResult(false);
}

edited Feb 10 '17 at 06:49

answered Nov 07 '13 at 21:05

David L

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4

Since the final release of the Framework its: return Task.FromResult(false); – uli78 Feb 10 '17 at 06:20
1

@uli78 this question was specifically for windows phone 7, but great point! I've updated my answer, thank you! – David L Feb 10 '17 at 06:50
In case it is in this form, you should add await: public async Task MyTask() { return await Task.FromResult(false); } – Anand - MCSA MCPD MCTS MCSD Nov 15 '22 at 10:45
@Anand-MCSAMCPDMCTSMCSD in the OP's (and your) example, there is no reason to generate an additional state machine to await. – David L Nov 15 '22 at 13:39

score 11 · Answer 2 · answered Nov 07 '13 at 21:07

11

Prior to .NET 4.5, you can use TaskCompletionSource<TResult> to simulate the FromResult method.

public static Task<TResult> FromResult<TResult>(TResult result)
{
    var completionSource = new TaskCompletionSource<TResult>();
    completionSource.SetResult(result);
    return completionSource.Task;
}

answered Nov 07 '13 at 21:07

Sam Harwell

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But why would i want to simulate it? Thanks anyway. – Vitalii Vasylenko Nov 07 '13 at 21:08
@VitaliiVasylenko Because prior to .NET 4.5, there is no `FromResult` method. – Sam Harwell Nov 07 '13 at 21:09
But.. well.. http://msdn.microsoft.com/en-us/library/hh194922(v=vs.110).aspx - i guess, it is? – Vitalii Vasylenko Nov 07 '13 at 21:10
@VitaliiVasylenko You are looking at 4.5, [go up a level and change the .net version to 4.0](http://msdn.microsoft.com/en-us/library/system.threading.tasks.task_methods%28v=vs.100%29.aspx) and you will see `FromResult` is not an available method. – Scott Chamberlain Nov 07 '13 at 21:12
@ScottChamberlain But we are talking about 4.5? – Vitalii Vasylenko Nov 07 '13 at 21:16
@280Z28 Well, anyway, thanks for your answer - its always good to keep analogues in mind, and to know what's inside. – Vitalii Vasylenko Nov 07 '13 at 21:17

Return Task instantly

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