Return Task<bool> 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?

Solution 1

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);
}

Solution 2

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;
}

Comments

[Vitalii Vasylenko]

But why would i want to simulate it? Thanks anyway.

[Sam Harwell]

@VitaliiVasylenko Because prior to .NET 4.5, there is no FromResult method.

[Vitalii Vasylenko]

But.. well.. msdn.microsoft.com/en-us/library/hh194922(v=vs.110).aspx - i guess, it is?

[Scott Chamberlain]

@VitaliiVasylenko You are looking at 4.5, go up a level and change the .net version to 4.0 and you will see FromResult is not an available method.

[Vitalii Vasylenko]

@ScottChamberlain But we are talking about 4.5?

[Vitalii Vasylenko]

@280Z28 Well, anyway, thanks for your answer - its always good to keep analogues in mind, and to know what's inside.

[Taiseer Joudeh]

@SamHarwell Thank you, this was very useful.

[Bionic]

Since the final release of the Framework its: return Task.FromResult(false);

[David L]

@uli78 this question was specifically for windows phone 7, but great point! I've updated my answer, thank you!