Task.WhenAll Not Working

Question

I have several tasks which involve binding collections to ItemsCollections for which I want to do concurrently using Task.WhenAll (or whatever the appropriate method is, if I am wrong). My current code is:

Private Async Function BindAll() As Task
	System.Diagnostics.Debug.WriteLine("BindAll - Start")
	Dim bindingtasks As New List(Of Task)
	If Me.chkDisplayRound.IsChecked Then bindingtasks.Add(New Task(AddressOf Me.BindRoundNumbers))
	If Me.players IsNot Nothing AndAlso Me.players.Any Then
		bindingtasks.Add(New Task(AddressOf Me.BindNames))
		bindingtasks.Add(New Task(AddressOf Me.BindScores))
		If Me.radCumulative.IsChecked Then bindingtasks.Add(New Task(AddressOf Me.BindTotals))
		System.Diagnostics.Debug.WriteLine("BindAll - Checkpoint 1")
		Await Task.WhenAll(bindingtasks)
		System.Diagnostics.Debug.WriteLine("BindAll - Checkpoint 2")
		'Scroll to make the current score visible
		Await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, Sub() Me.scrScores.ScrollToRequired(Me.GetRequiredVisible()))
		Me.stkEnterNames.Visibility = Visibility.Collapsed
	Else : Me.stkEnterNames.Visibility = Visibility.Visible
	End If
	System.Diagnostics.Debug.WriteLine("BindAll - End")
End Function

My code locks up at Await Task.WhenAll(bindingtasks) (between Checkpoint 1 and Checkpoint 2). If I call the methods that I am making into Tasks (BindRoundNumbers(), BindNames(), BindScores(), and BindTotals()) individually, everything is fine, but I want to call them concurrently. What am I doing wrong? Thanks.

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Nathan Sokalski [e-mail] http://www.nathansokalski.com/

Answer 1

Hi Nathan,

While you bind those tasks into bindingtask collection, have you call the Task.Start() method? 

For Each task As var In tasks
     task.Start()
Next

Or you can use Task.Factory.StartNew to initialize a task and run it immediately. See this for more information: StartNew Method (Action), then you could use the following code to deal with the result.

            await Task.Factory.ContinueWhenAll(tasks.ToArray(), continueworks =>
            {
                //do somthing to the reuslt
                foreach (var work in continueworks)
                {
                    int result = work.Result;
                }
            });

--James

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Answer 2

You should use the Task.Run or TaskFactory.StartNew methods when you create a new task and not the Task constructor. Using any of these metods to create a task will start the task immediately after it has been created:

    Private Async Function BindAll() As Task
        System.Diagnostics.Debug.WriteLine("BindAll - Start")
        Dim bindingtasks As New List(Of Task)
        If Me.chkDisplayRound.IsChecked Then bindingtasks.Add(Task.Run(AddressOf Me.BindRoundNumbers))

        If Me.players IsNot Nothing AndAlso Me.players.Any Then
            bindingtasks.Add(Task.Run(AddressOf Me.BindNames))
            bindingtasks.Add(Task.Run(AddressOf Me.BindScores))
            If Me.radCumulative.IsChecked Then bindingtasks.Add(Task.Run(AddressOf Me.BindTotals))
            System.Diagnostics.Debug.WriteLine("BindAll - Checkpoint 1")
            Await Task.WhenAll(bindingtasks)
            System.Diagnostics.Debug.WriteLine("BindAll - Checkpoint 2")
            'Scroll to make the current score visible
            Await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, Sub() Me.scrScores.ScrollToRequired(Me.GetRequiredVisible()))
            Me.stkEnterNames.Visibility = Visibility.Collapsed
        Else : Me.stkEnterNames.Visibility = Visibility.Visible
        End If
        System.Diagnostics.Debug.WriteLine("BindAll - End")
    End Function