Run multiple tasks with continuation

Question

I found some strange behavior running tasks within a for loop and awaiting them all. As mentioned below, what the code does is starting a "loop" for definite number of tasks, each task creates a "ToDo" item and each task has a continuation that assigns each created task to a person and finally but them in a listBox using Invoke so the UI thread is called. this works fine, but i do not get the expected data in the listBox. I do not expect them to be ordered, but i do expect them to be paired, eg :

Person_8, Todo 8 Person 5, Todo 5 etc...

And they should only appear ones in the listBox of course ! But instead, i get strange output (and output is never the same for each run), here are some examples running the code:

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And here is the code:

private async void buttonCreateToDo_Click(object sender, EventArgs e){
    await CreateToDoAsync();
}

private async Task CreateToDoAsync(){
    List<Task> taskList = new List<Task>();
    for (int i = 1; i < 10; i++){
        var task = Task.Run(() => CreateToDo(i));
        Task continuation = task.ContinueWith((antecedent) => Invoke(new AssignTaskDelegate(AssignTask), (new Person() {
            Name = $"Person_{i}",
            ToDoForPerson = antecedent.Result
        })));
        taskList.Add(task);

    }
    await Task.WhenAll(taskList.ToArray());
}

private ToDo CreateToDo(int toDoId) {
    return new ToDo(){
        Id = toDoId,
        Description = $"Todo {toDoId}"
    };
}

private void AssignTask(Person person){
    listBoxToDo.Items.Add($"{person.Name}, {person.ToDoForPerson.Description}");
}

Answer 1

Your issue is that for loop runs much faster than the creation of the tasks and so by the time the tasks run the variable i has reached the end of the loop.

To fix this you need to take a copy of i inside the loop and use that.

Try this code:

private async Task CreateToDoAsync()
{
    List<Task> taskList = new List<Task>();
    for (int i = 1; i < 10; i++)
    {
        var local_i = i;
        var task = Task.Run(() => CreateToDo(local_i));
        Task continuation = task.ContinueWith((antecedent) => Invoke(new AssignTaskDelegate(AssignTask), (new Person()
        {
            Name = $"Person_{local_i}",
            ToDoForPerson = antecedent.Result
        })));
        taskList.Add(task);
    }
    await Task.WhenAll(taskList.ToArray());
}

Now, in preference I'd use Microsoft's Reactive Framework (NuGet "System.Reactive") to do this work. Your code would look like this:

private async Task CreateToDoAsync()
{
    var query =
        from i in Observable.Range(1, 9)
        from t in Observable.Start(() => CreateToDo(i))
        select new Person() { Name = $"Person_{i}", ToDoForPerson = t };

    await query.ObserveOn(listBoxToDo).Do(x => AssignTask(x));
}

Done. That's it.

When I run my code (with the AssignTask outputting to the Console) I get this:

Person_1, Todo 1
Person_2, Todo 2
Person_3, Todo 3
Person_6, Todo 6
Person_7, Todo 7
Person_4, Todo 4
Person_5, Todo 5
Person_8, Todo 8
Person_9, Todo 9

The .ObserveOn(listBoxToDo) works for Windows Forms to marshall back to the UI thread.

Answer 2

You are using variable "i" from the loop inside the task. "i" might have changed while your task started.