Chaining Tasks with Different Return Types

Question

Is it possible to chain tasks with different return types or no return type at all? For example in pseudo-code:

Task<double>.ContinueWith(Task<string>).ContinueWith(Task<String>).ContinueWith(Task)

Or also here is real-code example:

private double SumRootN(int root)
{
    double result = 0;
    for (int i = 1; i < 10000000; i++)
    {
        result += Math.Exp(Math.Log(i) / root);
    }
    return result;
}

private void taskSequentialContinuationButton_Click(object sender, RoutedEventArgs e)
{
    Task<double> task = null;

    this.statusText.Text = "";
    this.watch = Stopwatch.StartNew();
    for (int i = 2; i < 20; i++)
    {
        int j = i;
        if (task == null)
        {
            task = Task<double>.Factory.StartNew(() => { return SumRootN(j); });
        }
        else
        {
            task = task.ContinueWith((t) => { return SumRootN(j); });
        }

        task = task.ContinueWith((t) => 
        { // I don't want to return anything from this task but I have to, to get it to compile
            this.statusText.Text += String.Format("root {0} : {1}\n", j, t.Result);
            return t.Result;
        }, TaskScheduler.FromCurrentSynchronizationContext());
    }

    task.ContinueWith((t) => 
        { // I also don't want to return anything here but I don't seem to have to here even though intellisense expects a Task<double>??
            this.statusText.Text += String.Format("{0}ms elapsed\n", watch.ElapsedMilliseconds);
        }, TaskScheduler.FromCurrentSynchronizationContext());
}

See the inline comments for the oddities of the chaining.

Answer 1

If you want to chain Tasks with different return types, you can just put each of them in a different variable:

Task<Type1> task1 = Task.Factory.StartNew(() => Compute1());
Task<Type2> task2 = task1.ContinueWith(_ => Compute2());
Task<Type3> task3 = task2.ContinueWith(_ => Compute3());

For your specific case, where you compute something in a loop and want to report on the UI thread after each iteration, you can do it like this:

var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();

Task.Factory.StartNew(() =>
{
    for (int i = 2; i < 20; i++)
    {
        // perform computation

        Task.Factory.StartNew(() =>
        {
            // report result
        }, CancellationToken.None, TaskCreationOptions.None, uiScheduler);
    }
});