Chaining Tasks with Different Return Types

.net, .net-4.0, c#, task-parallel-library

Solution

If you want to chain `Task`s 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);
    }
});

Problem

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.

Original source