How to wait for multiple Tasks without blocking the GUI?

Question

In a C# .NET 4.0 Windows Forms GUI application, in response to a button being clicked I need to spawn a large number of processes and show them in a list box. As each process ends it should be removed from the list and when all processes are done, something should happen.

Since it's 4.0 I think using Task is the best route but I can't just run up a bunch of tasks and call Task.WaitAll because that would block the main UI thread, as the button click handler wouldn't return.

What is the nicest way to implement this? I can see two main options:

1. The button handler starts one task, which runs all the other tasks and waits for them to complete.
2. I don't use Task.WaitAll - as each Task completes it checks (or tells the application to check) if all the tasks are complete or not.

But is there a standard/preferred solution here using the TPL?

Also - can a Task update my form GUI or do I need to decouple Task-GUI interaction, eg when a Task completes it fires an event on the Form ?

Pseudo-code would be great - I'm new to TPL and much of the fun stuff available in .NET 4.0.

Answer 1

If you were using .NET 4.5 you would use something like await Task.WhenAll(...) . It can handle IEnumerable<Task> , Task[] and also IEnumerable<Task<TResult>> .

If there is no way you can use .NET 4.5, you can come up with a solution of yourself, that will involve Task.ContinueWith and a counter of how many tasks are left to run, that is updated using Interlocked.Decrement . Also, you can look at how it's done inside .NET 4.5 by looking at this source. http://referencesource.microsoft.com/#mscorlib/system/threading/Tasks/Task.cs,69351c6da968e5d1 . It's a bit lengthy, but it basically is what I said: tasks are continued with some other tasks that update internal counter.