What happens to work scheduled by Task.Run() after the program terminates?

Question

I was doing some tests with the TPL and async/await and noticed something that I find unexpected: I was scheduling work to run using lambdas and Task.Run, for instance:

Task.Run(()=>Console.WriteLine("Nice program"));

And then I realized that if program immediately returns the work is never executed. Is that the expected behavior in any .NET application (WPF, Forms, etc.)? Is there any documentation that discusses this?

This means that Task.Run is actually a no-go for fire-and-forget scenarios.

Answer 1

This means that Task.Run is actually a no-go for fire-and-forget scenarios.

Well, you don't want to forget - you want to wait until it's completed. So use the Task that's returned to you.

To do that, you'll need to keep track of all uncompleted tasks that you launch this way, and then use something like Task.WaitAll(tasks) in a non-background thread. You potentially don't need to remember the tasks themselves - you just need to have a counter which is decremented when each task completes, and then you just need to wait for that to get to zero.

It's hard to give more concrete advice than that without knowing more about your scenario, to be honest... but something like that would certainly work.

You can easily encapsulate this in your own convenience methods, of course.

Answer 2

By definition, after program terminates, no code can run. This has nothing to do with Task.Run().

If what you're actually asking is something like:

Tasks run on background threads, so if the main thread completes (e.g. after the user closes the main window), they are not guaranteed to run to completion or even start, how can I fix that?

Then there are two options: either don't let the main thread complete (e.g. by calling Task.WaitAll() as Jon Skeet suggests), or run your important Tasks on a foreground thread. To do that, you can use QueuedTaskScheduler from ParallelExtensionsExtras.