How to add a task to the collection that Task.WhenAll is waiting for?

Question

Isn't it possible to add a task to the list that Task.WhenAll is waiting already?

I want to wait for all the tasks to finish, and there is a possibility to create new tasks after initialization. But here Task.WhenAll doesn't wait for the new task that is added to the collection later.

List<Task> tasks = new List<Task>();

var task1 = Task.Run(async () =>
    {
        Debug.WriteLine("task #1 started");
        await Task.Delay(TimeSpan.FromSeconds(20));
        Debug.WriteLine("task #1 finished");
    });

var task2 = Task.Run(async () =>
{
    Debug.WriteLine("task #2 started");
    await Task.Delay(TimeSpan.FromSeconds(30));
    Debug.WriteLine("task #2 finished");
});

var task3 = Task.Run(async () =>
{
    Debug.WriteLine("task #3 started");
    await Task.Delay(TimeSpan.FromSeconds(10));
    var inner = Task.Run(async () =>
    {
        Debug.WriteLine("inner task started");
        await Task.Delay(TimeSpan.FromSeconds(40));
        Debug.WriteLine("inner task finished");
    });

    tasks.Add(inner);

    Debug.WriteLine("task #3 finished");
});

tasks.Add(task1);
tasks.Add(task2);
tasks.Add(task3);

await Task.WhenAll(tasks);
Debug.WriteLine("All finished");

Output:

task #2 started
task #3 started
task #1 started
task #3 finished
inner task started
task #1 finished
task #2 finished
All finished
inner task finished < didn't wait for this to finish

Answer 1

This is because tasks.Add(inner) is executed after await Task.WhenAll(tasks). To answer your question, first you need to clearify the releationship between task #3 and task inner. IMO the inner task should be part of task #3. That is, task #3 can't finish until the inner task finishes.

await inner;   //instead of tasks.Add(inner);