Calling Task.wait may not wait if the task has not yet started?

Question

I was reading Jeffrey Richter\'s clr via c# book and felt uncomfortable reading that task wait may not always wait and I quote

\"When a thread calls

Answer 1

I think this is unfortunately phrased. It's not that the Wait call returns before the task has finished executing; it's that the thread calling Wait may end up executing the task itself, rather than just blocking idly.

Sample code:

using System;
using System.Threading;
using System.Threading.Tasks;

class Test
{
    static void Main()
    {
        // Make sure DemonstrateIssue is already called in a ThreadPool
        // thread...
        Task task = Task.Run((Action) DemonstrateIssue);
        task.Wait();
    }

    static void DemonstrateIssue()
    {
        Console.WriteLine("DemonstrateIssue thread: {0}",
                          Thread.CurrentThread.ManagedThreadId);
        Action action = () => Console.WriteLine("Inner task thread: {0}",
            Thread.CurrentThread.ManagedThreadId);
        Task task = new Task(action);
        // Calling Start will just schedule it... we may be able to Wait
        // before it actually executed
        task.Start();
        task.Wait();
    }
}

Output every time I've run it:

DemonstrateIssue thread: 3
Inner task thread: 3

This takes advantage of the fact that the thread pool doesn't spin up threads immediately on demand - it waits for a while to see if an existing thread will become available before starting another one. If you add Thread.Sleep(5000); before the call to task.Wait(), you'll see the two tasks end up on different threads.