Asynchronous Programming and await in a loop

Question

I started to learn Asynchronous programming in C# 5 and .Net 4.5 and there is something I don't understand.

private static int count;

public static void Main()
{
    LoopTest();

    Console.ReadKey(false);
}

static async void LoopTest()
{
    count = 0;

    for (var index = 0; index < 10; ++index)
    {
        Console.WriteLine( "({0}) In Loop before await, Index {1}, Thread: {2}", count++, index, Thread.CurrentThread.ManagedThreadId);
        await Task.Factory.StartNew(() => Thread.Sleep(10));
    }
}

The result is:

 (0) In Loop before await, Index 0, Thread: 9  
 (1) In Loop before await, Index 1, Thread: 10    
 (2) In Loop before await, Index 2, Thread: 11   
 (3) In Loop before await, Index 3, Thread: 10  
 (4) In Loop before await, Index 4, Thread: 11  
 (5) In Loop before await, Index 5, Thread: 10  
 (6) In Loop before await, Index 6, Thread: 12  
 (7) In Loop before await, Index 7, Thread: 11  
 (8) In Loop before await, Index 8, Thread: 10  
 (9) In Loop before await, Index 9, Thread: 12

So are there different Threads accessing the same loop? is there a Race-Condition on the index variable?

Please replace await Task.Factory.StartNew(() => Thread.Sleep(10)); with await Task.Delay(10). You are supposed to let go of your thread for 10 ms, not hold on to it. — Toni Petrina
– Toni Petrina, Commented Jan 22, 2013 at 15:03
@ToniPetrina Thread.Sleep is used to hold on the thread as to simulate 10 ms thread usage. — Syaiful Nizam Yahya
– Syaiful Nizam Yahya, Commented Jan 25, 2014 at 16:23

Servy · Accepted Answer · 2013-01-17 15:01:21Z

No, there is no race condition. You start a new task, that will run in the background, but your code will not continue until that task finishes, so you can be sure that your code will only be executed in one thread at any one time.

However, because this is a console application there is no current SyncronizationContext. (As opposed to a winform, WPF, ASP, Silverlight, etc. application in which there will be.) This means that the continuations from any await call will run in the thread pool, not in the calling thread. So yes, you're running in one of the thread pool threads in each iteration of the loop, and it could be a different one each time, but you can be sure that, excepting whatever you put inside the new task, you will "finish" running on any given thread before you start working on the next, which ensures that there is only ever one thread running code at any given time (which means no race conditions).

Pete · Accepted Answer · 2013-01-17 14:54:50Z

4

What you're doing isn't really asycnhronous

await Task.Factory.StartNew(() => Thread.Sleep(10));

This creates a task and then waits for that task to complete. You may as well just call Thread.Sleep(10) here.

A more appropriate way to do this would be:

Task myTask = Task.Factory.StartNew(() => Thread.Sleep(10));
... do stuff here ...
await myTask; // Now wait for the task to complete

If you want to do a number of tasks in a loop, you should look at the Parallel.Foreach() method.

answered Jan 17, 2013 at 14:54

Pete

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3 Comments

m1o2 Over a year ago

Thanks for your answer! but I have a few questions. if I'm creating a task on the current Thread then who is creating the rest of the threads ? or why the ManagedThreadId is keep changing ?

Servy Over a year ago

The example is just a toy example in which he's experimenting and trying to explain the results. This doesn't explain those results, and is attempting to say the test is wrong, when there is nothing it's trying to accomplish to begin with short of explaining how await works.

Pete Over a year ago

Sorry, I removed my mistaken "on the current thread" phrase from the answer. The tasks are run in a thread pool and won't necessarily create as many threads as you have tasks. It will try to manage resources for you, which is one of the advantages of using it over threads.

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