Does Task delay create new Thread?

user_14176971 0
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I would like to ask whether the Task.Delay calls Task.Run underneath and it is reason of change of thread id or there is different logic behind it?

async Task Main()
{
     Console.WriteLine(Thread.CurrentThread.ManagedThreadId); //11
     await Task.Delay(200);
     Console.WriteLine(Thread.CurrentThread.ManagedThreadId); //22
}
.net-core asynchronous c#
1 Answers

No, the Task.Delay doesn't call the Task.Run underneath. It just returns a Task that completes after the specified duration, and invokes any continuations that are attached to it on the ThreadPool. It uses a System.Threading.Timer internally. Its implementation looks like this:

public class Task
{
    public static Task Delay(int millisecondsDelay)
    {
        var tcs = new TaskCompletionSource();
        _ = new Timer(_ => tcs.SetResult(), null, millisecondsDelay, -1);
        return tcs.Task;
    }
}

So when you await the Task.Delay, the continuation after the await runs on the same thread where the Timer callback is invoked¹, which happens to be a ThreadPool thread.

Note: The above implementation is presented as an example. It's not the actual implementation, and it's not guaranteed to work correctly. The Timer instance is not stored anywhere, and so it might become garbage collected before the callback is invoked, resulting in a never-completing Task.

¹ Assuming that no SynchronizationContext is in place.

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