在c＃5.0中，“async / await”函数是否始终在运行开始时在主线程上运行

Question

I find that when I call an "async" function, at first, the thread is the main thread. When it meets the await Task, then the thread will be changed to another one.

static void Main(string[] args)
{
    println("Mainthread ID " + Thread.CurrentThread.GetHashCode());
    var s = DoSomethingAsync();
    println("Waiting for DoSomethingAsync Complete ");
    Thread.Sleep(2000);
    println(s.Result.ToString());
}

static async Task<int> DoSomethingAsync()
{
    println("DoSomethingAsync Thread ID " + Thread.CurrentThread.GetHashCode());
    int val = await Task.Factory.StartNew(() =>
    {
        Thread.Sleep(1000);
        println("Task Thread ID " + Thread.CurrentThread.GetHashCode());
        return 100;
    });

    println("DoSomethingAsync end " + Thread.CurrentThread.GetHashCode());
    return val;

}

And the result is as follow:

11:40:34 383 Mainthread ID 1
11:40:34 398 DoSomethingAsync Thread ID 1
11:40:34 400 Waiting for DoSomethingAsync Complete
11:40:35 400 Task Thread ID 3
11:40:35 400 DoSomethingAsync end 3
11:40:36 401 100

We can see the mainthread id is 1. DoSomethingAsync Thread ID is also 1. They are same. So at the beginning of running "async" function, it's the main thread that do this part.

This means when the main thread meet the "async" function, it will be blocked to run something until it meet the await Task.The "async" function doesn't make things asynchronous instantly. Is it right?

I know that if there's no await Task in the "async" function ,this "async" function will be run synchronously.

Answer 1

This means when the main thread meet the "async" function, it will be blocked to run something until it meet the await Task.The "async" function doesn't make things asynchronous instantly. Is it right?

Yes, that is right. Your async method will run until hitting its first await keyword, which will yield control back to the caller after that. In your example, you don't await on that method, so it will continue to execute the next method right away.

As a side note, a Console Application doesn't really have a Main Thread , as it uses the default ThreadPoolSynchronizationContext . That means that you'll see any arbitrary thread id after your awaited async method.

Answer 2

The method will start running in whatever thread calls the method. If the method is called from the main thread, it'll start running in the main thread. If it's called from a thread pool thread it'll start running in a thread pool thread. It is no different from any other method in that regard.

Continuations after any await operation will leverage the current SyncrhonizationContext to determine how they are scheduled.