C# Multithreading – BackgroundWorker vs. Async/Await

No, async + await != sync, because of continuation

From MSDN 'Asynchronous Programming with Async and Await (C# and Visual Basic)'

Async methods are intended to be non-blocking operations. An await expression in an async method doesn’t block the current thread while the awaited task is running. Instead, the expression signs up the rest of the method as a continuation and returns control to the caller of the async method.

For example async execution will not block UI thread, and Some TextBox.Text will be updated after download has finished

private async void OnButtonClick()
{
   SomeTextBox.Text = await new WebClient().DownloadStringTaskAsync("http://stackoverflow.com/");
}

You could rewrite your code this way in a pure async/await paradigm.

    private async void button1_Click(object sender, EventArgs e)
    {
        int r = await longRunningWork();
        textBox1.Text += (r.ToString());
    }

    private async Task<int> longRunningWork()
    {
        await Task.Delay(15000);
        return (new Random()).Next(10);
    }

The fact a thread is created is up to the CLR to decide. What the await does in the button1_Click method is that it returns the control to the calling thread of button1_Click (UI thread) which can do other work. The compiler introduces automatic code everytime it sees await to add some goto at the end of the awaitable method whenever it is completed to continue the execution where it left.

My guess here why I think your code spawn a new thread is because you call Thread.Sleep(15000); which is a synchronous call, hence if the CLR wants to give control back to the UI thread, it knows it has to spawn a new thread.

Even your original longRunningWork method is synchronous so it definitely needs a new thread.