Why am I still on the Main Thread when I specified ConfigureAwait(false)?

Question

I am trying to understand how async/await keywords works. I have a textblock on a WPF window bind to a TextBlockContent string property and a button which trigger on click ChangeText().

Here is my code :

public async void ChangeText()
{
     string mystring = await TestAsync();
     this.TextBlockContent= mystring;
}

private async Task<string> TestAsync()
{
     var mystring = await GetSomeString().ConfigureAwait(false);
     mystring = mystring + "defg";
     return mystring;
}

private Task<string> GetSomeString()
{
    return Task.Run(() =>
    {
        return "abc";
    });
}

From my readings, I understood that ConfigureAwait set to false would allow me to specify that I do not want to save the Current context for the "rest" of the Task that needs to be executed after the await keyword.

After debugging i realize that when i have this code, sometime the code is running on the Main thread after the line : await GetSomeString().ConfigureAwait(false); while i specifically added the configure await.

I would have expected it to always run on a different thread that the one it was in before it entered the Task.

Can someone help me understand why?

Thank you very much

Answer 1

It could well be that the task you're running is completing immediately - which means that the await doesn't even need to schedule a continuation. An async method only schedules a continuation when it needs to; if you await something which has already completed, it just keeps going - in the same thread.

ConfigureAwait is a hint that you don't need to continue in the same context; it's not a requirement that you mustn't continue on the same context.

If you change your task to something that doesn't complete immediately - e.g. call Thread.Sleep(1000) (which of course you wouldn't normally do) I'd expect you to see a change in behaviour.

It's possible that I've missed something else and that this isn't the cause of the problem, but it would at least be worth trying that first.