Question

I am playing with async await feature of C#. Things work as expected when I use it with UI thread. But when I use it in a non-UI thread it doesn’t work as expected. Consider the code below

private void Click_Button(object sender, RoutedEventArgs e)
    {
        var bg = new BackgroundWorker();
        bg.DoWork += BgDoWork;
        bg.RunWorkerCompleted += BgOnRunWorkerCompleted;
        bg.RunWorkerAsync();
    }

    private void BgOnRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs runWorkerCompletedEventArgs)
    {
    }

    private async void BgDoWork(object sender, DoWorkEventArgs doWorkEventArgs)
    {
        await Method();
    }


    private static async Task Method()
    {
        for (int i = int.MinValue; i < int.MaxValue; i++)
        {
            var http = new HttpClient();
            var tsk = await http.GetAsync("http://www.ebay.com");
        }
    }

When I execute this code, background thread don’t wait for long running task in Method to complete. Instead it instantly executes the BgOnRunWorkerCompleted after calling Method. Why is that so? What am I missing here?

P.S: I am not interested in alternate ways or correct ways of doing this. I want to know what is actually happening behind the scene in this case? Why is it not waiting?

Answer 1

So, BgDoWork is called on a background thread by the BackgroundWorker

It calls Method, which starts the loop and calls http.GetAsync

GetAsync returns a Task and continues it’s work on another thread.

You await the Task which, because the Task has not completed, returns from Method

Similarly, the await in BgDoWork returns another Task

So, the BackgroundWorker sees that BgDoWork has returned and assumes it has completed.

It then raises RunWorkerCompleted

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Basically, don’t mix BackgroundWorker with async / await!