My async Task always blocks UI

Question

In a WPF 4.5 application, I don't understand why the UI is blocked when I used await + a task :

    private async void Button_Click(object sender, RoutedEventArgs e)
    {
        // Task.Delay works great
        //await Task.Delay(5000);

        double value = await JobAsync(25.0);

        MessageBox.Show("finished : " + value.ToString());
    }

    private async Task<double> JobAsync(double value)
    {
        for (int i = 0; i < 30000000; i++)
            value += Math.Log(Math.Sqrt(Math.Pow(value, 0.75)));

        return value;
    }

The await Task.Delay works great, but the await JobAsync blocks the UI. Why ? Thank you.

Answer 1

You should be getting a warning about JobAsync - it contains no await expressions. All your work is still being done on the UI thread. There's really nothing asynchronous about the method.

Marking a method as async doesn't make it run on a different thread - it's more that it makes it easier to join together asynchronous operations, and come back to the appropriate context.

I suspect it would be a good idea to take a step back and absorb some of the materials about async on MSDN... this is a good starting point...