Async method is working as syncronous mehtod

Question

I am trying to call a async method of ViewModel from another method in View but it is behaving as syncronous.

View Model:

public async Task<bool> GetReadyForUnlockWithQR()
    {
        try
        {
            SmilaHash = GetRandomeHashKey();
            var data = JsonConvert.SerializeObject(GetSmilaInfo());
            var res = await apiClient.PostAsync<String>("readyforunlockwithqr", data);
            if (res != null)
            {
                JObject json = JObject.Parse(res);
                if (json["doUnlock"] != null)
                {
                    LoginStatus = json.SelectToken("doUnlock").Value<bool>();
                }
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message);
            CancelPendingRequests();
            throw ex;
        }
        return false;
    }

I have my api methods defined in a custome APIClient file. The above request may take a minute to complete. I don't want to stop the UI and perform my further operations in View. Following is my View:

private async void UnlockButton_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            await ViewModel.GetReadyForUnlockWithQR();
            DisplayQRCode();
        }
        catch(Exception ex)
        {
            if (ex is HttpRequestException)
            {
                Debug.WriteLine("Server not reachable");
                MessageBox.Show("Server not reachable");
            }
            else if (ex is OperationCanceledException)
            {
                Debug.WriteLine("Timeout exception");
                QRCodeImage.Source = null;
                QRCodeCanvas.Visibility = Visibility.Hidden;
            }
            else
            {
                Debug.WriteLine(ex.Message);
            }
        }
    }

I above code ideally the DisplayQRCode() function should work immediately after await ViewModel.GetReadyForUnlockWithQR(); but it is not happening. The DisplayQRCode() is waiting to receive response from ViewModel.GetReadyForUnlockWithQR() Why is this not behaving as logical asyn code.

*"The DisplayQRCode() is waiting to receive response from ViewModel.GetReadyForUnlockWithQR()"* - Because you are using `await` when calling the `async` operation. Did you not want to do that? — David, Feb 23 '22 at 14:41
No, I don't want to wait, the response can take over minute to arrive — Panks, Feb 23 '22 at 14:44
You must not remove `await`. Just call `DisplayQRCode()` *before* `await ViewModel.GetReadyForUnlockWithQR();`. See [Asynchronous programming with async and await](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/) for an introduction. This may also be helpful: https://stackoverflow.com/q/14455293/1136211 — Clemens, Feb 23 '22 at 14:45
Why did you tag this as both ASP.NET and WPF? And I'm not a desktop developer...but isn't MessageBox specific to Windows Forms? — mason, Feb 23 '22 at 15:30

score 2 · Accepted Answer · answered Feb 23 '22 at 15:45

The DisplayQRCode() is waiting to receive response from ViewModel.GetReadyForUnlockWithQR() Why is this not behaving as logical asyn code.

The asynchronous method is behaving serially (not "synchronously"), which is exactly what await is supposed to do.

You can think of await as "asynchronous wait": the method is paused and will not continue pass the await until the task completes, but it waits asynchronously, so the thread is freed (the method returns to its caller).

I above code ideally the DisplayQRCode() function should work immediately after await ViewModel.GetReadyForUnlockWithQR(); but it is not happening.

If you want to do that, then you can call GetReadyForUnlockWithQR but don't await the task until after DisplayQRCode completes:

var getReadyTask = ViewModel.GetReadyForUnlockWithQR();
DisplayQRCode();
await getReadyTask;

Async method is working as syncronous mehtod

1 Answers1