Using "await" in a converter (IValueConverter) not working?

Question

I have a converter in my Windows Phone application but it seems you cannot use "await" methods in it?

public object Convert(object value, Type targetType, object parameter, string language)
{
    _IDataService = ServiceLocator.Current.GetInstance<IDataService>();

    string imageUrlId = (string)value;

    byte[] imageByte = await iDataService.GetImage(imageUrlId);

    return LoadImageAsync(imageByte);
}

If I make the method async Task it says IValueConverter has no method Task async. This converter returns type ImageSource.

Kzryzstof · Answer 1 · 2018-12-09T12:22:50.073

4

You can't change the signature of the Convert method since you have to provide an implementation for all the methods of the IValueConverter interface.

What I usually do is loading in the images of the view-model asynchronously from the backend, not on the UI thread. With the proper bindings, the images will be displayed on your UI as soon as it is loaded.

edited Dec 09 '18 at 12:22

answered Dec 12 '15 at 13:46

Kzryzstof

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I see, and can I use await async in one of my model classes? In the get from a property? – Bayern Dec 12 '15 at 14:28
No, you can't use the async keyword on a property. (See http://stackoverflow.com/questions/12384309/async-property-in-c-sharp-4-5) Just make your GetImage method of your IDataService interface asynchronous. Loads the image from your view-model by calling the async GetImage and assign the BitmapImage to one of the property of the view-model. :) – Kzryzstof Dec 12 '15 at 14:32
Yes I got it! But I need to go to the detail page and then back to see the images, when I login I see my ListView with empty images, going to a detail page and back they are shown. – Bayern Dec 12 '15 at 20:51
I'm glad you got it :) – Kzryzstof Dec 13 '15 at 13:55

score 1 · Answer 2 · edited May 23 '17 at 12:02

1

I have an older answer here that mostly addresses this problem (but see below - one of the helper types is somewhat dated).

The core of the problem is that asynchronous work is actually improper for a value converter to do. What you're really doing is telling WPF "to cast this type to this other type, first call a web service..."

So, IMO any asynchronous behavior is better done in other parts of the system (ViewModel, service layer, etc). Asynchronous work can be easily represented in a ViewModel by using my NotifyTask type.

However, my older answer does have a working "asynchronous value converter". Note that the TaskCompletionNotifier type in that answer has a newer implementation.

edited May 23 '17 at 12:02

Community

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answered Dec 12 '15 at 20:54

Stephen Cleary

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Now I need to know how to update the xaml UI after a background task is done. – Bayern Dec 12 '15 at 20:59
@Bayern: That's what the `NotifyTask` type is for. I have an [MSDN article](https://msdn.microsoft.com/en-us/magazine/dn605875.aspx) with example usage. – Stephen Cleary Dec 12 '15 at 21:49
Ahaha that looks interesting! That will work for this problem? http://stackoverflow.com/questions/34244838/images-downloaded-in-a-async-method-shown-after-coming-back-from-a-detail-page – Bayern Dec 12 '15 at 22:01
@Bayern: From what I can tell of the question, it looks like yes. `NotifyTask` is specifically for updating MVVM UIs with the results of background operations. – Stephen Cleary Dec 13 '15 at 01:47
OK then I will try this. I also found out that when I scroll down and back up the images are also shown. – Bayern Dec 13 '15 at 09:30

score -2 · Answer 3 · answered Dec 12 '15 at 16:50

-2

You could use .result () after async function call

Like, byte[] imageByte = iDataService.GetImage(imageUrlId).result ();

answered Dec 12 '15 at 16:50

ITGenius

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You could, but generally you never should. – Aberro May 17 '20 at 21:38

Using "await" in a converter (IValueConverter) not working?

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