BitmapAssetValueConverter - unit testing

[jaromir-dobes]

Hello!
In my code, I use BitmapAssetValueConverter to set the image source dynamically, during the application runtime.
The code looks like the following (I'm leaving out exception handling for brevity):

    public class BitmapAssetValueConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            switch (value)
            {
                case null:
                    return null;
                case string rawPath when targetType == typeof(IBitmap):
                {
                    return new Bitmap(rawPath);
                }
                default:
                    return null;
            }
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotSupportedException();
        }
    }

In my .NET Core 3.1 project, I use Avalonia.Desktop and Avalonia.ReactiveUI packages.
I've got a few unit tests going through this code. But I'm not sure they are correct.
In one of the tests I put in a valid path to a PNG file and verify that the returned object is a bitmap with expected attributes.
Without any Avalonia initialization, the Bitmap constructor throws a NullReferenceException.
If I add the following code, my tests (multiple tests executing this code, running in parallel, to be precise) often work, but sometimes they fail with InvalidOperationException : Call from invalid thread

AppBuilder.Configure<App>().UsePlatformDetect().UseReactiveUI().SetupWithoutStarting();

My questions are:

• are there any recommendations on unit testing of the code involving UI elements?
• is my way of initializing Avalonia for the tests correct? Does Avalonia require anything like this?
• are there any restrictions on the tests as regards their execution (in a sequence, in parallel, any other)?

Many thanks, all suggestions and ideas appreciated.

[maxkatz6]

are there any recommendations on unit testing of the code involving UI elements?

Use Avalonia.Headless platform. There is an example of integration with xUnit - https://github.com/kekekeks/Avalonia-unit-testing-with-headless-platform

is my way of initializing Avalonia for the tests correct? Does Avalonia require anything like this?

Not sure how exactly you initialize Avalonia for tests, this one line it not enough to start main loop.

are there any restrictions on the tests as regards their execution (in a sequence, in parallel, any other)?

I doubt that it's possible to run Avalonia app on multiple UI threads (for parallel tests). Avalonia internal code expects to have only one UI thread, at least there is single static Dispatcher.UIThread.