Question regarding ConfigureAwait(false)

[MartyIX]

Hi,

I have been trying to figure out a question of mine regarding:

HackerNews/src/HackerNews/ViewModels/NewsViewModel.cs

Lines 76 to 106 in aaad245

	try
	{
	await foreach (var story in GetTopStories(StoriesConstants.NumberOfStories).ConfigureAwait(false))
	{
	StoryModel? updatedStory = null;
	try
	{
	updatedStory = story with { TitleSentiment = await _textAnalysisService.GetSentiment(story.Title).ConfigureAwait(false) };
	}
	catch (Exception)
	{
	//Todo Add TextAnalysis API Key in TextAnalysisConstants.cs
	updatedStory = story;
	}
	finally
	{
	if (updatedStory is not null && !TopStoryCollection.Any(x => x.Title.Equals(updatedStory.Title)))
	InsertIntoSortedCollection(TopStoryCollection, (a, b) => b.Score.CompareTo(a.Score), updatedStory);
	}
	}
	}
	catch (Exception e)
	{
	OnPullToRefreshFailed(e.ToString());
	}
	finally
	{
	IsListRefreshing = false;
	}
	}

Basically, my question is: Is it OK to use ConfigureAwait(false) here:

HackerNews/src/HackerNews/ViewModels/NewsViewModel.cs

Line 78 in aaad245

await foreach (var story in GetTopStories(StoriesConstants.NumberOfStories).ConfigureAwait(false))

?

The reason I'm asking is that IsListRefreshing is not necessarily set on the UI thread:

HackerNews/src/HackerNews/ViewModels/NewsViewModel.cs

Line 104 in aaad245

IsListRefreshing = false;

And I wonder whether one can set view model properties (used in data bindings) on non-UI threads. Is it or is it not?

Best regards!

[brminnick]

Good question!

Short answer: Yes, in .NET MAUI it's safe to update properties in the ViewModel that are bound to the view from any thread.

.NET MAUI (and Xamarin.Forms) marshall binding updates to the Main (UI) Thread, so we're totally safe setting a property in our ViewModel from any thread because MAUI's binding engine will bring the updates to the UI Thread for us.

You'll want to stay aware around this, though. There has been talks amongst the engineering team to no longer marshall every binding to the UI Thread, and I've seen some instances in newer versions of .NET MAUI where it wasn't properly marshaling CollectionView updates to the UI Thread. It'd be a substantial breaking change, and I don't they'd break all of our code, but it could happen!

[MartyIX]

Thank you for the answer. Much appreaciated!

[MartyIX]

.NET MAUI (and Xamarin.Forms) marshall binding updates to the Main (UI) Thread, so we're totally safe setting a property in our ViewModel from any thread because MAUI's binding engine will bring the updates to the UI Thread for us.

Do you happen to know where in MAUI's source code I could find this marshalling?

[brminnick]

In BindingExpression.OnPropertyChanged(), the binding expression is marshaled to the UI Thread via Dispatcher.DispatchIfRequired():
https://github.com/dotnet/maui/blob/1b86f6583ed97c82de67a8c29ee0ad7193b45059/src/Controls/src/Core/BindingExpression.cs#L750